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Trimeresurus
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How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Fri Mar 12, 2021 2:14 pm

Considering that they don't even flare at touchdown. How is it so that at that attitude and speed, the lift can't overcome the weight? And why doesn't that lead to a stall but to a controlled descent?
 
chimborazo
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Fri Mar 12, 2021 2:29 pm

It is a controlled descent. Lift is still being generated and holding the plane weight- it’s just holding it in a descent profile. Just ends with no flare as you note. They basically crash it into the deck and go to full throttle as they do so in case the arrester cable breaks. Note the very strong landing gear on carrier aircraft- designed to take the descent with little to no flare. All because it’s such a short flight deck, there is no space/time to noticeably slow the descent over “the runway”.
 
GalaxyFlyer
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Fri Mar 12, 2021 3:51 pm

The wing on approach to a carrier is producing about 90% of its potential lift, if it weren’t, it fall into the water. Where did you get the idea it wasn’t producing lift?
 
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dennypayne
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Fri Mar 12, 2021 5:10 pm

Keep in mind that excessive angle of attack is what causes a stall. Pitch and angle of attack are not the same thing. You can have a very high pitch if you also have enough power applied to keep the AOA below the region where the wing stalls.
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Florianopolis
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Fri Mar 12, 2021 6:58 pm

Let's go back to the basics.

Imagine you're a student pilot sitting your Cessna 172. Your instructor points to the control yoke in front of you, and says, "You push and pull on this, and what are you controlling?" You say, "I pull back and I climb, I push forward and we descend." Your instructor frowns and says, "Ooh no, my sweet summer child."

The elevator has no direct control over whether you climb or descend. The elevator only controls pitch attitude. You pull back and you raise the nose. Push forward and the nose comes down.

Whether you climb or descend depends on your thrust and airspeed.

See here. You can descend even when you have the nose pointed up, if you are going slow.
Image


Also, take a look at this video. https://www.youtube.com/watch?v=ivi76OfDLvE

This guy is coming down to land on the carrier, and has to go around. He adds full power, but doesn't really change his pitch attitude. The airplane climbs. He had the same pitch attitude the whole time, but the amount of thrust determined if he was descending to land or climbing on a go-around.
 
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Fri Mar 12, 2021 6:59 pm

Trimeresurus wrote:
Considering that they don't even flare at touchdown. How is it so that at that attitude and speed, the lift can't overcome the weight? And why doesn't that lead to a stall but to a controlled descent?


Simply put, because they are flying fairly slowly, in a high-drag configuration.
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Trimeresurus
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Fri Mar 12, 2021 7:06 pm

chimborazo wrote:
It is a controlled descent. Lift is still being generated and holding the plane weight- it’s just holding it in a descent profile. Just ends with no flare as you note. They basically crash it into the deck and go to full throttle as they do so in case the arrester cable breaks. Note the very strong landing gear on carrier aircraft- designed to take the descent with little to no flare. All because it’s such a short flight deck, there is no space/time to noticeably slow the descent over “the runway”.


How does it manage to keep descending at such a pitch up attitude? 8 degrees of angle of attack and still descending. How does that happen? Do they not use flaps in carrier landings? Even at flaps zero, I don't see how an airplane can descend in a controlled manner in a positive pitch angle when the thrust force vector is pointing upwards and the AoA is at an angle that provides the maximum lift which also has a upwards component(at least multiplied by the sine of the AoA). Fighter jets not having trim tabs at the elevators/stabilizers make it even more confusing. Could you design a wing that even at 8 degrees of AoA, it didn't generate enough lift for level flight? Yes, but than that aircraft would probably need to pull an Apollo 11 if it wanted to climb even at a moderate rate, which isn't the case with F-18.

I hope there's an answer beyond "God pushes it down during the approach"
 
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Florianopolis
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Fri Mar 12, 2021 7:18 pm

Trimeresurus wrote:
How does it manage to keep descending at such a pitch up attitude? 8 degrees of angle of attack and still descending. How does that happen?


