Extending an aircraft landing gear that has gotten stuck

  • #1

Main Question or Discussion Point

Imagine an aircraft of which the its wing is connected to its landing gear by means of a hinge (joint), and then the gear suddenly gets stuck midway. If, subsequently, the aircraft makes a turn around its longitudinal axis (see picture), it seems to hold true that then the angle between the objects is likely to change. The question is “How does rotation of an aircraft around its longitudinal axis influence the angle between the wing and a landing gear that is stuck; rotation in which direction will make the gear get extended?”

I have brainstormed about how to solve it and those are my ideas:

Option A) Visualize a free body diagram and ask yourself which forces are acting on the gear and how their magnitude and direction can be changed in such a way that the gear gets extended to the left. There is a x and y component of the gravity force working on the gear. And then (conceptually) figuring out what happens with those components when the airplane tilts to the left and right, in order to answer what happens with the angle.

Option B) Seeing the question as a torque/moment question and, therefore, answering “How does the clockwise and anti-clockwise rotation of an aircraft around its longitudinal axis change the angle between the wing and a landing gear that is stuck?”

Option C) A combination of both option A and B.

I would really appreciate it if I could be told how to go further or can be provided sources. Please keep in mind that I am a beginner in this area.

Thanks a lot in advance for your valuable opinions!


(Picture retrieved from: http://www.thaitechnics.com/fly/control.html)
 

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  • #2
Here an illustration as addition to my question to make it more clear.
20180829_172909.jpeg
 

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  • #3
russ_watters
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There are different effects depending on the interpretation of the problem:
1. Force(torque) due to an accelerating rotation.
2. Force (torque) due to gravity at different bank angles.
3. Force due to g-force in the turn.
 
  • #4
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Not really sure I understand the question. But I'm assuming:
1. Your idea of how to get the gear unstuck is to rotate the aircraft in such a way that the weight of the gear will help bring it down.
2. You're rotating ONLY about its longitudinal axis. Of course generally when an airplane turns about it's longitudinal axis it will also turn about its vertical axis as the lift vector is now pointing off to the side causing the nose to point a different direction. However a pilot can counteract this by using opposite rudder (this is called a side-slip). But for simplicity sake you're only considering rotation about the longitudinal axis.
3. You're not taking into the account the drag forces on the plane or gear or anything.

In this case I would say that turning the plane such that the landing gear becomes more horizontal would cause a higher bending moment on the linkage, which would help un-stuck the gear, so in your diagram above, turning right (from the pilot perspective) would do it.

I think you're on the right track. Basically simplify it: Pretend you're holding a rod with a weight on the end. If you held it vertically there would be no bending moment on your hand. If you held it horizontally that would be the worst case. The bending moment on your hand would be the mass x gravitation acceleration x the horizontal distance the center of gravity is away from your hand. So what changes when you rotate it from vertical to horizontal? Only the horizontal distance the c.o.g. is from your hand. Find the relationship between the angle and the distance and you've solved your homework problem. :)
 
  • #5
sophiecentaur
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It strikes me that applying the rudder, hard in the direction of the stuck gear might help. There could be significant aerodynamic drag on the gear in the required direction to lower it if the craft is 'crabbing' and if the gear is already partly lowered.
 
  • #6
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A high positive G maneuver and a sharp roll to the right would take advantage of the gear's linear and angular momentum in an effort to free the gear.
 
  • #7
CWatters
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One way to help the wheels down is to make them "heavier" buy increasing the "g-force" they experience. Look up how a banked turn would achieve that.

In reality pilots have tried all sorts of manouvers to try and increase the downward force on the undercarriage, including bumping the good wheels on the ground.
 
  • #8
jrmichler
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In reality pilots have tried all sorts of manouvers to try and increase the downward force on the undercarriage, including bumping the good wheels on the ground.
Hopefully, they first go through the emergency gear extension procedure. Beech Bonanzas have a hand crank, Piper Commanches have a Johnson bar, other airplanes have other systems.

I once found out the hard way that a Commanche Johnson bar will jam against a flashlight setting on the center console, leaving the landing gear stuck halfway up. The airplane owner was sitting in the back seat excreting bricks because he had made a gear up landing a year earlier in that airplane.
 
  • #9
First and foremost, really thanks a lot everybody for providing me this awesome wide spectrum of ways on how to deal with this problem. As being someone in preparation for applying to a pilot trainee program, I am always curious about every detail. Especially, the fact that you are providing terminology (such as banked G maneuvers) helps me a lot.

