rheinwaldner wrote:

It could even be an active hydraulic that extends the gear at the time of rotation, but all the suggestions have one thing in common: they push the airframe upward at the time of rotation because the wing at the possible pitch angle can not provide enough lift alone.

There is no energy leveraged from "springing up" the gear.
What you get is
"while unloading the gear via creating lift the extension of the sprung gear leg
per tonne unloading is (much) bigger in this design than on the other 737 models"

the same happens on the A330/A340 gear leg.
initial unloading from lift pushed the rear of the bogie down.
( only on the A330/A340 case the bogie then is stowed in the tilted position.)

the plane now rotates around a point further aft and down from the previous position ( relative to the fuselaqe ).
( But that is what you want. the spur cylinder on the 77W does the same ( only it is an "active" design element.
My guess would be that for the 3 axle bogie the rear wheels would be overloaded before the spring forces
are overcome and all wheels turn load bearing.)

WIederling wrote:

There is no energy leveraged from "springing up" the gear.
What you get is "while unloading the gear via creating lift the extension of the sprung gear leg per tonne unloading is (much) bigger in this design than on the other 737 models"

Not sure how that is different from what I said, except that a compressed spring does leverage energy the moment it extends. But as I know that you know this kind of stuff, I am not sure what else you have meant...

rheinwaldner wrote:

But as I know that you know this kind of stuff, I am not sure what else you have meant...

The plane is not "catapulted" into the air.
Spring energy is just sufficient to lift it 10" to 1'.
But that gives another degree of tailstrike free rotation.

I never used the term "catapulted" by myself. The spring energy of course only provides the delta between lift of the wing and the weight.

Assume that at the pitch angle, where the tail starts to scratch, 80% of the weight is carried by the wing. The remaining 20% of the weight might be W=200kN. You now need a spring which applies the 200kN in compressed state. For these 20% of the weight, we could say that they are pushed in the air by the spring.

That the spring is fully relaxed after the 9" of lift, that they provide is not a given. The cylinders could also simply be locked in the extended position to prevent further extension.

I just watched the video and it seems to me as there is nothing catapulting or storing energy and utilizing the wing for added ability to aid in the spring. It looks to be a pretty standard hydraulic cylinder, which, when given a signal to go longer, goes longer, kind of like Concorde. There is probably some fun logic about when it extends based around V speeds or rotation angles and rates and the "special lever" is probably no more than a mechanical actuator/interlock for a valve for the hydraulics which means the oil forces the secondary cylinder up when the gear is retracted.

Is it exciting? No, but good engineering isn't, that's a sign of good engineering.

Fred

So to summarize the physics...This new LG mechanism for the 737-10 simply allows the gear to extend when the aircraft is rotating during its takeoff run -- allowing a greater nose up angle of attack (AOA). Otherwise the 737-10 still has the same ground clearance when the wing is not generating lift on the ground.

So the 737 can rotate earlier and leave the ground with less runway length because as the wing takes the weight off the gear -- but not enough to leave the ground -- the gear extends, which allows for a greater AOA producing more lift. Thus the 737 takes off using less runway length because it is getting lift from a greater rotation angle, not more speed.

Previous 737 models (9-Max) needed more speed and runway to provide lift at takeoff because they could not rotate to a greater AOA on the runway to avoid tail strikes.

With the difference between max and minimum weights being over 50% of MTOW the extension must be active, use of hydraulics makes the most sense. In hydraulics, cushions can be a free piston with the tail few inches of the cylinder being air or nitrogen. It can also be an adjustable regulator and back pressure regulator set to the desired pressure for the flight. For this, I would have a low setting for taxi & gate, a high setting for all runway operations.

Anyone know the actual 737 system hydraulic pressure is and what the pnu / bleed air pressures are. I think they are 3,000 PSI & around 250 PSI. At 250 psi a 25,000 lb cylinder would have an area of 100 sq inches, about a 12" diameter, the hydraulic cylinder would be 8.4 sq inches or 3.5" diameter.

QuarkFly wrote:

So to summarize the physics...This new LG mechanism for the 737-10 simply allows the gear to extend when the aircraft is rotating during its takeoff run -- allowing a greater nose up angle of attack (AOA). Otherwise the 737-10 still has the same ground clearance when the wing is not generating lift on the ground.

So the 737 can rotate earlier and leave the ground with less runway length because as the wing takes the weight off the gear -- but not enough to leave the ground -- the gear extends, which allows for a greater AOA producing more lift. Thus the 737 takes off using less runway length because it is getting lift from a greater rotation angle, not more speed.

Previous 737 models (9-Max) needed more speed and runway to provide lift at takeoff because they could not rotate to a greater AOA on the runway to avoid tail strikes.

Exactly this. The new design allows for an additional 9 inches of extension unloaded and additional trail when fully compressed.

JayinKitsap wrote:

With the difference between max and minimum weights being over 50% of MTOW the extension must be active, use of hydraulics makes the most sense. In hydraulics, cushions can be a free piston with the tail few inches of the cylinder being air or nitrogen. It can also be an adjustable regulator and back pressure regulator set to the desired pressure for the flight. For this, I would have a low setting for taxi & gate, a high setting for all runway operations.

Anyone know the actual 737 system hydraulic pressure is and what the pnu / bleed air pressures are. I think they are 3,000 PSI & around 250 PSI. At 250 psi a 25,000 lb cylinder would have an area of 100 sq inches, about a 12" diameter, the hydraulic cylinder would be 8.4 sq inches or 3.5" diameter.

The regular 737 oleo struts are separate oil-nitrogen reservoirs that can be charged and discharged. They are not tied into the A or B systems. Only Boeing knows the actual pressures at this point until we see a maintenance manual.

JayinKitsap wrote:

With the difference between max and minimum weights being over 50% of MTOW the extension must be active, use of hydraulics makes the most sense. In hydraulics, cushions can be a free piston with the tail few inches of the cylinder being air or nitrogen. It can also be an adjustable regulator and back pressure regulator set to the desired pressure for the flight. For this, I would have a low setting for taxi & gate, a high setting for all runway operations.