This is a subtle point, but I'm not sure you're getting it: Angle of attack is only part of whether you are climbing or descending, along with airspeed and thrust.
 
chimborazo
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Fri Mar 12, 2021 7:30 pm

Trimeresurus wrote:
chimborazo wrote:
It is a controlled descent. Lift is still being generated and holding the plane weight- it’s just holding it in a descent profile. Just ends with no flare as you note. They basically crash it into the deck and go to full throttle as they do so in case the arrester cable breaks. Note the very strong landing gear on carrier aircraft- designed to take the descent with little to no flare. All because it’s such a short flight deck, there is no space/time to noticeably slow the descent over “the runway”.


How does it manage to keep descending at such a pitch up attitude? 8 degrees of angle of attack and still descending. How does that happen? Do they not use flaps in carrier landings? Even at flaps zero, I don't see how an airplane can descend in a controlled manner in a positive pitch angle when the thrust force vector is pointing upwards and the AoA is at an angle that provides the maximum lift which also has a upwards component(at least multiplied by the sine of the AoA). Fighter jets not having trim tabs at the elevators/stabilizers make it even more confusing. Could you design a wing that even at 8 degrees of AoA, it didn't generate enough lift for level flight? Yes, but than that aircraft would probably need to pull an Apollo 11 if it wanted to climb even at a moderate rate, which isn't the case with F-18.

I hope there's an answer beyond "God pushes it down during the approach"


Save the DC-8 iirc, various regional jets, some Russian stuff, most aircraft have a positive nose angle to horizon when on a normal descent. It often feels like the nose has gone below the horizon on an airliner when larger angles of flaps are added, that’s an illusion based on the deceleration effect making it feel like the aircraft pitches forward a lot more than it actually does. Most are still nose up relative horizon. The descriptions above are useful. A combination of lift due to airspeed and angle of attack along with thrust are generating the lift. There is still enough lift to carry the weight of the plane (it’s not stalling aerodynamically) but not enough lift to maintain a given level at that weight so it descends.
 
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Florianopolis
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Fri Mar 12, 2021 7:53 pm

chimborazo wrote:
Save the DC-8 iirc, various regional jets, some Russian stuff, most aircraft have a positive nose angle to horizon when on a normal descent.


I don't want to go too off-topic, but the big difference is the leading edge devices. As you extend and lower the trailing-edge flaps, the wing camber and chord change and you have to pitch the nose down (also I'm thinking the center of lift moves). With slats or kruegers coming out on the leading edge, the direction of the wing chord doesn't change so much, so the airplane's pitch doesn't have to change as much.

Final approach in a CRJ-200 is rather like a lawn dart. But they added slats to the new CRJs, and you'll see a CRJ-700/900 is much more nose-up on approach.
 
hivue
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Fri Mar 12, 2021 9:27 pm

The airplane is pitched to be at proper AoA with zero trim. There is an instrument the pilot sees in the cockpit to help keep him at this "on speed AoA." This AoA is producing close too max lift. Maintaining proper pitch angle is vital so that the hook can catch a wire rather than skipping or passing above it entirely. The "up-and-down" of the airplane is controlled using power only. If the Fresnel lens system ("meatball") shows the airplane is too low the pilot will add power to increase altitude. Vice versa if the airplane is too high. (As far as how the airplane keeps from climbing at the high pitch for on speed AoA, if there's not enough power any airplane is going to descend no matter what the pitch.) The LSO monitors all of this, provides commands to the pilot when necessary, and has the final say-so regarding whether the pilot can land or not. The airplane is not flared at landing. The carrier box (runway) is way too short for luxuries like that.
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GalaxyFlyer
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Fri Mar 12, 2021 10:44 pm

Trimeresurus wrote:
chimborazo wrote:
It is a controlled descent. Lift is still being generated and holding the plane weight- it’s just holding it in a descent profile. Just ends with no flare as you note. They basically crash it into the deck and go to full throttle as they do so in case the arrester cable breaks. Note the very strong landing gear on carrier aircraft- designed to take the descent with little to no flare. All because it’s such a short flight deck, there is no space/time to noticeably slow the descent over “the runway”.