@brianrtw The question that I put forward is (indeed) exceptionally featuring two fairly standard answers, namely: “a bank to the left” and “a bank to the right. Thus, all your assumptions are totally right.

It seems to me that the concept of “bending moments” is enough for answering this particular question. Besides, I am pretty sure that being informed about the “banked G maneuvers” is absolutely of great value for solving more complex versions of the question. So, thanks a lot for that.

I am going to draw a free body diagram to experiment with bending moments on stuck undercarriage and return my conclusions and see if one would be so kind to check if I am on the right way.
 
  • #10
With regard to the same question, I concluded the following:
When the bank angle in a rightward turn increases, the stuck undercarriage will make a wider angle with the work line of the gravitational force. I assume that, the wider this angle, the bigger the X component of the gravitational force, and thus, the more movement the gravitational force is likely to bring about in the X direction.

I have drawn one complete FBD and a simplified torque illustration.
I wonder if my simplified illustration - in which I represent Fgx as the perpendicular distance from the WHEEL of the undercarriage to the work line of the gravitational force - is a proper method (??) for answering my topic question. It seems way more time efficient compared to drawing the entire FBD. By choosing that method I will have neglected the c.g. of the undercarriage and seen the WHEEL as the point where the gravitational force applies. When making torque calculations, it should be OK to do so, right.. or not?

Curious about your valuable opinions. Thanks a lot in advance!
@brianrtw @jrmichler @CWatters @Ryoko @sophiecentaur @russ_watters

PS: I have one extra question in the simplied diagram; maybe something for the pilots among us..
FBD for stuck undercarriage.png

Simplified torque diagram for stuck undercarriage.png
 

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  • #11
jrmichler
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As being someone in preparation for applying to a pilot trainee program
This is the last thing you need to know at this point. You will be taught the proper procedure when you get that far along. MUCH more important is your attitude. Always know your limits relative to the airplane, the weather, the terrain, etc. Know the applicable regulations. Know the aircraft speeds, limits, normal and emergency procedures. Memorize the cockpit so you find the proper switch/control without having to hunt for it. Learn a proper scan. Always be willing to not go, or to terminate a flight, when (not if) conditions are outside your personal safe limits.

A gear failure is a rare occurrence that rarely causes injury, while exceeding your limits will kill you.

And not all landing gears swing sideways. Look at the Cessna 210, DC-3, and Twin Beech for examples.
 
  • #12
This is the last thing you need to know at this point. You will be taught the proper procedure when you get that far along. MUCH more important is your attitude. Always know your limits relative to the airplane, the weather, the terrain, etc. Know the applicable regulations. Know the aircraft speeds, limits, normal and emergency procedures. Memorize the cockpit so you find the proper switch/control without having to hunt for it. Learn a proper scan. Always be willing to not go, or to terminate a flight, when (not if) conditions are outside your personal safe limits.

A gear failure is a rare occurrence that rarely causes injury, while exceeding your limits will kill you.

And not all landing gears swing sideways. Look at the Cessna 210, DC-3, and Twin Beech for examples.
Thanks a lot for your honorable view and piece of advice. It absolutely makes sense and I am definitely gonna take it along in my preparation. Part of the application is a physics test and it is said to be very important; therefore I need to have acquaintance with physics and thus I try to learn as much as possible. But, I totally do understand the fact that those factors you mentioned are utmost important.
 
  • #13
CWatters
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The bank angle will increase the torque produced by the weight of the landing gear but...

If the undercarriage leg was made horizontal by banking (without turning) then the torque would be mgl/2. However if you bank and pull 3g in the turn the torque would be roughly three times that at 3mgl/2. So I would suggest that pulling a high g turn is quite a bit more effective than "just" banking.

I say "roughly" because the centripetal force won't be quite in the same direction.
 
  • #14
CWatters
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This is the last thing you need to know at this point. You will be taught the proper procedure when you get that far along. MUCH more important is your attitude. Always know your limits relative to the airplane, the weather, the terrain, etc. Know the applicable regulations. Know the aircraft speeds, limits, normal and emergency procedures. Memorize the cockpit so you find the proper switch/control without having to hunt for it. Learn a proper scan. Always be willing to not go, or to terminate a flight, when (not if) conditions are outside your personal safe limits.