Anyone know the actual 737 system hydraulic pressure is and what the pnu / bleed air pressures are. I think they are 3,000 PSI & around 250 PSI. At 250 psi a 25,000 lb cylinder would have an area of 100 sq inches, about a 12" diameter, the hydraulic cylinder would be 8.4 sq inches or 3.5" diameter.

Didn't find answers to your question, but it did help me figure some things out.

Did some googling and found a few things:

1) Good old Wikipedia ( https://en.wikipedia.org/wiki/Oleo_strut ) gives a good description of the operation of an oleo strut:

An oleo strut consists of an inner metal tube or piston, which is attached to the wheel axle, and which moves up and down in an outer (or upper) metal tube, or cylinder, that is attached to the airframe. The cavity within the strut and piston is filled with gas (usually nitrogen, sometimes air—especially on light aircraft) and oil (usually hydraulic fluid), and is divided into two chambers that communicate through a small orifice.

When the aircraft is stationary on the ground, its weight is supported by the compressed gas in the cylinder.[1] During landing, or when the aircraft taxis over bumps, the piston slides up and down. It compresses the gas, which acts as a spring, and forces oil through the orifice, which acts as a damper. A tapered rod is used on some designs to change the size of the orifice as the piston moves, providing greater resistance as compression of the strut increases. Additionally, a check valve is sometimes used to uncover additional orifices so that damping during compression is less than during rebound.

Nitrogen is usually used as the gas instead of air, since it is less likely to cause corrosion. The various parts of the strut are sealed with O-rings or similar elastomeric seals, and a scraper ring is used to keep dust and grit adhering to the piston from entering the strut.

2) http://www.b737.org.uk/landinggear.htm is all about the 737 gear, and has a picture showing the strut along with oil and gas filling valves.

3) viewtopic.php?t=758489 is a thread from this very forum from 10 years ago with an interesting discussion of the 737NG oleo struts

In all of these discussions, I didn't find any evidence of any sort of external pump to add energy if/when needed, or any regulator more elaborate than an orifice (i.e. hole) and perhaps a metering pin (the 'tapered rod' in the quote above).

I think I can answer my own earlier question: what happens if there is a rejected take off at Vr-1?

Sitting at the gate, the aircraft rests on the (highly) compressed nitrogen in the strut. The energy to compress it came from the last landing. On the takeoff roll the weight on wheels decreases so the nitrogen expands and forces oil through the orifice to establish a new equilibrium. The energy stored in the nitrogen is what forces the oil through the orifice. If there is a RTO at Vr-1 the opposite happens: the weight of the aircraft increases, which forces the nitrogen to compress, which forces the oil through the orifice in the opposite direction.

Jay's comments and the research it triggered helped me put the pieces together, hopefully without any major errors.

The 10 year old thread had an interesting dialog:

roseflyer wrote:

To use engineering terms, the Nitrogen asks like a spring and the fluid asks like a damper. Moving fluid through an orifice does not store energy. It's a spring-mass-damper model just like every freshman engineer learns about in kinematics class!

Quoting JetMech (Reply 12):
On 747's, I have seen more than 3,000psi when the aircraft was heavily loaded. Even when the aircraft is lightly loaded, you may still have over 1,000 psi in the strut. Apparently on the 737, you can get even higher static strut pressures.

737 can get up to 4,000.

Quoting JetMech (Reply 14):
I remember reading a book about landing gear where there was a particular oleo that used a stack of Belleville (springs) washers in place of pressurised gas. I'm sure it was on one of these two planes you mention

That's how a motorcycle suspension works. Airplanes have the fluid at the bottom that is being moved through orifices by a piston.

So there is no spring per se to launch the plane at takeoff. There is nitrogen at up to 4,000 pounds per square inch of pressure to provide springiness, but it is subject to the need to force the oil through the orifice. Thus, there is an upward force at takeoff but I'm not sure I'd describe it as catapult action.

Quite interesting the mechanics, a compressed gas over a hydraulic fluid. The diagram shows both a nitrogen and hydraulic charge valves, but I believe these are just for maintenance.

Ok, as far as I understood all the posts, everybody agrees that a significant upward force at rotation is needed. About how to achieve that we had mentioned several possibilities: a spring, an active hydraulic solution or a simple oleo strut.

Problem with the oleo strut is that it dampens the spring. The moment it starts moving, the force drops (see the "As the strut compresses, the spring rate increases dramatically" remark in Wikipedia). To prevent bouncing during the landing, it compresses like a normal spring, but it does not extend easily once compressed. So for the purpose of the MAX 10 (where we exactly need the force while the gear is extending) an oleo is not the solution. The MAX 10 needs a significant expanding force during rotation.

A simple spring would also not be an obvious solution, because of the requirement that any kind of bouncing during landing shall be suppressed. A strong spring would not comply with that requirement. The only way it could be used, is that the compressed force of the spring is significant but still rather low in comparison to the airframes weight (say 1/10). We can only guess the needed force at rotation but I guess 1/10 of the weight is in the range of the possible values.

So a hydraulic solution might indeed be the most probable solution.

If we look at the ingredients of the planned solution (mentioned in the link, that I posted before) we find these:

The engineering team ended up adding a spring-loaded lever to extend the telescoping landing gear, plus a “shrink link” mechanism that pulls the extension inward when it needs to be folded back up. And to power the movement, the engineers took advantage of a retraction actuator that was already in the design for the other members of the 737 MAX family.

