karamara
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Need For Rotation

Mon Apr 08, 2013 10:41 pm

What exactly would happen if a pilot doesn't rotate the nose up when takeoff speed is reached? Would the aircraft eventually lift off all gears at the same time if the runway was long enough? What is the need for nose up other than a high angle of attack
 
vikkyvik
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RE: Need For Rotation

Mon Apr 08, 2013 11:29 pm

Quoting karamara (Thread starter):
What is the need for nose up other than a high angle of attack

That would pretty much be the reason right there. Well, that and you generally use a nose-up attitude to climb, with climb rate generally being governed by excess thrust (that is, you have more thrust than you need to fly level, so you point some of it upward to push the airplane up).

If there was no rotation, you'd have to achieve a much higher speed to enable liftoff. Many airplanes probably would rotate of their own volition at some point, just due to the increased moment. If I remember correctly, if you increase thrust while in level flight, the airplane will tend to increase attitude and start climbing.

Not sure if all that is true for neutrally stable / statically unstable aircraft.
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PapaChuck
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RE: Need For Rotation

Mon Apr 08, 2013 11:29 pm

It depends on the aircraft. An aircraft like a Cessna 172, properly trimmed, will tend to lift off in ground effect and climb away by itself once it gains enough speed. A tailwheel aircraft, if the tail is held on the ground, will naturally want to take off since the wing will be at an optimum angle of attack.

However, a big jet like a 747 would be a different story. An airplane like that requires a few degrees nose up to fly even at cruising speeds. Kept level, it wouldn't matter how fast you were going. If you can't achieve the required angle of attack, you're not going to fly. I suppose it would be possible to get airborne in ground effect, but not before you exceed your tire speed limits. Over-speeding and shredding all your tires before getting up in ground effect would introduce a whole new set of problems.

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timz
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RE: Need For Rotation

Tue Apr 09, 2013 12:20 am

You could attach the wing to the fuselage at the right angle for the aircraft to lift off the runway pointed horizontally like a B-52-- then when you land and bring the nosewheel down to the runway the wing is still trying to lift the main wheels off the ground. Which disimproves braking.
 
vikkyvik
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RE: Need For Rotation

Tue Apr 09, 2013 3:03 am

Quoting PapaChuck (Reply 2):
Over-speeding and shredding all your tires before getting up in ground effect would introduce a whole new set of problems.

And, of course, very quickly running out of runway.  
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N243NW
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RE: Need For Rotation

Tue Apr 09, 2013 3:43 am

I would imagine that at extreme nose-up stabilizer settings, most large jets would become much more prone to "self-rotation" than with the stabilizer in the normal takeoff range. However, couple such an extreme nose-up trim with a lot of thrust and the aircraft will likely return to earth even more dramatically than it left.  

[Edited 2013-04-08 20:48:52]
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BreninTW
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RE: Need For Rotation

Tue Apr 09, 2013 3:44 am

Quoting karamara (Thread starter):
What exactly would happen if a pilot doesn't rotate the nose up when takeoff speed is reached? Would the aircraft eventually lift off all gears at the same time if the runway was long enough? What is the need for nose up other than a high angle of attack

Isn't that essentially what happens with low clearance aircraft such as the C-130 and AN124? The aircraft don't really have room to rotate without smacking the tail.

Of course, once off the ground, they do rotate to improve climb.
 
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Starlionblue
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RE: Need For Rotation

Tue Apr 09, 2013 3:45 am

Quoting N243NW (Reply 5):

I would imagine that at extreme nose-up stabilizer settings, most large jets would become much more prone to "self-rotation" than with the stabilizer in the normal takeoff range. However, couple such an extreme nose-up trim with a lot of thrust and the aircraft will likely return to earth even more dramatically than it left the earth.

Like this? http://www.youtube.com/watch?v=OcuBW2nX9bY. I know, I know, that was due to CG.
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SAAFNAV
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RE: Need For Rotation

Tue Apr 09, 2013 4:30 am

Quoting BreninTW (Reply 6):

Isn't that essentially what happens with low clearance aircraft such as the C-130 and AN124? The aircraft don't really have room to rotate without smacking the tail.

