How do flaps work?
Posted by CommunicationFit4360@reddit | flying | View on Reddit | 53 comments
Hey, im getting read to go to school to get my A and P license and Im trying to understand how the controls work im just confused on the flaps at the back end of the plane. From what I understand the flaps will extend down. This will then compress the air on the bottom trailing edge and also cause a lower pressure area on the top of the wing at the trailing edge. Im just confused why both of these actions dont end up pushing/pulling ghr trailing edge up and therefore pushing the leading edge down which would decrease the angle of attack and push the nose of the plane downwards? Thank you in advance 🙏
nerobro@reddit
right. So your intuition is right.
Most POH have instructions for how much elevator trim to input when you're deploying flaps, for that reason. Or otherwise have flaps and elevator trim linked.
Flaps.. do a lot of things. Plain flaps, and split flaps both end up "increasing the effective AOA" so you get more lift, at a lower airplane angle. And by providing a cambered airfoil they also provide a lot of drag.. which is kinda what you want.
Fowler, and slotted flaps, \~start off\~ by making the wings bigger first. Which also moves the COP back a little, but much less than when we're talking big sugar scoop flap deployment. The first 5-10-even 15 deg of flap deployment is often just making the wings bigger. Bigger wings let you fly slower.
Planes in general are a ballancing act. The wings push up, the tailplane pushes down, and between those two ballancing you get a plane that wants to fly level.
CommunicationFit4360@reddit (OP)
Thank you very much
nerobro@reddit
how deep do you wanna go, as long as you're not doing math "how lift works" is pretty fun to discuss
CommunicationFit4360@reddit (OP)
Id love to go deeper if possible! I have a book that trys to explain the differint flap types though i dont fully understand them. My limit is definitely any form of calculus
nerobro@reddit
I got you. I'll post late tonight.
CommunicationFit4360@reddit (OP)
Awesome thank you
nerobro@reddit
So here's how I think about.. air... and how airplanes fly.
The thing that made flying make sense to me, was the newtonian idea of flight. For you to stay airborne, you need to impart enough force on enough air stay aloft.
Almost everything i'm going to talk about relates to wings that are flying, that is wings, that are not stalled, and have a connected slipstream. We'll get back to that.
Airplane wings are "human scale". But airplanes, are much bigger. If you try to do the math on the air a wing affects being just the air touching it, or near it, none of the math works. But.. it turns out, when you're in a bath of air, your influence goes a lot further.
The first thing I need to share is how a thing moving through a fluid behaves. The thing to try, and imagine, is if you move your hand through a pool of water, lots more water is affected than just what your hand touches. This "air is affected far from the object" feeds into a lot of aerodynamic things.
The closer you are to the middle of the wing, the more air, above and below the wing you get to have an influence on. The bigger the wing, in area, the more air you get to grab and move.
Lift is distributed across the wing. The tips, make the least lift, and the area near the center makes the most lift. This is because as you get near the tips of the wing, air starts spilling around the tip. This is where wingtip vortices come from.
This also means, the area near the center, is the place to put the most effective lift devices. That's why slats, flaps, are always at the center, first. It's also why you don't see lots of drooping ailerons... as the rewards for that kind of effort is quite small in comparison to the flap area near the center of the wing.
Now we get to talk about wing shape. A square wing has an eliptical lift distribution. That is, a hershey bar wing, has lift that starts at zero and goes to it's max at the middle, and back to none at the far tip. What could you do to optimize that?
Long thin wings are more efficient than short thick wings. The most practical way you can think of it, is the longer the wing the less air ends up spilling around the tip. This is part of where tapered, and oval wings got their start. There's structural reasons for that too, but they all really aim to emphasize lifting surface at the middle of a wing.
The downside to this, is as you reduce the size of wingtips, you're also increasing their effective loading. This can cause stalls to start at the wingtip... which leads to planes flipping and diving towards the ground on landing approach. This is why we rig ailerons to point up a bit on some planes, it's also why other planes have significant washout.
I think that's enough to chew on.... The Daniel Raymer books are real good if you wanna learn more. Also "flying on your own wings" and really anything published by the EAA.
CommunicationFit4360@reddit (OP)
This is amazing, thank you so much, I definitely dont understand it all but im gonna try to figure it out, I do have one question, by middle of the wing do you mean closer to the middle of the plane overall or closer to the middle of a singular wing?
PhilRubdiez@reddit
Page 40 in Aerodynamics for Naval Aviatiors has all the information you could want. Bonus: it’s from the FAA, so you get the federal answer.
dumptruckulent@reddit
400+ pages? I ain’t reading all that shit.
Long-Speed-5746@reddit
This is absolutely hilarious Sir, I wonder how long the natops is for the ah1z.
dumptruckulent@reddit
I ain’t reading all that shit either
PhilRubdiez@reddit
I had to like glance at it for Aerodynamics in college. I was a chump and bought the physical copy (which looks great in my bookcase #topgun #letsdosomepilotshit) when the digital was available free.