As I read it now, the hint for a spring loaded solution does not explain away the clear description of a powered solution.

rheinwaldner wrote:

an oleo is not the solution. The MAX 10 needs a significant expanding force during rotation

It needs the Take off weight - net lift. This massive force you keep alluding to doesn't exist. An oleo on the max 9 has no problem providing this non massive force, not sure why you think it can't on the max 10. Go look at oleos on trailing link gear and view the sizing. The max 10 is a trailing link gear that requires torsion links only because the outer and middle cylinders aren't would frely rotate otherwise. The solution is simple. Complex solutions would be either too unreliable or expensive or both. They're not going to give the reliability crown to the a320 series.

RandallStephens wrote:

It needs the Take off weight - net lift. This massive force you keep alluding to doesn't exist.

Good that we agree on that. I dont say its massive. Its enough, that the gear needs to be reconstructed vs. the MAX9.

RandallStephens wrote:

An oleo on the max 9 has no problem providing this non massive force, not sure why you think it can't on the max 10. Go look at oleos on trailing link gear and view the sizing.

At rotation a normal oleo does only release the stored energy delayed (as an oleo is a dampened spring vs. a normal spring). So neither on the MAX9, nor on any other aircraft nor on the MAX10 the oleo can be the source for actively applied force during rotation.

RandallStephens wrote:

Go look at oleos on trailing link gear and view the sizing.

Look again the video of the MAX10. It is different. You cant compare with other trailing links. Normally the trailing link and the lower torsion link are one part. The MAX10 has a trailing link, which is is not driven by the main oleo. You can see it in the Boeing video at 0:38.

There are two other solutions, that I could imagine, that would tick off the simplicity requirement:
- 2017 in the early explanations, they said aoa is simply increased by moving the rotation point aft. That would be a very simple passive solution, useful if adding just a little aoa would already be sufficient. Maybe thats the case.

- Or - It should be possible to build oleos, where the dampening characteristics is actively bypassed for the short moment of rotation. Such a device would support the rotation long before the aoa is reached, at which the wing will carry all the weight.

As nobody really provided evidence for one or another solution, I guess we have to wait to find out.

rheinwaldner wrote:

At rotation a normal oleo does only release the stored energy delayed (as an oleo is a dampened spring vs. a normal spring). So neither on the MAX9, nor on any other aircraft nor on the MAX10 the oleo can be the source for actively applied force during rotation.

You are correct, the word actively is the important part. There is something (pilot, computer, squirrel ... it doesn’t matter) that actively decides when the energy is released. I would be very surprised if the release was a simple spring released as how would you retract the spring and know it was going to reload. Hydraulics make sense as it would fulfil the needs of the system and be a know quantity in terms of maintenance and design.

An oleo is already a spring (albeit a damped one) so adding another spring would make it just a different spring rate over a longer stroke.

An additional spring with a release mechanism would require logic to determine the time at which it was released, a system to load the spring with that force would likely need hydraulics for that high of a force or like was said would have to rely on it being loaded by the previous landing would need to ensure that there was enough energy imparted on the spring to load it otherwise you could have a wonky plane on landing and be too high for the exit requirement and need to load the it at the gate.

In essence springs are good at energy and for movements you need actuators.

Fred



Sent from my iPhone using Tapatalk

flipdewaf wrote:

I would be very surprised if the release was a simple spring released as how would you retract the spring and know it was going to reload.

I dont say, there will be a spring, I only say it could work this way:
- The force generated by the spring in compressed state would still far below the equivalent empty weight (say the compressed spring would lift 5000kg). As the fuselage behind the wig is only extends 26 inches, a rather low force could be sufficient to bridge the aoa gap.
- The spring would be reloaded at landing, stay fully compressed all the time and release the energy again at rotation again. As the empty weight on the main gear would far more than 5000kg, there would be no issues at all.
- The problem would be the undampened characteristics at landing. Possibly the resulting bouncing would not be tolerable.

flipdewaf wrote:

Hydraulics make sense as it would fulfil the needs of the system and be a know quantity in terms of maintenance and design.

I agree about the advantages of an active hydraulic solution. It does tick off all the requirements. The proposal seems not to be the favorites for some posters though.

flipdewaf wrote:

An oleo is already a spring (albeit a damped one) so adding another spring would make it just a different spring rate over a longer stroke.

As there exist different kind of oleos, I wonder whether by a small change, the spring force could be made available undampened for a short period (of rotation).

flipdewaf wrote:

You are correct, the word actively is the important part. There is something (pilot, computer, squirrel ... it doesn’t matter) that actively decides when the energy is released. I would be very surprised if the release was a simple spring released as how would you retract the spring and know it was going to reload. Hydraulics make sense as it would fulfil the needs of the system and be a know quantity in terms of maintenance and design.

An oleo is already a spring (albeit a damped one) so adding another spring would make it just a different spring rate over a longer stroke.

An additional spring with a release mechanism would require logic to determine the time at which it was released, a system to load the spring with that force would likely need hydraulics for that high of a force or like was said would have to rely on it being loaded by the previous landing would need to ensure that there was enough energy imparted on the spring to load it otherwise you could have a wonky plane on landing and be too high for the exit requirement and need to load the it at the gate.

In essence springs are good at energy and for movements you need actuators.

flipdewaf wrote:

You are correct, the word actively is the important part. There is something (pilot, computer, squirrel ... it doesn’t matter) that actively decides when the energy is released. I would be very surprised if the release was a simple spring released as how would you retract the spring and know it was going to reload. Hydraulics make sense as it would fulfil the needs of the system and be a know quantity in terms of maintenance and design.

An oleo is already a spring (albeit a damped one) so adding another spring would make it just a different spring rate over a longer stroke.

An additional spring with a release mechanism would require logic to determine the time at which it was released, a system to load the spring with that force would likely need hydraulics for that high of a force or like was said would have to rely on it being loaded by the previous landing would need to ensure that there was enough energy imparted on the spring to load it otherwise you could have a wonky plane on landing and be too high for the exit requirement and need to load the it at the gate.