You do positively rotate with a C130. Some of the longer ones are prone to tail strike, but you still have to rotate.

IIRC, the nose wheel is 139kts, so that would be the first limitation you run into.
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KELPkid
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RE: Need For Rotation

Tue Apr 09, 2013 5:12 am

Some planes (the DeHavilland Beaver comes to mind) just seem to be able to levitate up with the nose in a level attitude...  
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roseflyer
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RE: Need For Rotation

Tue Apr 09, 2013 12:22 pm

It really depends on the plane. A 737 would never rotate if the pilot did not put in elevator input unless the plane is trimmed for a climb. With no trim, a 737 generates a pitch down attitude on the ground. This is common on large transport aircraft.
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Aesma
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RE: Need For Rotation

Tue Apr 09, 2013 4:01 pm

I started flying on MS flight simulator and with the Cessna 172 it takes off by itself, so I wondered for a long time about rotation, especially after seeing the A340-600 at the Paris Air Show the time it made an incredible take-off with the pilot pulling the stick full aft.

Quoting Roseflyer (Reply 10):
It really depends on the plane. A 737 would never rotate if the pilot did not put in elevator input unless the plane is trimmed for a climb. With no trim, a 737 generates a pitch down attitude on the ground. This is common on large transport aircraft.

I'm guessing the 330/340 family would be the same since they have a notorious nose-down attitude on the ground. In fact I asked here if they had to change the fly by wire parameters for the A330F since it has the nose jacked up.
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CosmicCruiser
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RE: Need For Rotation

Tue Apr 09, 2013 4:05 pm

There was a Jetstar crash on T/O in '76 at Midway that resulted in 4 fatalities due to lack of nose up trim set. The jet went thru the field boundary fence. Witnesses said the nose wheel came off the ground slightly twice before leaving the runway.
 
freeze3192
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RE: Need For Rotation

Tue Apr 09, 2013 6:00 pm

Quoting KELPkid (Reply 9):
Some planes (the DeHavilland Beaver comes to mind) just seem to be able to levitate up with the nose in a level attitude...

We call a Flaps 15 takeoff in the Dash 8 a levitation takeoff. Really pronounced on the -300. I
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KELPkid
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RE: Need For Rotation

Tue Apr 09, 2013 9:00 pm

Quoting Roseflyer (Reply 10):
It really depends on the plane. A 737 would never rotate if the pilot did not put in elevator input unless the plane is trimmed for a climb. With no trim, a 737 generates a pitch down attitude on the ground. This is common on large transport aircraft.

I wonder how the 737 would behave with the CG at the aftmost acceptable position    I'm sure CG plays into this, as most GA planes that I've flown are rather "eager" to get airborne when your CG is aft...   Whether or not the 737 could be dispatched this way is another matter entirely...  
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KELPkid
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RE: Need For Rotation

Tue Apr 09, 2013 9:02 pm

Quoting freeze3192 (Reply 13):
We call a Flaps 15 takeoff in the Dash 8 a levitation takeoff. Really pronounced on the -300. I

Well, it is a distant cousin of the Beaver, wouldn't be suprised if some of the same designers had a hand in the design of the DHC-8-300 and the Beaver...   When did Bombardier take over? I don't think that happened until at least the Q400...
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hivue
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RE: Need For Rotation

Wed Apr 10, 2013 8:14 pm

Quoting karamara (Thread starter):
What is the need for nose up other than a high angle of attack

My (non-expert) understanding is that no airfoil will generate aerodynamic lift without a positive angle of attack. The aircraft is rotated specifically to set up sufficient positive AOA.
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RE: Need For Rotation

Wed Apr 10, 2013 11:40 pm

Quoting hivue (Reply 16):
My (non-expert) understanding is that no airfoil will generate aerodynamic lift without a positive angle of attack. The aircraft is rotated specifically to set up sufficient positive AOA.