How does it manage to keep descending at such a pitch up attitude? 8 degrees of angle of attack and still descending. How does that happen? Do they not use flaps in carrier landings? Even at flaps zero, I don't see how an airplane can descend in a controlled manner in a positive pitch angle when the thrust force vector is pointing upwards and the AoA is at an angle that provides the maximum lift which also has a upwards component(at least multiplied by the sine of the AoA). Fighter jets not having trim tabs at the elevators/stabilizers make it even more confusing. Could you design a wing that even at 8 degrees of AoA, it didn't generate enough lift for level flight? Yes, but than that aircraft would probably need to pull an Apollo 11 if it wanted to climb even at a moderate rate, which isn't the case with F-18.

I hope there's an answer beyond "God pushes it down during the approach"


With few exceptions, all carrier planes use flaps and slats to increase the coefficient of of lift, swept wings with slats operate over a much larger range of AoA than your straight wing flap only plane. See the examples already given. Don’t confuse AoA with fuselage angle, 8 degrees nose up with 3.5 degrees of down flight path angle equals something close 11.5 degrees AoA. Jets operate at high angles of attack when they land. There’s no magic, just elemental aerodynamics. Carrier planes land closer to stall AOA making it appear strangely nose high.
 
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Starlionblue
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sat Mar 13, 2021 1:35 am

Trimeresurus wrote:
chimborazo wrote:
It is a controlled descent. Lift is still being generated and holding the plane weight- it’s just holding it in a descent profile. Just ends with no flare as you note. They basically crash it into the deck and go to full throttle as they do so in case the arrester cable breaks. Note the very strong landing gear on carrier aircraft- designed to take the descent with little to no flare. All because it’s such a short flight deck, there is no space/time to noticeably slow the descent over “the runway”.


How does it manage to keep descending at such a pitch up attitude? 8 degrees of angle of attack and still descending. How does that happen? Do they not use flaps in carrier landings? Even at flaps zero, I don't see how an airplane can descend in a controlled manner in a positive pitch angle when the thrust force vector is pointing upwards and the AoA is at an angle that provides the maximum lift which also has a upwards component(at least multiplied by the sine of the AoA). Fighter jets not having trim tabs at the elevators/stabilizers make it even more confusing. Could you design a wing that even at 8 degrees of AoA, it didn't generate enough lift for level flight? Yes, but than that aircraft would probably need to pull an Apollo 11 if it wanted to climb even at a moderate rate, which isn't the case with F-18.

I hope there's an answer beyond "God pushes it down during the approach"


On an approach, thrust is decreased. This decreases speed. Lower speed means less lift is produced given the same AoA. Lift becomes lower than weight, and thus the aircraft descends. However, as noted above, AoA is high and high lift devices are out, so you're getting quite a bit of lift.

Very simply, if lift is lower than weight, the aircraft descends.


It doesn't really matter what the pitch attitude is.

Thrust is indeed pointed slightly down, but the effect is nowhere near sufficient to make the aircraft climb.



As mentioned, it sounds like you're confusing pitch angle, angle of attack, and angle of incidence. To add to the above, here's a summary.

- Angle of incidence. The angle between the mean chord of the wing and the longitudinal axis of the aircraft. Basically how the wing is mounted with regards to the fuselage.

- Pitch angle. The angle between the horizontal and the longitudinal axis of the aircraft. If the noise is pointed up, the pitch angle is positive. The pitch angle is shown on the attitude indicator.

- Angle of attack. The angle between the free stream airflow and the mean chord of the wing. AoA is the ONLY factor in a stall. Fast jets, unlike most civilian jets, typically use AoA as a reference during approaches.

Importantly, and as shown in Florianopolis' reply above, AoA is not directly related to pitch angle. There have been accidents where the aircraft has been pointed almost straight down, while still being stalled.



Here's an example which shows high pitch angle on approach from a steadier viewpoint. SAAB Viggen used pretty much the same "carrier landing" technique in order to achieve short runway performance. You can see in this video that it has high pitch attitude and flies a steep glide path. The canards act as massive vortex generators. On approach, trailing edge flaps on the canards massively increase the effect.

(Keep watching to see the aircraft reverse into a three point turn and take off again.)

https://youtu.be/fBE41A9VT3Q
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r6russian
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sat Mar 13, 2021 2:26 am

Florianopolis wrote:
Imagine you're a student pilot sitting your Cessna 172. Your instructor points to the control yoke in front of you, and says, "You push and pull on this, and what are you controlling?" You say, "I pull back and I climb, I push forward and we descend." Your instructor frowns and says, "Ooh no, my sweet summer child."


ahh the good ole pitch for airspeed, power for altitude. seems so ass backwards to non pilots who dont know pitch and power are not 2 separate controls, theyre linked together whether we like it or not.
 