In essence springs are good at energy and for movements you need actuators.

flipdewaf wrote:

You are correct, the word actively is the important part. There is something (pilot, computer, squirrel ... it doesn’t matter) that actively decides when the energy is released. I would be very surprised if the release was a simple spring released as how would you retract the spring and know it was going to reload. Hydraulics make sense as it would fulfil the needs of the system and be a know quantity in terms of maintenance and design.

An oleo is already a spring (albeit a damped one) so adding another spring would make it just a different spring rate over a longer stroke.

An additional spring with a release mechanism would require logic to determine the time at which it was released, a system to load the spring with that force would likely need hydraulics for that high of a force or like was said would have to rely on it being loaded by the previous landing would need to ensure that there was enough energy imparted on the spring to load it otherwise you could have a wonky plane on landing and be too high for the exit requirement and need to load the it at the gate.

In essence springs are good at energy and for movements you need actuators.

flipdewaf wrote:

You are correct, the word actively is the important part. There is something (pilot, computer, squirrel ... it doesn’t matter) that actively decides when the energy is released. I would be very surprised if the release was a simple spring released as how would you retract the spring and know it was going to reload. Hydraulics make sense as it would fulfil the needs of the system and be a know quantity in terms of maintenance and design.

An oleo is already a spring (albeit a damped one) so adding another spring would make it just a different spring rate over a longer stroke.

An additional spring with a release mechanism would require logic to determine the time at which it was released, a system to load the spring with that force would likely need hydraulics for that high of a force or like was said would have to rely on it being loaded by the previous landing would need to ensure that there was enough energy imparted on the spring to load it otherwise you could have a wonky plane on landing and be too high for the exit requirement and need to load the it at the gate.

In essence springs are good at energy and for movements you need actuators.

flipdewaf wrote:

You are correct, the word actively is the important part. There is something (pilot, computer, squirrel ... it doesn’t matter) that actively decides when the energy is released. I would be very surprised if the release was a simple spring released as how would you retract the spring and know it was going to reload. Hydraulics make sense as it would fulfil the needs of the system and be a know quantity in terms of maintenance and design.

An oleo is already a spring (albeit a damped one) so adding another spring would make it just a different spring rate over a longer stroke.

An additional spring with a release mechanism would require logic to determine the time at which it was released, a system to load the spring with that force would likely need hydraulics for that high of a force or like was said would have to rely on it being loaded by the previous landing would need to ensure that there was enough energy imparted on the spring to load it otherwise you could have a wonky plane on landing and be too high for the exit requirement and need to load the it at the gate.

In essence springs are good at energy and for movements you need actuators.

GalaxyFlyer wrote:

There’s NO “spring” action or upward lift component produced by the gear. The link merely lengthens the strut dimension during rotation allowing the plane to achieve an AOA sufficient to generate the required lift required to lift-off at the desired Vr without scrapping the longer fuselage.

I'm in agreement with this.

The spring-loaded lever to extend the telescoping landing gear, plus a “shrink link” mechanism that pulls the extension inward when it needs to be folded back up seem to be related to getting the gear into and out of the locked position while the aircraft is airborne, as opposed to rotation time.

We see in https://www.youtube.com/watch?v=F4IGl4OizM4 at 0:18 the gear is shortened at retraction time, using the retraction actuator mentioned earlier.

At 0:39 the narrator describes the new lever that is used at takeoff, which is different than the extension lever. The forward end of the new lever is attached to an linkage (active or passive?), the rearward end is attached to the landing gear axle, and the pivot point is at the bottom of the landing gear strut. I agree this is serving to "lengthen the strut dimension during rotation".

I'm pretty convinced no other major elements (such as a hydraulic fluid pump to provide a burst of energy) are present and active at takeoff/touchdown time. None of the sources are mentioning it. No other aircraft seem to need this, from MAX9 down. As mentioned earlier, fixed gear aircraft don't have it, and with retracting gear aircraft the Vmu test drags the tail on a skid down the runway and it still takes off. An additional active element doesn't fit the picture.

Revelation wrote:

I agree this is serving to "lengthen the strut dimension during rotation".

How is it lengthened? How much force is needed, to lengthen it? If you say none, what effect on the airframe would it have then (correct answer is: none too)?

There is only the possibility, that an aft moving rotation point improves the aoa. Though officially stated as explanation in 2017, the wheel displacement of the presented gear is so small, that it is not credible.

Other than that, the only effect that would help taking off the MAX 10, is an upward pushing force at rotation, that makes up the missing lift from the wing.


And...sorry for the embarrasing copy-past error in my last post...

rheinwaldner wrote:

Revelation wrote:

I agree this is serving to "lengthen the strut dimension during rotation".

How is it lengthened? How much force is needed, to lengthen it? If you say none, what effect on the airframe would it have then (correct answer is: none too)?

There is only the possibility, that an aft moving rotation point improves the aoa. Though officially stated as explanation in 2017, the wheel displacement of the presented gear is so small, that it is not credible.

Other than that, the only effect that would help taking off the MAX 10, is an upward pushing force at rotation, that makes up the missing lift from the wing.

In my post I noted I didn't understand if the linkage that moves the lever is active (i.e. activated by externally provided energy) or passive (i.e. activated by internally stored energy created by the previous landing being released, aided by the weight of the wheels). The video doesn't make that clear one way or the other. If I had to guess, I'd go with passive. I agree that one wants to keep these systems as simple as possible because they take a lot of high forces during their operation.

I'm not sure about the "missing lift" you mention. If that is a factor, then where does the missing lift come from with the MAX9 or C172?

My sense is that the wing producing a lot of lift at Vr and the extra gear length and rotation angle just helps stabilize the aircraft and allows it to get a higher AoA during the rotation, as opposed to providing missing lift. You seem to think the wings are not producing as much lift at Vr and the aircraft needs force provided by the gear during rotation.

Revelation wrote:

If that is a factor, then where does the missing lift come from with the MAX9 or C172?

These planes have the tail clearance to apply sufficient pitch angle, so the lift coming from the wing alone will suffice. The MAX10 would scratch its tail before that aoa can be established.