Correct. Without positive AoA, no lift. However if we want to nitpick this is only valid for a completely symmetrical airfoil. Given the shape of an airliner airfoil (camber etc) you get lift even with zero AoA if you define AoA as airflow relative to the mean chord line. I believe the term is "effective AoA", meaning you take into account the shape of the airfoil. You do need positive effective AoA.

If the wings are mounted at an incidence that gives the wing a sufficient positive AoA while rolling on the ground with the nosewheel touching the asphalt, no need to rotate. The B-52 is an example as mentioned above.
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hivue
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RE: Need For Rotation

Wed Apr 10, 2013 11:58 pm

Quoting Starlionblue (Reply 17):
If the wings are mounted at an incidence that gives the wing a sufficient positive AoA while rolling on the ground with the nosewheel touching the asphalt, no need to rotate. The B-52 is an example as mentioned above.

I believe that the B-17's wings were arranged so that the airplane could take off and land 3-point whereas most tail-draggers needed to lift the tail off first to avoid a stall.
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RE: Need For Rotation

Thu Apr 11, 2013 1:39 am

Quoting KELPkid (Reply 14):
Quoting Roseflyer (Reply 10):
It really depends on the plane. A 737 would never rotate if the pilot did not put in elevator input unless the plane is trimmed for a climb. With no trim, a 737 generates a pitch down attitude on the ground. This is common on large transport aircraft.

I wonder how the 737 would behave with the CG at the aftmost acceptable position I'm sure CG plays into this, as most GA planes that I've flown are rather "eager" to get airborne when your CG is aft... Whether or not the 737 could be dispatched this way is another matter entirely...


There is insufficient lift because of the nose down attitude. Flaps and leading edge slats give some lift, but not enough to get the airplane off the ground with the 2-4 degrees of nose down attitude. The nose down attitude is useful because it shortens braking distance.
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RE: Need For Rotation

Thu Apr 11, 2013 4:22 am

Quoting Roseflyer (Reply 10):
It really depends on the plane. A 737 would never rotate if the pilot did not put in elevator input unless the plane is trimmed for a climb. With no trim, a 737 generates a pitch down attitude on the ground. This is common on large transport aircraft.
Quoting KELPkid (Reply 14):
I wonder how the 737 would behave with the CG at the aftmost acceptable position I'm sure CG plays into this, as most GA planes that I've flown are rather "eager" to get airborne when your CG is aft... Whether or not the 737 could be dispatched this way is another matter entirely...
Quoting Roseflyer (Reply 19):
There is insufficient lift because of the nose down attitude. Flaps and leading edge slats give some lift, but not enough to get the airplane off the ground with the 2-4 degrees of nose down attitude. The nose down attitude is useful because it shortens braking distance.

Once upon a time on a run back to Anchorage from some runway out west in Alaska, as we sped down the runway in a 737-200 belonging to a long defunct airline, the nose came off without any pull on the yoke about 10 kts below our planned rotation speed. Why? The stabilizer trim was set correctly for the weight and balance we were given but the actual CG was a lot farther aft due to a lot more weight in the aft cargo area. If the stab trim is either set to far aft or forward for the "actual" CG the 737 will either rotate on its own or take a lot higher force to get the airplane off the runway.
 
vikkyvik
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RE: Need For Rotation

Thu Apr 11, 2013 4:34 am

Quoting Starlionblue (Reply 17):
However if we want to nitpick this is only valid for a completely symmetrical airfoil.

I wouldn't call that a nitpick at all. Given that most airliners, at least, have nonsymmetrical airfoils (and I'd guess the majority of current airplanes), it's a completely valid and important point.

I don't personally recall hearing "effective AoA", but you could be right on that.

Quoting Roseflyer (Reply 19):
There is insufficient lift because of the nose down attitude. Flaps and leading edge slats give some lift, but not enough to get the airplane off the ground with the 2-4 degrees of nose down attitude.

That may be true, but I believe the question is whether it would rotate on its own, given an aft CG, but with stab trim set for a center or forward CG. At least, that's how I read it.
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roseflyer
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RE: Need For Rotation

Thu Apr 11, 2013 12:25 pm

Quoting vikkyvik (Reply 21):
That may be true, but I believe the question is whether it would rotate on its own, given an aft CG, but with stab trim set for a center or forward CG. At least, that's how I read it.