Woodreau
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sat Mar 13, 2021 4:13 am

carrier aircraft unlike commercial airliners and general aviation, during approach and landing don't fly airspeed - Vref or anything like that. instead they fly AoA using the indexer that is a prominent instrument just to the left of the HUD.

they're just looking for a circle that says they're "on speed" and not arrows which tell them their alpha is too high or too low.
The same indicator is duplicated on the nosegear so that the LSO on the carrier can see the indexer as well.

Because they fly alpha, they never exceed critical angle of attack, so they aren't stalled.

Trimeresurus wrote:
How does it manage to keep descending at such a pitch up attitude? 8 degrees of angle of attack and still descending. How does that happen?


On a 320 flaps 3 landing, at Vref airspeed, the nose is 5 degrees nose up, with the FPA indicating 3 degrees nose down. so my AoA during final approach in an A320 is 8 degrees of AoA and still descending to land on the runway.

I could be wrong though.
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sat Mar 13, 2021 7:45 am

Woodreau wrote:
carrier aircraft unlike commercial airliners and general aviation, during approach and landing don't fly airspeed - Vref or anything like that. instead they fly AoA using the indexer that is a prominent instrument just to the left of the HUD.

they're just looking for a circle that says they're "on speed" and not arrows which tell them their alpha is too high or too low.
The same indicator is duplicated on the nosegear so that the LSO on the carrier can see the indexer as well.

Because they fly alpha, they never exceed critical angle of attack, so they aren't stalled.

Trimeresurus wrote:
How does it manage to keep descending at such a pitch up attitude? 8 degrees of angle of attack and still descending. How does that happen?


On a 320 flaps 3 landing, at Vref airspeed, the nose is 5 degrees nose up, with the FPA indicating 3 degrees nose down. so my AoA during final approach in an A320 is 8 degrees of AoA and still descending to land on the runway.

I could be wrong though.


I hope you're not wrong. Because if you are we have to start questioning more things in the FCOM. ;)
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kalvado
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sat Mar 13, 2021 12:36 pm

Woodreau wrote:

On a 320 flaps 3 landing, at Vref airspeed, the nose is 5 degrees nose up, with the FPA indicating 3 degrees nose down. so my AoA during final approach in an A320 is 8 degrees of AoA and still descending to land on the runway.

I could be wrong though.

As far as I understand, pitch and AoA are related, but not the same. Zero pitch can, but don't have to, correspond to zero AoA. Moreover, to have cabin floor more or less level in cruise, zero pitch has to correspond to positive AoA
Even more difference with flaps down, which should increase AoA as airfoil chord changes, as trailing end moves down. I would expect landing at MLW to be much closer to stall - which should be 14-15 degrees AoA. 8 deg AoA would mean you can still lower the speed by 30% and stay airborne - reducing brakes and tyres wear among other things. That alone would be enough for beancounters to consider.
 
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Starlionblue
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sat Mar 13, 2021 1:42 pm

kalvado wrote:
Woodreau wrote:

On a 320 flaps 3 landing, at Vref airspeed, the nose is 5 degrees nose up, with the FPA indicating 3 degrees nose down. so my AoA during final approach in an A320 is 8 degrees of AoA and still descending to land on the runway.

I could be wrong though.

As far as I understand, pitch and AoA are related, but not the same. Zero pitch can, but don't have to, correspond to zero AoA. Moreover, to have cabin floor more or less level in cruise, zero pitch has to correspond to positive AoA
Even more difference with flaps down, which should increase AoA as airfoil chord changes, as trailing end moves down. I would expect landing at MLW to be much closer to stall - which should be 14-15 degrees AoA. 8 deg AoA would mean you can still lower the speed by 30% and stay airborne - reducing brakes and tyres wear among other things. That alone would be enough for beancounters to consider.


Zero pitch does not need to correspond to zero AoA. Most wings are mounted with a positive angle of incidence, in which case even with a symmetrical airfoil, zero pitch gives positive AoA. (Assuming stable flight above stall speed and so forth.)