Revelation wrote:

My sense is that the wing producing a lot of lift at Vr and the extra gear length and rotation angle just helps stabilize the aircraft and allows it to get a higher AoA during the rotation, as opposed to providing missing lift.

Once the plane has the extended gear, the aoa becomes sufficient, about that there is no debate. But what makes the gear longer before the wing carries all the weight? Hint: it must be some kind of lift.

Revelation wrote:

You seem to think the wings are not producing as much lift at Vr and the aircraft needs force provided by the gear during rotation.

If you stretch a fuselage more and more, you reach and pass the point, where the tail clearance allows to establish the needed aoa to lift of at MTOW on the runways of this world. If that aoa cant be reached anymore, and the wings dont carry the weight yet (as a result of the lower lift force), you need to compensate the missing lift by something else. This is my point, since I posted this:

rheinwaldner wrote:

Let me show a diagram, that I have made for this purpose:


During rotation, angle of attack rises from zero to P2. At P2 the wing is able to lift and accelerate upwards the whole aircraft (Lift@P2 = weight). The MAX10's problem is that the tailstrike zone includes angle of attacks starting at P1. So what the gear needs to accomplish, is providing enough lift already at P1, to push the airframe upwards (otherwise the tail will strike).

rheinwaldner wrote:

RandallStephens wrote:

Go look at oleos on trailing link gear and view the sizing.

Look again the video of the MAX10. It is different. You cant compare with other trailing links. Normally the trailing link and the lower torsion link are one part. The MAX10 has a trailing link, which is is not driven by the main oleo. You can see it in the Boeing video at 0:38.

Normally they are one part on trailing link gear, but because the gear has to shrink in this case, they can't be. This design is more complex but much more elegant. Torsion links are well understood because they're used on all conventional landing gear designs. You can see the pivot, trailing link, and lower oleo attachment at https://www.youtube.com/watch?v=F4IGl4OizM4 look around 38 seconds. The max 10 seems to have two separate chambers in the strut, one to control the lower chamber and the other to control the trailing link.

rheinwaldner wrote:

Revelation wrote:

If that is a factor, then where does the missing lift come from with the MAX9 or C172?

These planes have the tail clearance to apply sufficient pitch angle, so the lift coming from the wing alone will suffice. The MAX10 would scratch its tail before that aoa can be established.

Revelation wrote:

My sense is that the wing producing a lot of lift at Vr and the extra gear length and rotation angle just helps stabilize the aircraft and allows it to get a higher AoA during the rotation, as opposed to providing missing lift.

Once the plane has the extended gear, the aoa becomes sufficient, about that there is no debate. But what makes the gear longer before the wing carries all the weight? Hint: it must be some kind of lift.

Revelation wrote:

You seem to think the wings are not producing as much lift at Vr and the aircraft needs force provided by the gear during rotation.

If you stretch a fuselage more and more, you reach and pass the point, where the tail clearance allows to establish the needed aoa to lift of at MTOW on the runways of this world. If that aoa cant be reached anymore, and the wings dont carry the weight yet (as a result of the lower lift force), you need to compensate the missing lift by something else. This is my point, since I posted this:

rheinwaldner wrote:

Let me show a diagram, that I have made for this purpose:


During rotation, angle of attack rises from zero to P2. At P2 the wing is able to lift and accelerate upwards the whole aircraft (Lift@P2 = weight). The MAX10's problem is that the tailstrike zone includes angle of attacks starting at P1. So what the gear needs to accomplish, is providing enough lift already at P1, to push the airframe upwards (otherwise the tail will strike).

We all agree that the gear needs to do what it is designed to do, which is support the weight of the airplane on the ground. I'm not sure why you think we need a chart to show this????

RandallStephens wrote:

We all agree that the gear needs to do what it is designed to do, which is support the weight of the airplane on the ground. I'm not sure why you think we need a chart to show this????

Because the chart does not show that.

It shows the situation at rotation before the wing supports the full weight and after the AoA can't be increased anymore due to lack of tail clearance.

You can drive your 737 MAX 10 from Los Angeles to New York, at the tire limit speed and at the geometrically highest possible aoa: if this new gear does not provide the missing lift, it will not depart 1 millimeter from earth...

Honestly I dont know how I could explain it in a more simple way for you to understand.

rheinwaldner wrote:

RandallStephens wrote:

We all agree that the gear needs to do what it is designed to do, which is support the weight of the airplane on the ground. I'm not sure why you think we need a chart to show this????

Because the chart does not show that.

It shows the situation at rotation before the wing supports the full weight and after the AoA can't be increased anymore due to lack of tail clearance.

You can drive your 737 MAX 10 from Los Angeles to New York, at the tire limit speed and at the geometrically highest possible aoa: if this new gear does not provide the missing lift, it will not depart 1 millimeter from earth...

Honestly I dont know how I could explain it in a more simple way for you to understand.

I think you've explained it well. Your earlier quote:

rheinwaldner wrote:

If you stretch a fuselage more and more, you reach and pass the point, where the tail clearance allows to establish the needed aoa to lift of at MTOW on the runways of this world. If that aoa cant be reached anymore, and the wings dont carry the weight yet (as a result of the lower lift force), you need to compensate the missing lift by something else.

is pretty concise too. It's clear the MAX10 needed more AoA (the whole point of the extending/rotating gear), it's not clear to me at least that it is in this corner case of never being able to get off the ground.

RandallStephens wrote:

It needs the Take off weight - net lift. This massive force you keep alluding to doesn't exist.

I think you're describing the same thing.

Your (Take off weight - net lift) is his chart's (P2-P1).

The disagreements seem to be (a) the magnitude of this difference and (b) if active external force is needed to extend the gear to allow an increase in AoA to achieve the lift needed to overcome this difference.

At this point I'm pretty evenly divided. MAX9s bad field performance is partially due to inability to increase AoA without tail strike. Clearly a mechanism had to be added to MAX 10 that extends the gear which allows an increase in its AoA. Whether the force to do this extension comes from internal stored energy or external added energy is not clear to me.