Good question. If the airplane is not trimmed correctly for its CG then I don't know what would happen.
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hivue
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RE: Need For Rotation

Thu Apr 11, 2013 2:19 pm

Quoting vikkyvik (Reply 21):
Quoting Starlionblue (Reply 17):
However if we want to nitpick this is only valid for a completely symmetrical airfoil.

I wouldn't call that a nitpick at all. Given that most airliners, at least, have nonsymmetrical airfoils (and I'd guess the majority of current airplanes), it's a completely valid and important point.

This is getting into the area of how airfoils work to keep a plane in the air, a topic that is complex at best and has been discussed many times in these forums.

See the following:
Airbus A330 Nose Down Attitude (by A380Heavy Feb 13 2010 in Civil Aviation)?threadid=4711515&searchid=4715141&s=aerodynamic+lift#ID4715141

-- particularly tdscanuck's reply 35.
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BoeingGuy
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RE: Need For Rotation

Thu Apr 11, 2013 3:14 pm

Quoting Roseflyer (Reply 22):
Good question. If the airplane is not trimmed correctly for its CG then I don't know what would happen.

Boeing airplanes will give the crew a Takeoff Configuration Warning when the thrust levers advance to takeoff power if the trim is set outside of the correct range for its current CG and weight.
 
vikkyvik
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RE: Need For Rotation

Thu Apr 11, 2013 3:54 pm

Quoting hivue (Reply 23):
This is getting into the area of how airfoils work to keep a plane in the air, a topic that is complex at best and has been discussed many times in these forums.

See the following:
Airbus A330 Nose Down Attitude (by A380Heavy Feb 13 2010 in Civil Aviation)?threadid=4711515&searchid=4715141&s=aerodynamic+lift#ID4715141

-- particularly tdscanuck's reply 35.

Not sure what you mean. Nothing I wrote is at all in conflict with what Tom stated in that thread.
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7BOEING7
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RE: Need For Rotation

Thu Apr 11, 2013 4:44 pm

Quoting vikkyvik (Reply 21):
That may be true, but I believe the question is whether it would rotate on its own, given an aft CG, but with stab trim set for a center or forward CG. At least, that's how I read it.
Quoting Roseflyer (Reply 22):
Good question. If the airplane is not trimmed correctly for its CG then I don't know what would happen.
Quoting BoeingGuy (Reply 24):
Boeing airplanes will give the crew a Takeoff Configuration Warning when the thrust levers advance to takeoff power if the trim is set outside of the correct range for its current CG and weight.

If the airplane is not trimmed correctly for its actual CG, either due to an incorrect weight and balance or an incorrect stabilizer trim setting (within the Greenband--i.e. no takeoff warning) the airplane could either rotate on its own or be a lot harder to rotate. Airplanes basically have two wings which generate lift--the main wing and the stabilizer. With an aft CG if the stabilizer is set incorrectly it will generate a down force and rotate the airplane without the pilot doing anything.
 
hivue
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RE: Need For Rotation

Thu Apr 11, 2013 7:06 pm

Quoting vikkyvik (Reply 25):
Not sure what you mean. Nothing I wrote is at all in conflict with what Tom stated in that thread.

I didn't say that it necessarily was. However, if what you and Stationblue are saying is that "nonsymmetrical" airfoils can generate sufficient lift at 0 AoA for an airplane to fly then that sounds to me like it is.
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vikkyvik
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RE: Need For Rotation

Thu Apr 11, 2013 8:08 pm

Quoting hivue (Reply 27):
I didn't say that it necessarily was. However, if what you and Stationblue are saying is that "nonsymmetrical" airfoils can generate sufficient lift at 0 AoA for an airplane to fly then that sounds to me like it is.

I think you're misunderstanding what Tom said, or what we're saying.

In that thread, Tom said:

If you have a wing at it's Cl=0 angle of attack, it doesn't matter if you go Mach 10, it won't generate any lift.