With an asymmetrical wing, as all airliner wings have, zero AoA still gives positive lift because of the airfoil shape. Only with a symmetrical wing does zero AoA mean zero lift.
Last edited by Starlionblue on Sat Mar 13, 2021 1:56 pm, edited 3 times in total.
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chimborazo
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sat Mar 13, 2021 1:47 pm

And all the above can be nicely demonstrated by driving along with your hand out the window of a car. Hold it “flat” for zero AoA and speed up and there is no noticeable change. Tilt your hand up to say 5 degrees while maintaining the same relative position and you will feel the tendency for your hand to raise - lift is created by the AoA increase. Maintain the same angle and increase speed further and you will feel more lift. Then “Lower the nose” - reduce your hand angle back towards 0 degrees - and your hand will tend to move forwards because you’ve been bracing it in position and now the lift has gone that energy from your muscles holding your hand in position will accelerate your hand forward (not quite the same as what’s going on with an aircraft but it demonstrates the principle of what’s happening with AoA, relative airflow speed and hence lift).
 
chimborazo
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sat Mar 13, 2021 1:51 pm

And SB’s point of pitch and AoA can be shown by cupping your hand slightly fingers down
 
hivue
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sat Mar 13, 2021 2:01 pm

hivue wrote:
The airplane is pitched to be at proper AoA with zero trim.


Correcting my mistake: The airplane is pitched to on speed AoA with trim only so that the stick is in the neutral/ undeflected position.
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Trimeresurus
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sat Mar 13, 2021 2:53 pm

Starlionblue wrote:
Importantly, and as shown in Florianopolis' reply above, AoA is not directly related to pitch angle. There have been accidents where the aircraft has been pointed almost straight down, while still being stalled.


Yes, but only momentarily. If the aircraft suddenly pointed it's nose downwards while traveling at a level attitude, then during the few seconds of the AoA increase wings may stall. However, carrier jets keep that AoA for much longer time. My question is, high AoAs lead to greater lift, and this lift causes the aircraft to climb and AoA to become neutral(or whatever it was designed for level flight) again, why doesn't this happen with the F-18. Airliners only reach the AoA that the F-18 has during the whole approach during flare, and only the sudden reduction of thrust ables them to touch down and not float over the runway like an ice skater. This makes the F-18 wings look terribly inefficient at generating lift, considering they apply full power right before touchdown and not even the ground effect causes them to float. So at what speed would an F-18 manage to keep constant altitude at a level AoA? 300 knots?

Power for the pitch is more related to CG and the downwash of the horizontal stabilizers(and I am not sure if F-18 has a similar CG as the 172 and causes similar increase in pitch with applied power), my question is solely about the wings.
 
GalaxyFlyer
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sat Mar 13, 2021 3:03 pm

If you pitch down from level attitude, you decrease, not increase, AoA. You’re nose down pitch “unloads” the wing reducing G and AoA. You’re confusing visual pitch (body) angle with the AoA, angle of incidence (angle of wing to fuselage) and flight path. It’s not a sudden reduction of thrust that causes the touch down, it’s the flight path. Land a heavily loaded plane like the C-5 at MLW, you should have some amount of approach thrust at touchdown, if you’re right at Vref.

A carrier landing touches down with approach thrust in a controlled 3.5 degree flight path, full thrust only applied at touchdown. If the pilot applied full thrust just before touchdown, he’d either miss the wire and bolted, or much worse, engage the CDP airborne crashing on to the deck. Killed, maybe, certainly with spinal pain or injury. F-18 wings are very efficient at high AoA and has great nose pointing power.

“Power for pitch” is only tangentially related to CoG. In the landing F-18 example, pitching down, lessening AoA, would cause a large increase in descent rate and a ramp strike. Bad day there
 
kalvado
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sat Mar 13, 2021 3:46 pm

Starlionblue wrote:
kalvado wrote:
Woodreau wrote:

On a 320 flaps 3 landing, at Vref airspeed, the nose is 5 degrees nose up, with the FPA indicating 3 degrees nose down. so my AoA during final approach in an A320 is 8 degrees of AoA and still descending to land on the runway.

I could be wrong though.