A gear failure is a rare occurrence that rarely causes injury, while exceeding your limits will kill you.
+1 to all of that.

I used to fly gliders. As some point, perhaps a few dozen flights after going solo you will start to gain confidence in your abilities. That's when things get dangerous. In my case I'd made perhaps 50 solo flights all local to the field, conditions were great and I was enjoying myself hunting weak thermals... until I failed to notice the wind had shifted around. At some point I realised I was rather a long way down wind and unsure if I had enough height to get back. I did but was bricking it for a few mins.

Incidentally I highly recommend a few gliding lessons if learning to fly powered aircraft.
 
  • #15
+1 to all of that.

I used to fly gliders. As some point, perhaps a few dozen flights after going solo you will start to gain confidence in your abilities. That's when things get dangerous. In my case I'd made perhaps 50 solo flights all local to the field, conditions were great and I was enjoying myself hunting weak thermals... until I failed to notice the wind had shifted around. At some point I realised I was rather a long way down wind and unsure if I had enough height to get back. I did but was bricking it for a few mins.

Incidentally I highly recommend a few gliding lessons if learning to fly powered aircraft.
Yes, exactly, I was thinking of going for some gliding lessons a while ago already. I have already experienced a couple of times how it is to fly in a small Cessna, had the chance to operate it in air and make some maneuvers etc. Assuming that there is a good change that you have experienced the Cessna too : is there much difference in making standand maneuvers between a small Cessna and a glider? Much difference present in g-force for example?
 
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  • #16
jrmichler
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A friend once got two hours free flying time when he shut the engine off in a Cessna 150, slowed until the prop stopped, and glided. He was in mountain country on a breezy day, so had plenty of updrafts to work with. Airplanes are rented by the time on a Hobbs hour meter. The Hobbs meter is connected to the engine oil pressure switch.

Flying a low power airplane in mountain country is a lot like flying a glider. On a breezy day at typical density altitudes, normal updrafts and downdrafts exceed 1000 FPM, while a typical normally aspirated Cessna or Piper will have climb capability less than 200 FPM. It's great fun, provided that you have professional instruction in mountain flying, all loose objects are strapped down, and you are not susceptible to airsickness.
 
  • #17
A friend once got two hours free flying time when he shut the engine off in a Cessna 150, slowed until the prop stopped, and glided. He was in mountain country on a breezy day, so had plenty of updrafts to work with. Airplanes are rented by the time on a Hobbs hour meter. The Hobbs meter is connected to the engine oil pressure switch.

Flying a low power airplane in mountain country is a lot like flying a glider. On a breezy day at typical density altitudes, normal updrafts and downdrafts exceed 1000 FPM, while a typical normally aspirated Cessna or Piper will have climb capability less than 200 FPM. It's great fun, provided that you have professional instruction in mountain flying, all loose objects are strapped down, and you are not susceptible to airsickness.
Wow hahaha, that's a quite extreme way of cutting on costs.

That sounds like some strong up and downdrafts. Does the technique of making use of updrafts have a specific name?
 
  • #18
CWatters
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Yes, exactly, I was thinking of going for some gliding lessons a while ago already. I have already experienced a couple of times how it is to fly in a small Cessna, had the chance to operate it in air and make some maneuvers etc. Assuming that there is a good change that you have experienced the Cessna too : is there much difference in making standand maneuvers between a small Cessna and a glider? Much difference present in g-force for example?
I've had many flights in powered light aircraft as well as gliders. Some aerobatic. G-force are similar but depends on the aircraft. Most of the twin seat gliders I've flow could be intentionally spun but I don't think so many powered training aircraft can be, but perhaps I'm wrong. Spin recovery is one of the training exercises you either love or hate. Ditto negative g. Personally I loved it.
 
  • #19
I've had many flights in powered light aircraft as well as gliders. Some aerobatic. G-force are similar but depends on the aircraft. Most of the twin seat gliders I've flow could be intentionally spun but I don't think so many powered training aircraft can be, but perhaps I'm wrong. Spin recovery is one of the training exercises you either love or hate. Ditto negative g. Personally I loved it.
No, I don't think so as well, as I once heard that there are limitations to their bank angles, which sounds logical.
I think it is a good added value to have some extra knowledge and experience of how to level the plane in extreme maneuvers and it also kind of trains your multidimensional perception.

Definitely gonna give gliding a try! I
 

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