Also, happy early Birthday.
jedensuscg@reddit
Well, technically it's on the FAA website and is distributed by them now, but it's a NAVAIR, so it's a Navy publication.
johnisom@reddit
Well, it actually does push the leading edge down, it creates a nose down movement. But the angle of attack is not decreased, because the wing chord line changed and has a different angle now. So to keep the same angle of attack, the nose must come down.
BigJellyfish1906@reddit
When you extend flaps:
So yes, the trailing edge area experiences a stronger upward force from that added pressure difference.
But that force does not act in isolation at the trailing edge. It acts over the whole wing through the new pressure field. The total aerodynamic force, or lift, still acts roughly perpendicular to the airflow, not like a hinge trying to pry the wing up or down.
Extending the flaps also changes the pitching moment. The extra lift is concentrated farther aft, which moves the overall lift vector backward. That shift creates a nose-down pitching moment. It happens not because the flap is physically pushing the trailing edge up, but because the center of lift moves rearward.
That is why most airplanes pitch nose-down when flaps come out even though total lift increases.
CommunicationFit4360@reddit (OP)
Thank you, this makes alot of sense and helped me understand it alot more
3minence@reddit
Remember, all the axis move around the centre of gravity. Depending on the arm between the centre of pressure, and the centre of gravity, this would result in various amounts of pitch down moment.
However, some aircraft, like the DA40 that I fly, pitch up with flaps. This is because the additional camber on the wing causes the relative airflow over the horizontal stabiliser to shift, increasing the angle of attack, and increasing the downforce.
BigJellyfish1906@reddit
Because the flaps are too close to the center of gravity of the plane to pivot the whole thing like that. Simple as that. If those exact forces were out at the tail, it would raise the tail just like the elevator does.
beretta01@reddit
How exactly is a rainbow made? How exactly does a sun set? How exactly does a posi-trac rear-end on a Plymouth work?……………
NotSayinItWasAliens@reddit
If you ain't first, you're last.
phxcobraz@reddit
IT JUST DOES
Additional_Shoe8605@reddit
Idk i tossed a spool into my roadrunner
poser765@reddit
I don’t know any of those answers. I’m sure Marisa does.
Lamathrust7891@reddit
Pull lever up plane fly slower. Push lever down plane fly faster
piper pilot.
druuuval@reddit
Push button, stick fly out of hand to floor, CFI poo pants on go around.
Lamathrust7891@reddit
"positive rate"
druuuval@reddit
I feel like I heard that way more often than right rudder.
Dark_KingPin@reddit
You just brought back some great memories
Chance_Storage_9361@reddit
Not a pilot yet, so take this with a grain of salt. But it makes sense to me that since the flaps are on the back of the wing, that it will move the center of the wing to the rear, essentially creating a torque moment Around the center of mass of the airplane, pushing the nose down.
Working_Football1586@reddit
There are free air foil design apps in the app store that will let you design a wing and you can change the shape like the flaps are out and it will show you how the pressure changes. Its a good visual I used to use for my students
CommunicationFit4360@reddit (OP)
Thank you! Do you have any suggestions on the most accurate ones?
Working_Football1586@reddit
I used to use Wind Tunnel Free but I dont think its available anymore
Reasonable_Blood6959@reddit
Generally extending flaps does indeed create a pitch down moment, because when you extend flaps you move the centre of pressure aft.
If you didn’t do anything the nose would drop, but all it takes it a bit of elevator pressure, and then trim out the forces.
Ill_Writer8430@reddit
To add to this, increasing flap setting will lower the deck angle for a given AoA (and create a moment to that effect) but after that change the plane will maintain a roughly similar AoA to before the change in flap setting once stabilised.Â
WhiteoutDota@reddit
The plane will either pitch up or down dependent on whether the center of lift is above or below the center of gravity
Reasonable_Blood6959@reddit
So first I said “generally” because there are definitely some exceptions, so I’m not disputing you there.
Secondly, I know I said I was grabbing popcorn, but never in my OTT POF instruction was a Cp/Cg ever mentioned moving Up or Down.
I was always taught that the centre of lift and centre of gravity act only along the longitudinal axis.
The Cp/Cg therefore don’t move “up” or “down”, only forward or aft.
I’ve recently lost my medical, and I’m about to go into ATPL TKI-ing, so I’m very open to my mind being challenged
WhiteoutDota@reddit
Are you talking about flaps, which are located on the trailing edge of the wing, or an elevator which is located on the empennage (tail/rear)? Or perhaps you are curious about the elevator trim, also located on the tail?