It's also not clear to me that MAX10 is so limited by pitch and wheel speed that additional lift isn't being generated as rotation happens which aids the further extension of the gear, or if as suggested it'd never take off without external energy to avoid the tail striking.

Hairs are being split, but hopefully we're converging on a better understanding.

GalaxyFlyer wrote:

it lengthens the gear enough to allow the WING to produce the lift needed to get airborne.

All agreed, but how does it lengthen? This question is not answered in your posts...

GalaxyFlyer wrote:

The MAX 10 gear doesn’t provide the “missing” lift; it lengthens the gear enough to allow the WING to produce the lift needed to get airborne at the desired speed and reducing the potential for tail strike. The link is there to make the gear act as if it were longer while still fitting in the current wells. Think of it this way, the optimum answer would be longer gear struts on all three gear providing plenty of clearance, this is the next best thing.

GF

Hang on a moment, were people really thinking that that extra extension was using the energy to launch the aircraft in to the air rather than allow a greater AoA for takeoff?

Oh my! I assumed we all knew about why Boeing were doing this.

Fred


Sent from my iPhone using Tapatalk

flipdewaf wrote:

Hang on a moment, were people really thinking that that extra extension was using the energy to launch the aircraft in to the air rather than allow a greater AoA for takeoff?

It is kind of the same: the gear provides supplementary lift (besides the much larger but not yet sufficient lift coming from the wing), to elevate the whole aircraft the few inches until the AoA allows taking off.

GalaxyFlyer wrote:

The MAX 10 gear doesn’t provide the “missing” lift; it lengthens the gear enough to allow the WING to produce the lift needed to get airborne at the desired speed and reducing the potential for tail strike. The link is there to make the gear act as if it were longer while still fitting in the current wells. Think of it this way, the optimum answer would be longer gear struts on all three gear providing plenty of clearance, this is the next best thing.

GF

The gear has to apply a force to push the aircraft farther off the ground. If it just passively extended, why would it be needed? If that were the case, the -900/9 MAX could just "levitate" a few inches and then rotate.

rheinwaldner wrote:

flipdewaf wrote:

Hang on a moment, were people really thinking that that extra extension was using the energy to launch the aircraft in to the air rather than allow a greater AoA for takeoff?

It is kind of the same: the gear provides supplementary lift (besides the much larger but not yet sufficient lift coming from the wing), to elevate the whole aircraft the few inches until the AoA allows taking off.

The gear doesn't provide lift. It supports the weight not borne by the wing, just like every other landing gear does. I'm starting to think that this is your not having English as a first language and you're trying to say something different?

planecane wrote:

GalaxyFlyer wrote:

The MAX 10 gear doesn’t provide the “missing” lift; it lengthens the gear enough to allow the WING to produce the lift needed to get airborne at the desired speed and reducing the potential for tail strike. The link is there to make the gear act as if it were longer while still fitting in the current wells. Think of it this way, the optimum answer would be longer gear struts on all three gear providing plenty of clearance, this is the next best thing.

GF

The gear has to apply a force to push the aircraft farther off the ground. If it just passively extended, why would it be needed? If that were the case, the -900/9 MAX could just "levitate" a few inches and then rotate.

It's not passive in the sense that it's static, there is a dynamic load in the lower cyLinder which is created by compressed nitrogen which is trying to force the oleo to the stops, just like any other gear. It's not all gravity powered extension.

rheinwaldner wrote:

GalaxyFlyer wrote:

it lengthens the gear enough to allow the WING to produce the lift needed to get airborne.

All agreed, but how does it lengthen? This question is not answered in your posts...

The high pressure nitrogen in the chamber forces the piston down when enough force has been removed from the lower trailing link. When you get enough weight of the wheel the gear extends on its own, like all oleo struts do.

GalaxyFlyer wrote:

The strut doesn5 provide

If the struts doesn't provide, the aircraft would be lifted the 9.3 inch by the wing alone. If that would be the case, Boeing would not need a new gear.

RandallStephens wrote:

It's not passive in the sense that it's static, there is a dynamic load in the lower cyLinder which is created by compressed nitrogen which is trying to force the oleo to the stops

That's the spring part in the oleo. It contains energy in compressed state. The "load" that you mention is called force and as it is applied vertically it is also called lift.

RandallStephens wrote:

The gear doesn't provide lift. It supports the weight not borne by the wing, just like every other landing gear does. I'm starting to think that this is your not having English as a first language and you're trying to say something different?

I am pretty sure that my language deficits are less of a problem, than your lack of understanding in physics.

Let me ask some control question, to check whether we share the same basics:

1. Are you aware, that lift by the wing at rotation builds up proprotionally to AoA? If at 13° the wing would start accelerating the aircraft upwards alone, at 6.5° it would carry 50% of the weight.
2. Are you aware, that the MAX 10 with a normal gear could not reach the AoA needed for the wing to carry 100% of the weight in all cases?
3. Are you aware, that after being lifted 9.3 inches, the AoA allows to carry the weight of the aircraft alone?
4. Are you aware, that to accelerate something in a direction you need a net positive force in that direction?
5. Are you aware, that therefore to lift a 737 even 9.3 inches, you need more lift than there is weight?
6. Are you aware, that the overall lift can be the sum of severall forces?
7. Are you aware, that due to 2. and 5. you need another force to lift the aircraft just for the first 9.3 inches?

If you can agree up to this point, then we can start discussing about the source of this other force.

RandallStephens wrote:

The high pressure nitrogen in the chamber forces the piston down when enough force has been removed from the lower trailing link. When you get enough weight of the wheel the gear extends on its own, like all oleo struts do.