As far as I understand, pitch and AoA are related, but not the same. Zero pitch can, but don't have to, correspond to zero AoA. Moreover, to have cabin floor more or less level in cruise, zero pitch has to correspond to positive AoA
Even more difference with flaps down, which should increase AoA as airfoil chord changes, as trailing end moves down. I would expect landing at MLW to be much closer to stall - which should be 14-15 degrees AoA. 8 deg AoA would mean you can still lower the speed by 30% and stay airborne - reducing brakes and tyres wear among other things. That alone would be enough for beancounters to consider.


Zero pitch does not need to correspond to zero AoA. Most wings are mounted with a positive angle of incidence, in which case even with a symmetrical airfoil, zero pitch gives positive AoA. (Assuming stable flight above stall speed and so forth.)

With an asymmetrical wing, as all airliner wings have, zero AoA still gives positive lift because of the airfoil shape. Only with a symmetrical wing does zero AoA mean zero lift.

Thanks, very good correction. Looks like things are even more interesting though. Since the wing is 3D, aerodynamic AoA may vary along with the wing with geometry. To reduce that to a single number, AoA is measured with respect to some arbitrary line; and some old thread on this forum says that Airbus uses cabin floor level as a reference.
Is indicated AoA changes as flaps are lowered? Aerodynamically, that should matter- but would it be too much information for pilots?
 
GalaxyFlyer
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sat Mar 13, 2021 4:10 pm

The AoA indication will change when high lift devices are extended and it’s shown on either the indexer lights or on the garage, in either dimensionless units or as related to stall AoA for the configuration. Most civil ones I’ve seen go 0 to 1, with approach Vref being about .6. The C-5 used units, approach around 7 units and lift off or go around being about 9 units. If you were in the pattern, slowing, at 7 units extend approach slat/flap and AoA would decrease to about 5 units at the same indicated airspeed, as you slowed, it would increase again toward 7 for final flaps. Then approach at 7 units. The Global used the 0-1 system and .6 for approach, same drill as each increment of high lift devices were extended. Finally, it is normalized for configuration, 1 will always be stall AoA regardless of flaps. The C-5, stall was up around 14, IIRC, but would vary according to configuration.

Every builder has a system of reference lines, fuselage (longitudinal), waterlines (vertical) and buttlines (lateral). INUs along with AoA have to referenced to a fuselage reference line.
 
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sat Mar 13, 2021 4:18 pm

Starlionblue wrote:
Very simply, if lift is lower than weight, the aircraft descends.

That is perhaps a misunderstanding. Lift is reduced during descent, but also during climb.

Remember Newton: F = m * a , force is mass times acceleration.

Unless you're accelerating, the sum of all forces is 0 (lift + drag + weight + thrust). Acceleration is a change in velocity, so a constant descent or climb requires no acceleration. This is why Airbus designed the sidestick as a tool to control acceleration, not attitude.
Image
http://www.aviationchief.com/airbus-control-laws.html

So how do you descend and climb then? Simply, it's your engine (or rather drag) that makes it all work.
Climb:
Image
Descent:
Image
https://www.aviation.govt.nz/licensing- ... escending/
 
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sat Mar 13, 2021 8:56 pm

Trimeresurus, may I recommend getting yourself a copy of Wolfgang Langewiesche's book, Stick and Rudder. Pick up a used copy or try here: https://www.amazon.com/Stick-Rudder-Exp ... 0070362408

From the Back Cover
WHAT'S IN STICK AND RUDDER:

* The invisible secret of all heavier-than-air flight--the Angle of Attack. What it is, and why it can't be seen. How lift is made, and what the pilot has to do with it.
* Why airplanes stall
* How do you know you're about to stall?
* The elevator and the throttle. One controls the speed, the other controls climb and descent. Which is which?
* The paradox of the glide. By pointing the nose down less steeply, you descend more steeply. By pointing the nose down more steeply, you can glide further.
* What's the rudder for? The rudder does NOT turn the airplane the way a boat's rudder turns the boat. Then what does it do?
 
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Starlionblue
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sun Mar 14, 2021 12:55 am

Trimeresurus wrote:
Starlionblue wrote:
Importantly, and as shown in Florianopolis' reply above, AoA is not directly related to pitch angle. There have been accidents where the aircraft has been pointed almost straight down, while still being stalled.