The Cl=0 angle of attack is the zero-lift angle of attack, which in the case of an unsymmetrical airfoil, will be a negative AoA. It will be less than Zero AoA, which is what you asked about, and what Starlion and I were talking about.
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hivue
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RE: Need For Rotation

Thu Apr 11, 2013 10:28 pm

Quoting vikkyvik (Reply 28):
I think you're misunderstanding what Tom said, or what we're saying.

Thanks. Perhaps I did, as I am certainly no aeronautical engineer. I will maintain, as I did in reply 16, that no airfoil will generate lift (or, at least, any worth mentioning) at 0 degrees AoA, but I could easily be wrong about that.
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jetstar
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RE: Need For Rotation

Thu Apr 11, 2013 10:29 pm

Quoting CosmicCruiser (Reply 12):
There was a Jetstar crash on T/O in '76 at Midway that resulted in 4 fatalities due to lack of nose up trim set. The jet went thru the field boundary fence. Witnesses said the nose wheel came off the ground slightly twice before leaving the runway.

I can tell you that on the Lockheed JetStar, we considered the trim setting one of the killer items, it was so critical that it was on the checklist 2 times.

While doing the taxi checklist the Captain had to run the trim in both directions for a few degrees to verify the operation of the pitch trim system from the Captains side, then the First Officer had to do the same using the right seat pitch trim switch and then set the trim for the proper take off position which was just about 4 degrees, 3.8 to be exact but the gauge was so small that it was hard to really see the difference between the 3.8 and 4, so we used 4 degrees as the take off setting and call out to the Captain to verify the trim was set correctly. The pitch trim setting was again verified when the checklist called for all trims setting to be verified, but we never moved the rudder or aileron trims.

One time we were out doing some flight training and after some air work we did a few touch and go’s, I was in the left seat and the Chief Pilot was in the right seat. His job upon landing was to set the flaps back up to the take off position and reset the pitch trim to 4 degrees nose up while I advanced the throttles to take off power. On our first T & G he called rotate and when I pulled the control wheel back the nose did not lift up. I don’t know why I did this, but for some reason I looked down and he had set the pitch trim to 3 degrees nose up instead of 4 degrees, so I immediately pushed the pitch trim switch to crank in so more nose up trim and I was able to rotate the airplane and complete the take off. The pitch trim gauge is located to the left of the throttles and slightly recessed so while it was easy for the left seat pilot to look down to see it, the right seat pilot has to lean over slightly to get a better view of the pointer.

When I noticed he had set the trim wrong I yelled at him that he set the trim wrong and after our flight training he said very little to me and did not mention or apologize for his error. He was a complete idiot and he was the reason why I quit that job.

Normally on landing the pitch trim would be about 0 degrees, so while taxing in after landing it was the First Officers job to reset the trim to 4 degrees nose up. One degree off in the nose down position was enough to possibly prevent the airplane from rotating, but from what I remember reading the accident report on that JetStar in MDW in 1976, they found the pitch trim in the landing configuration so there was absolutely no way that JetStar was going to leave the ground, no matter how far you pulled the control wheel back, if I remember correctly the Captain was a highly experienced JetStar Captain, but probably in a hurry and forgot to use or read the checklist.

The JetStar had no warning systems to alert you that the trim or flaps were not set right for takeoff, so you had to make sure you read the checklist carefully and sometimes complacency sets in, after reading the same checklist over and over for a few years, it was not that hard to accidentally skip over an item, it happened to me more than one in the Jetstar, so for all you pilots or future pilots out there, use your checklist each and every time you fly, don’t do it from memory.

JetStar
 
vikkyvik
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RE: Need For Rotation

Thu Apr 11, 2013 10:43 pm

Quoting hivue (Reply 29):
I will maintain, as I did in reply 16, that no airfoil will generate lift (or, at least, any worth mentioning) at 0 degrees AoA, but I could easily be wrong about that.

You are wrong about that.  