Ok, you seem to think, the oleo is the source of this force. I mentioned this possibility many posts ago. Some more control questions about the physical characteristics of oleos:

1. Are you aware, that an oleo also is spring?
2. Are you aware, that a compressed spring stores energy and can apply violent force?
3. Are you aware, that a compressed spring other than a solid is able to provide work (= move something)?
4. Are you aware, that the spring characteristics of an oleo could achieve what I was talking about from the beginning? Provide the force delta, partially catapult (only some inches of course)?
5. Are you aware, that besides the spring characteristics of an oleo, there is a dampening characteristics?
6. Are you aware, that the damping force counters the available force of the spring?
7. Are you aware, that the damping force is different for different stroke velocities?
8. Are you aware, that the damping force is different during compression than during extension of the oleo?
9. Are you aware, that the damping force is small during the compression and large during the extension of the oleo? (during landing the aircraft shall just sink down on the legs, but not be lifted again =
   suppress the bouncing tendencies if a spring would be in place alone)
10. Are you aware, that this last point means, that the supplementary force needed to lift the MAX 10 the first 9 inches, probably will not come from the oleo?

RandallStephens wrote:

It's not passive in the sense that it's static, there is a dynamic load in the lower cyLinder which is created by compressed nitrogen which is trying to force the oleo to the stops, just like any other gear. It's not all gravity powered extension.

Being a native speaker seems to not provide much help either?

It is passive due to there not being any active control involved.
( you mix the meaning of "static" and "passive" afaics.)

What the introduced lever does is change the spring constant of the gear arrangement.
( and in a nonlinear way at that.)
A compressible gas spring has an exponential load/displacement relation.
The introduced linkage reverts that to some part
giving you more travel for less unloading of the gear leg ( from already created lift.)

there seems to be a bit of a muddle concerning what is denominated "lift".
you get a raising lifting force from unloading the gear leg ( from the gas spring ).
you get a rising lifting force during rotation ( aerodynamic lift created by way of the wing profile in a changing incidence air stream.)

rheinwaldner wrote:

GalaxyFlyer wrote:

The strut doesn5 provide

If the struts doesn't provide, the aircraft would be lifted the 9.3 inch by the wing alone. If that would be the case, Boeing would not need a new gear.

RandallStephens wrote:

It's not passive in the sense that it's static, there is a dynamic load in the lower cyLinder which is created by compressed nitrogen which is trying to force the oleo to the stops

That's the spring part in the oleo. It contains energy in compressed state. The "load" that you mention is called force and as it is applied vertically it is also called lift.

"Lift" is usually defined as a component of the force generated by a flow around a body. It is perpendicular to the flow direction, in contrast to the second component, drag, that is parallel to the flow direction. Most people would not say that the gear provides lift. It does, however, lift the aircraft by those few inches once the sum of all forces is nonzero. (specifically, the sum perpendicular to the runway)

rheinwaldner wrote:

RandallStephens wrote:

The gear doesn't provide lift. It supports the weight not borne by the wing, just like every other landing gear does. I'm starting to think that this is your not having English as a first language and you're trying to say something different?

I am pretty sure that my language deficits are less of a problem, than your lack of understanding in physics.

Let me ask some control question, to check whether we share the same basics:

1. Are you aware, that lift by the wing at rotation builds up proprotionally to AoA? If at 13° the wing would start accelerating the aircraft upwards alone, at 6.5° it would carry 50% of the weight.

This point isn't all that relevant to this discussion here, but anyway, lift at 0° AoA is often not 0. Thus, the relationship between lift and AoA is not strictly proportional.

mxaxai wrote:

This point isn't all that relevant to this discussion here, but anyway, lift at 0° AoA is often not 0. Thus, the relationship between lift and AoA is not strictly proportional.

Just define AoA as deviation from the zero lift orientation and you are set.

mxaxai wrote:

...

That makes all sense. I agree with these statements...

Just to clarify, this new design feature on the MAX10 is not to provide a boost to get it in to the air it is to facilitate a higher AOA on take off so a higher lift coefficient can be attained without a tail strike.

Fred

flipdewaf wrote:

Just to clarify, this new design feature on the MAX10 is not to provide a boost to get it in to the air it is to facilitate a higher AOA on take off so a higher lift coefficient can be attained without a tail strike.
Fred

So for what did you suggest to use hydraulics? I mean how would you call it, if a hydraulic actuator provides the missing force to elevate the aircraft 9 inches into the air at rotation?

To me it seems since many posts we all talk about the same, just some guys of the group do not like the terms used by others. I will happily use the term you suggest, if you provide it.

GalaxyFlyer wrote:

rheinwaldner wrote:

flipdewaf wrote:

Hang on a moment, were people really thinking that that extra extension was using the energy to launch the aircraft in to the air rather than allow a greater AoA for takeoff?

It is kind of the same: the gear provides supplementary lift (besides the much larger but not yet sufficient lift coming from the wing), to elevate the whole aircraft the few inches until the AoA allows taking off.

No, the wheel assembly is on a lever (the trailing link), as lift is produced on rotation the oleo extends as it would on any strut.

Strange to find myself in the role of diplomat, but let me give it a try.

Everyone is agreeing that the energy to extend the gear comes from the compressed nitrogen in the strut.

The disagreement seems to be about when this energy is released.

One camp says the aircraft can't lift itself without achieving a large AoA enabled by full gear extension so the energy must be released before the takeoff point.

Another camp says the wings are developing more lift as the aircraft gains speed and the energy is being released continuously during rotation as AoA is being increased.

If you look at our favorite video at 0:11 it looks to me that weight is coming off the wheels and the gear is extending before any rotation happens which favors the later camp's theory.

Revelation wrote:

GalaxyFlyer wrote:

rheinwaldner wrote:

It is kind of the same: the gear provides supplementary lift (besides the much larger but not yet sufficient lift coming from the wing), to elevate the whole aircraft the few inches until the AoA allows taking off.

No, the wheel assembly is on a lever (the trailing link), as lift is produced on rotation the oleo extends as it would on any strut.