Yes, but only momentarily. If the aircraft suddenly pointed it's nose downwards while traveling at a level attitude, then during the few seconds of the AoA increase wings may stall. However, carrier jets keep that AoA for much longer time. My question is, high AoAs lead to greater lift, and this lift causes the aircraft to climb and AoA to become neutral(or whatever it was designed for level flight) again, why doesn't this happen with the F-18. Airliners only reach the AoA that the F-18 has during the whole approach during flare, and only the sudden reduction of thrust ables them to touch down and not float over the runway like an ice skater. This makes the F-18 wings look terribly inefficient at generating lift, considering they apply full power right before touchdown and not even the ground effect causes them to float. So at what speed would an F-18 manage to keep constant altitude at a level AoA? 300 knots?

Power for the pitch is more related to CG and the downwash of the horizontal stabilizers(and I am not sure if F-18 has a similar CG as the 172 and causes similar increase in pitch with applied power), my question is solely about the wings.


High AoA leads to higher lift, yes. However, on final approach the aircraft is relatively slow, which decreases lift. Also, all the draggy stuff like gear and high-lift devices is hanging out.

Greater lift is a relative term. Lift on approach is just enough to keep you descending on the glideslope. In order to climb, lift must be increased further. Given the high AoA already, to climb, thrust must be increased, leading to increased speed, leading to increased lift even with maintained AoA.

You can't look at an aircraft's pitch angle and assume a certain AoA. The AoA of an airliner and a fighter jet on approach may well be similar despite very dissimilar pitch angles.

If we don't reduce thrust during the flare we'd still touch down. It would just take quite a while and it wouldn't be pretty. ;) Approach thrust is not enough to keep the aircraft flying level.

What do you mean by "level AoA"?

I think you're still confusing pitch angle and angle of attack.





mxaxai wrote:
Starlionblue wrote:
Very simply, if lift is lower than weight, the aircraft descends.

That is perhaps a misunderstanding. Lift is reduced during descent, but also during climb.

Remember Newton: F = m * a , force is mass times acceleration.

Unless you're accelerating, the sum of all forces is 0 (lift + drag + weight + thrust). Acceleration is a change in velocity, so a constant descent or climb requires no acceleration. This is why Airbus designed the sidestick as a tool to control acceleration, not attitude.
Image
http://www.aviationchief.com/airbus-control-laws.html

So how do you descend and climb then? Simply, it's your engine (or rather drag) that makes it all work.
Climb:
Image
Descent:
Image
https://www.aviation.govt.nz/licensing- ... escending/


Lift is reduced in a climb only assuming a constant AoA at the same speed. Unless you're just doing a very small climb, you'd point the nose up to increase AoA.

Since AoA is increased, lift increases in your typical climb. More lift means more drag, which is offset by thrust.

The engine is part of things, but climbs are not really performed by engine thrust alone.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
kalvado
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sun Mar 14, 2021 2:46 am

Starlionblue wrote:
Lift is reduced in a climb only assuming a constant AoA at the same speed. Unless you're just doing a very small climb, you'd point the nose up to increase AoA.
Since AoA is increased, lift increases in your typical climb. More lift means more drag, which is offset by thrust.
The engine is part of things, but climbs are not really performed by engine thrust alone.

I would look at it from a slightly different perspective.
Climb or descent at a given pitch changes AoA. At a given pitch and airspeed there is a single (more or less) value of AoA which provides for weight = lift, and aircraft will choose the appropriate climb/descent rate to get there. If climb rate is such that AoA is higher than needed, climb rate will increase and AoA decreases. If climb rate is too high, climb rate will go down with an appropriate decrease of AoA etc.
Now, changes of AoA change drag, and it comes to the engine providing enough (too low or too much) thrust to keep airspeed constant (accelerate/decelerate)
Overall, things should converge to constant airspeed, AoA and climb/descent rate for given thrust and pitch, and stabilized cruise/descent flare etc. Or things may not converge, then there may be a problem like a little crash.
Carrier approach need to be at minimal speed to stop on deck (even arrester cables have limitations!), requiring high AoA and high pitch. Lots of drag follows, and high thrust engines to counteract that.
 
mxaxai
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Sun Mar 14, 2021 12:34 pm

Starlionblue wrote:
Lift is reduced in a climb only assuming a constant AoA at the same speed. Unless you're just doing a very small climb, you'd point the nose up to increase AoA.
Since AoA is increased, lift increases in your typical climb. More lift means more drag, which is offset by thrust.
The engine is part of things, but climbs are not really performed by engine thrust alone.