CommunicationFit4360@reddit (OP)
Sorry ill edit it it, i meant the trailing edge of the wing*
DatSexyDude@reddit
They flap! 🕊️
Mispelled-This@reddit
The main purpose of flaps is to trade increased drag for increased lift by changing the size and shape of the wing. This allows us to fly slower without stalling, which is useful when landing.
You are correct that flaps also lower the nose a bit. That makes seeing the runway easier, which is also useful when landing.
Finmin_99@reddit
Not a pilot, just an engineer who’s been looking more into aviation. A poor man’s way of explaining it is Bernoullis principle with conservative of mass and energy is how you roughly explain lift from an air foil. I think your confusion may be from looking at the tail in isolation. If all you had was a tail and you extend the flap in a fixed orientation relative to flow it’s coefficient of drag and lift increase. Lift is dependent upon speed so if your drag coefficient becomes to large from an increase in angle of attack it will effectively dominate your lift through a reduction of speed. AKA stall. If you extend the flaps are they increasing the lift or decreasing it? Depends on the flaps and the extension speed and angle of attack and etc...
To control anything you must consider the system as a whole. If you change state variable 1 that is intertwined with other state variables. Fancy way to say you can’t change your flaps in the back and not expect it to impact speed, angle of attack which are inputs to other systems in the plane which affects those same parameters that impact your tail. Chicken and the egg but for everything on the plane. Therefore it depends on the plane on what will happen.
If you were to design a trimming algorithm you calculate your current attitude and calculate the error to your desired attitude. From there you feed the error into a kalman filter to calculate your actuation of the flap to hopefully correct the error. It’s all based on some mathematical model some engineer built and tuned. Build in logic to limit your model. Can’t actuate to infinity thus some limitations must be implemented. It depends upon a multitude of sensors to provide fidelity to the mathematical model to calculate proper actuation.
If Boeing had half a brain cell they would have had their two angle of attack sensors and IMUs feed into a single kalman filter it could have helped prevent the 737 max crashes. Instead it was dependent on one angle of attack sensor.
smoothbrian@reddit
Simple answer: extending or retracting the flaps does try to change the pitch of the plane. We use the elevator to keep that from happening.
The airfoil would definitely try to rotate if it weren't attached to the plane. The stabilizers and flight controls hold the plane where we want it.
Using flaps increases lift and increases drag. On takeoff, some planes use a little bit of flaps to help themselves get airborne sooner. You wouldn't want the flaps down too far or else you won't climb as well because of all the extra drag. On landing, the extra lift lets the plane fly slower (so it stops sooner) and the extra drag lets it come in to land steeper which helps us clear obstacles.
ConnorDGibson123@reddit
Usually by a flap motor, hope these helps
VileInventor@reddit
Go in a plane ask to do one circuit and put the flaps in vfe while putting power to idle. You’ll see everything you’re asking about in real time.
KayaLyka@reddit
Fuckin magnets, how do they work??
Frederf220@reddit
Flaps reconfigure the shape of the wing. The extended flap configuration behaves identically to a wing built into that shape permanently.
Consider that angle of attack is between the chord line and the flow direction. How does the flap reconfiguration of the wing geometry change the chord line?
CommunicationFit4360@reddit (OP)
Ahh it would increase the overall angle of attack would that change sufficiently counteract the forces i descrobed before?
Frederf220@reddit
Those forces described may or may not arise depending on what things are or are not changed. Some aircraft experience a nose down moment on flap extension. There may be other factors like flow over tail surfaces that dwarf this effect so the total effect is a nose pitch up.
Guysmiley777@reddit
You're confused because the physics of lift is more complex than what you'll be taught in an A&P course and frankly the physics of lift is beyond the scope of pilot training courses as well.
There are many different types of flaps, the important thing to know is that flaps increase lift and in doing so they also increase drag.
The efficiency of the lift (meaning the ratio of the lift versus the drag) created by flaps is almost always worse than the efficiency of a clean wing with flaps retracted, but the trade-off is worth it to bring the wing's stall speed down.
If you want to know the WHYS of aerodynamics then you really need to study calculus and then Navier-Stokes equations. But none of that will directly make you a better A&P, that would be more of a thing to study because you're interested, not because you need to know.
ThatLooksRight@reddit
popcorn gif
throwaway5757_@reddit
YouTube is your best bet
rFlyingTower@reddit
This is a copy of the original post body for posterity:
Hey, im getting read to go to school to get my A and P license and Im trying to understand how the controls work im just confused on the flaps at the back end of the plane. From what I understand the flaps will extend down. This will then compress the air on the bottom trailing edge and also cause a lower pressure area on the top of the wing at the trailing edge. Im just confused why both of these actions dont end up pushing/pulling ghr trailing edge up and therefore pushing the leading edge down which would decrease the angle of attack and push the nose of the plane downwards? Thank you in advance 🙏
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