Strange to find myself in the role of diplomat, but let me give it a try.

Everyone is agreeing that the energy to extend the gear comes from the compressed nitrogen in the strut.

The disagreement seems to be about when this energy is released.

One camp says the aircraft can't lift itself without achieving a large AoA enabled by full gear extension so the energy must be released before the takeoff point.

Another camp says the wings are developing more lift as the aircraft gains speed and the energy is being released continuously during rotation as AoA is being increased.

If you look at our favorite video at 0:11 it looks to me that weight is coming off the wheels and the gear is extending before any rotation happens which favors the later camp's theory.

The aircraft can fly with a lower AoA but it needs to be going faster which means it requires more runway. If its just a straight sprung mechanism (sprung with a gas or a spring) then it could be completely contained in the inner strut which only has a pressure to move the strut beyond its stops when there is less than 20% (for arguments sake) on the mains. My inclination for this would be to use a physical spring as opposed to a gas strut type scenario as gasses at high pressures aren't easy. The lever is just to move the total assembly up to the stops even when there is zero weight on wheels.

rheinwaldner wrote:

flipdewaf wrote:

Just to clarify, this new design feature on the MAX10 is not to provide a boost to get it in to the air it is to facilitate a higher AOA on take off so a higher lift coefficient can be attained without a tail strike.
Fred

So for what did you suggest to use hydraulics? I mean how would you call it, if a hydraulic actuator provides the missing force to elevate the aircraft 9 inches into the air at rotation?

To me it seems since many posts we all talk about the same, just some guys of the group do not like the terms used by others. I will happily use the term you suggest, if you provide it.

I would have initially thought hydraulics to extend the additional strut would be better than a sprung mechanism due to control for an actively controlled system but the more I think the better a passive system sounds which is what a spring wold give you as they (like a gas) can store mechanical energy whereas a hydraulic system can't.

What I mean is that the gear system isn't replacing a force that isn't able to be generated by the wings it is generating a force that enables a yet greater force to be generated by the wings if the pilot so chooses.

Fred

flipdewaf wrote:

The aircraft can fly with a lower AoA but it needs to be going faster which means it requires more runway.

Yes, but the video Boeing chose to produce shows weight coming off the wheels with zero or very little rotation and energy being released in a continuous motion rather than some spring popping the airframe up 9.x inches all at once before any airframe elevation takes place.

I'm exaggerating a bit, but I'm doing it because I think the point of disagreement is right here.

As far as I can tell, the gear extension starts before rotation and is powered solely by the release of energy from the compressed nitrogen as wing lift allows, and this happens enough "ahead of schedule" so that the MAX-10 can use same or better angle of rotation as other family members without a tail strike.

The other point of view is the airframe cannot develop the lift needed to elevate till the gear extension happens and AoA can increase, but that's not what I'm seeing in the video.

There's a lot of info to wade through here. I will leave this here in the hopes that someone with time and expertise will look through, and see if there is any useful info to be shared. It is Boeing's patent application for what has become the MAX 10 gear, with lengthy descriptions, and with drawings. https://www.freshpatents.com/-dt2018072 ... 208298.php

Revelation wrote:

flipdewaf wrote:

The aircraft can fly with a lower AoA but it needs to be going faster which means it requires more runway.

Yes, but the video Boeing chose to produce shows weight coming off the wheels with zero or very little rotation and energy being released in a continuous motion rather than some spring popping the airframe up 9.x inches all at once before any airframe elevation takes place.

I'm exaggerating a bit, but I'm doing it because I think the point of disagreement is right here.

As far as I can tell, the gear extension starts before rotation and is powered solely by the release of energy from the compressed nitrogen as wing lift allows, and this happens enough "ahead of schedule" so that the MAX-10 can use same or better angle of rotation as other family members without a tail strike.

The other point of view is the airframe cannot develop the lift needed to elevate till the gear extension happens and AoA can increase, but that's not what I'm seeing in the video.

There has to be some rotation before extension. If the nose wheel doesn't come off the ground the AoA would decrease as the gear extends, wouldn't it?

I think based on the video, rotation starts thus increasing lift. Shortly thereafter, the gear extends allowing a greater AoA.

Revelation wrote:

flipdewaf wrote:

The aircraft can fly with a lower AoA but it needs to be going faster which means it requires more runway.

Yes, but the video Boeing chose to produce shows weight coming off the wheels with zero or very little rotation and energy being released in a continuous motion rather than some spring popping the airframe up 9.x inches all at once before any airframe elevation takes place.

Agree, I think there is a very likely scenario here in that the video doesn't accurately represent the physics or potential control logic and is there to show the principle of the gear extending.
It could be that the aircraft is creating enough lift simply due to the speed down the runway but I agree that this seems a bit much.

Revelation wrote:

I'm exaggerating a bit, but I'm doing it because I think the point of disagreement is right here.

Thats fair

Revelation wrote:

As far as I can tell, the gear extension starts before rotation and is powered solely by the release of energy from the compressed nitrogen as wing lift allows, and this happens enough "ahead of schedule" so that the MAX-10 can use same or better angle of rotation as other family members without a tail strike.

I guess this is what is confusing to me, if it is a passive system and relying on stored energy then a spring feels like a better option than compressed gas and if it is an active system then hydraulics seem like a better option. In my engineering experience gasses find every available leak location and this is going to be a very high pressure dynamic system and therefore needs charging externally or be an open system with a compressed supply on board

Revelation wrote:

The other point of view is the airframe cannot develop the lift needed to elevate till the gear extension happens and AoA can increase, but that's not what I'm seeing in the video.

Agree, but that might be that the video is wrong.

Fred

flipdewaf wrote:

Agree, but that might be that the video is wrong.

I agree. It could be showing the nominal condition and there may be other more extreme conditions where we could see different behavior.

I'm willing to wait till we get more information.

Thanks everyone for the interesting conversation and the high quality posts tolerating mechanical novices like me without letting any disagreement take us off course.