That is true for symmetrical airfoils only. Nonsymmetrical (unsymmetrical, whatever) airfoils will still generate lift at 0 deg AoA, and even down into the negatives a bit, until they hit their zero-lift-AoA.
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Starlionblue
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RE: Need For Rotation

Thu Apr 11, 2013 11:43 pm

Quoting hivue (Reply 29):
I will maintain, as I did in reply 16, that no airfoil will generate lift (or, at least, any worth mentioning) at 0 degrees AoA, but I could easily be wrong about that.

As vikkyvik points out, you are wrong. It is a matter of definitions. AoA is defined as the angle between airflow and the mean chord line.

Note in this image of a non-symmetrical airfoil how the mean camber line is above the mean chord line. At zero AoA, meaning airflow exactly parallel to the mean chord line, the shape of the wing means the wing will generate lift.



This is why there is a difference between actual AoA (defined by the mean chord line) and "effective AoA", defined by the aerodynamic properties of the airfoil. In the thread you reference, Tom calls this "zero effective AoA" the "Cl=0 angle of attack". I quote: "If you have a wing at it's Cl=0 angle of attack, it doesn't matter if you go Mach 10, it won't generate any lift." Note also Zeke's statement in Reply 36 of the same thread: "Cl is not zero at zero degrees angle of attack due to the camber."

Having said that, you also have to take angle of incidence into account. Before rotation most airliners have an effective AoA that does not generate enough lift for flight.

[Edited 2013-04-11 16:44:45]
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hivue
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RE: Need For Rotation

Fri Apr 12, 2013 12:20 am

Quoting vikkyvik (Reply 31):
Quoting Starlionblue (Reply 32):

Thanks much for the info, and the diagram. So just to make sure I have this straight, if the free stream airflow is running parallel to the mean chord line (Chord "C" in the diagram), then if you can get up enough airspeed (and have enough runway) the airplane will lift off the runway and fly without a rotation maneuver and without ever creating a positive actual AoA. This effect is due to the non-symmetrical shaping of the airfoil. Sound right?

[Edited 2013-04-11 17:23:34]
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roseflyer
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RE: Need For Rotation

Fri Apr 12, 2013 1:28 am

Quoting hivue (Reply 29):
Quoting vikkyvik (Reply 28):
I think you're misunderstanding what Tom said, or what we're saying.

Thanks. Perhaps I did, as I am certainly no aeronautical engineer. I will maintain, as I did in reply 16, that no airfoil will generate lift (or, at least, any worth mentioning) at 0 degrees AoA, but I could easily be wrong about that.

No airplane that takes off with flaps has 0 degrees AOA. Flap change the chord line. There is a nose down attitude on many airplanes, but just to clarify that the effective angle of attack is above zero with flap settings, but still not enough to get off the ground without elevator input unless the sirplane is miss trimmed.

[Edited 2013-04-11 18:39:20]
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RE: Need For Rotation

Fri Apr 12, 2013 3:49 am

Quoting hivue (Reply 33):
Thanks much for the info, and the diagram. So just to make sure I have this straight, if the free stream airflow is running parallel to the mean chord line (Chord "C" in the diagram), then if you can get up enough airspeed (and have enough runway) the airplane will lift off the runway and fly without a rotation maneuver and without ever creating a positive actual AoA. This effect is due to the non-symmetrical shaping of the airfoil. Sound right?

Correct.
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vikkyvik
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RE: Need For Rotation

Fri Apr 12, 2013 3:49 am

Quoting hivue (Reply 33):
Thanks much for the info, and the diagram. So just to make sure I have this straight, if the free stream airflow is running parallel to the mean chord line (Chord "C" in the diagram), then if you can get up enough airspeed (and have enough runway) the airplane will lift off the runway and fly without a rotation maneuver and without ever creating a positive actual AoA. This effect is due to the non-symmetrical shaping of the airfoil. Sound right?

Given an extremely long runway, and no tire speed limit, yes, your airplane may lift off the ground with enough speed. But that also depends on your trim setting/CG. If the airplane has a continued nose-down moment, it may never actually lift off, even if the wings are generating enough lift to equal the airplane weight.

As I stated before, climb rate is generally governed by excess thrust. So you have more thrust than you need, and you rotate the airplane to angle the thrust downward, and push the airplane upward.
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