That's the point, though. Assuming a constant speed, your AoA during climb is the same, or even less than during level flight. You only increase AoA to start climbing (increase in vertical speed -> acceleration). During the continuous portion of the climb your nose still points up, yes, but that's because of your flight path and does not indicate a higher AoA. A fighter jet could be climbing at 40°+ pitch but only a couple degrees of AoA, thanks to the generous amount of thrust installed.

Thrust during climb has to increase to directly counteract gravity, not additional drag.
 
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PITingres
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Tue Mar 16, 2021 3:25 pm

Starlionblue wrote:
Here's an example which shows high pitch angle on approach from a steadier viewpoint. SAAB Viggen used pretty much the same "carrier landing" technique in order to achieve short runway performance. You can see in this video that it has high pitch attitude and flies a steep glide path. The canards act as massive vortex generators. On approach, trailing edge flaps on the canards massively increase the effect.

(Keep watching to see the aircraft reverse into a three point turn and take off again.)

https://youtu.be/fBE41A9VT3Q


Thanks for that link. I guess I need to watch more fighter videos! "WTF. Seriously? Seriously? OMG. Sweet!"
Fly, you fools! Fly!
 
vikkyvik
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Tue Mar 16, 2021 7:04 pm

Starlionblue wrote:
Lift is reduced in a climb only assuming a constant AoA at the same speed. Unless you're just doing a very small climb, you'd point the nose up to increase AoA.

Since AoA is increased, lift increases in your typical climb. More lift means more drag, which is offset by thrust.

The engine is part of things, but climbs are not really performed by engine thrust alone.


A useful tool someone on this forum once mentioned to me is to take things to their limit case. So for a climb, increase the pitch until you are climbing at 90 degrees (vertical). All the climbing force is coming from the engines at that point.

It stands to reason that this happens gradually as you increase your pitch angle, and not just suddenly at 90 degrees.
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
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Starlionblue
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Wed Mar 17, 2021 1:32 am

vikkyvik wrote:
Starlionblue wrote:
Lift is reduced in a climb only assuming a constant AoA at the same speed. Unless you're just doing a very small climb, you'd point the nose up to increase AoA.

Since AoA is increased, lift increases in your typical climb. More lift means more drag, which is offset by thrust.

The engine is part of things, but climbs are not really performed by engine thrust alone.


A useful tool someone on this forum once mentioned to me is to take things to their limit case. So for a climb, increase the pitch until you are climbing at 90 degrees (vertical). All the climbing force is coming from the engines at that point.

It stands to reason that this happens gradually as you increase your pitch angle, and not just suddenly at 90 degrees.


That's a neat way of looking at it.

However, in all cases below 90 degrees pitch, the wing is still producing lift in the vertical. If that vector is higher than weight, the aircraft will climb. Engine thrust is obviously a factor, but for moderate climb angles, climbing isn't very much dependent on the aircraft "sitting" the engine thrust.

Aircraft can climb without changing pitch simply by increasing speed (and not trimming out).
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
mxaxai
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Re: How do the carrier landing aircraft manage to descend during approach with such a big positive pitch angle

Wed Mar 17, 2021 9:00 am

I think the problem here is that lift, thrust and drag are traditionally defined as forces perpendicular or parallel to the incoming airflow.

Whereas the force that is produced by the engines ("thrust") can point in any direction. For an extreme case of this, think of a VTOL aircraft. In the descending F-18 example, a large portion of the engine thrust actually acts as lift in the flight mechanics sense, i. e. perpendicular to the flight path.

Similarly, if you climb without a change in pitch, the engine thrust is at an angle to the airflow. It is no longer perpendicular to the lift generated by the wings. Hence, for a steady climb, they have to provide more lift to counteract the force created by the engines. The sum of all forces is still zero.

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