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DEPLOYABLE FLAPS AND LIFT


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Hello everyone!

So i read some old discussions about the "deploy" function of ailerons but they were all 1.1 so I am curious as to how the state of affairs is now on FLAPS and LIFT in general.

As I am sure everyone knows, flaps in real life are a way to extend a wing's lift and thus provide lower stall speeds. Since it creates drag as a "byproduct", it is only used in low speed situations, but thats when you want it (takeoff and landing) so thats ok. Basically that is it.

Is the game reflecting that, though? I made a plane with very large wings and control surfaces, with dedicated flaps, close (but not "on") to the CoG. I set the flaps to an action group and tested it in several takeoff and landing situations. Above all, i did several very slow acellerations on the ground with full stick backwards to effectivelly gauge the speed in which speed was enough to overcome weight. I did this with different takeoff weights, and with flaps on and off.

What i have found is this:

1 - the "deployed" ailerons do function as an effective airbrake. Much more effectivelly than the airbrake part, if you have enough surface area on the "flap". It is worth considering abandoning the airbrake stock part alltogether if putting a "hand-made" airbrake fits your design;

2 - i saw absolutelly no discermible change in stall speed when flaps are deployed. Game mechanics indeed do not appear do mimic the lift effect provided by deploying ailerons. I'm not complaining though. Nowhere in the descriptions it is stated that it does. But indeed the apparent usefullness of deploying control surfaces as a group is limited to braking, and does not affect the lift rating (too bad!).

This makes sense when you see how lift is treated by game mechanics. Parts have a "lift rating". More parts, more lift, thats basically it. Less lift and/or more weight, higher stall speeds. That is simple but not bad actually, since that's 95% how it works in real life (somethings like speed stalls do not appear to be contemplated but thats ok). What bugs me is that there is no way to calculate anything in the SPH regarding lift and stall speeds. What the hell is "lift rating = 1" supposed to mean?

With this in mind, my two cents are:

1 - perhaps when you deploy ailerons, the lift rating could be doubled for the duration of the deployment. That would mimic the effect flaps have when deployed. Keep the drag, thats correct anywhays;

2 - since we are at lift rating, shouldn't lift be a proper cientific measurment like everytiing else in KSP?  Even if a simplistic one, like for instance, "this surface generates 100kg of lift every 10 m/s of speed". This arbitrary lift rating concept is merely a stopgap and KSP deserves better;

3 - is anyone aware of a mod that properlu calculates lift? Likr kerbal engineer does for everything else?

I would love to hear your thougts on that.

Daniel

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I haven't done any specific tests but my overall impression pretty much matches what you are saying.  'Aileron' flaps don't seem to make much difference to the amount of lift, but they are pretty handy for slowing a craft down when coming in to land.

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I have a passenger jet that uses them. In level flight, deploying the flaps is usually good for about a 10-15 m/s climb in additional lift. They're mostly useful for slower landings by acting as air brakes and helping the nose stay up (the ailerons are ahead of the CoM, so they try to push the nose up). They can also be used to achieve tighter banked turns for the same Angle of Attack, though the difference isn't on that particular plane.

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2 minutes ago, Randox said:

 In level flight, deploying the flaps is usually good for about a 10-15 m/s climb in additional lift. 

So you think they DO provide more lift? Are you sure its just not an effect of the different configuration of control vectors, something we know the game mechanic indeed provide? I'm not sure this is true lift you're describing.

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Just now, Daniel Prates said:

So you think they DO provide more lift? Are you sure its just not an effect of the different configuration of control vectors, something we know the game mechanic indeed provide? I'm not sure this is true lift you're describing.

Insofar as the plane can maintain a stable altitude at a lower speed for the same pitch, or a higher rate of climb for the same pitch and speed; yes. In terms of greater intrinsic lift; no. It's not 'true lift', it's a useful change in geometry.

For that plane, the lift effects are not very useful. It just needs them to keep the nose up and allow for a proper landing flare.

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25 minutes ago, Randox said:

 In terms of greater intrinsic lift; no. It's not 'true lift', it's a useful change 

Yep, that I have verifyed also: a change in geometry. It figures that in some designs it may cause effects like keeping the nose higher in lower speeds, what is similar to some of the effects of deployed flaps (but for different reasons).  In my designs it did that too. Its like a one-setting trim or "bias". But a poor substitute for the full fledged effect of a proper flap. 

I suppose though that  this is indeed one of the few things to complain in ksp wings. Everything else is correctly there, even the different effects in pitch as the wing provides different lift with different speeds.

55 minutes ago, Theysen said:

Use FAR for properly working flaps and slats. I'd expect the behavior you are looking for

Thanks i'll check it out!

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I'd argue that you can get flaps to work in stock they way we want them to work.

First off, know that lifting surfaces in stock KSP can stall.  Wings in KSP generate lift roughly proportional to angle of incidence - up to a point.  Past that point, they become more and more drag and less and less lift.  Turn on the F-12 aero markers, play with your flap deploy limits, and you can just watch it.  There's a lot of documentation late in this thread with the details (where I recently had a similar question about flaps).  TL;DR: past ~7 degrees of AoI you are starting to stall.  

The main problem here is that the control surfaces at their default 100% tend to move a lot more than 7 degrees!  And if you crank that up to 150% thinking you'll get more lift - you don't.  You just get an airbrake.  So for a functioning flap, you should limit deflection to ~7 degrees if your wing is starting out flat.  If your wing starts at 5 degrees, no more than 2 extra degrees on the flap. It's not trivial to measure that - I use Part Angle Display to view relative angles accurately, and Editor Extensions Redux to control them.  Even then, you have to place a reference wing part next to the flap, set it to the desired angle and gizmo it right next to the flap, set deploy on the flap and tweak until the angle matches, then delete the reference part. Not trivial. EDIT - Of course there's an easier way, the parts' right click tooltips show max deploy angles, just do the math on the correct percentage from there.

Problem 2 is about pitch moment.  If you have a typical control surface on the trailing edge of the main wing, when deployed it will act to pitch the nose down - PREVENTING the earlier rotation and liftoff you're looking for.  Sometime you should try a flap on the leading edge that deploys UP - that can make a dramatic difference in rotation speed.  Perhaps ideally, deploy them in pairs - an 'up' surface on the leading edge, 'down' on the trailing edge, and adjust them so there's no overall change in pitch trim (and observe the limits above so it's not just an airbrake).  By this time it's complex enough that you'll want action groups - I like to bind the flap deploy to the gear group.

So it can work.  Extra parts, extra testing, might look rather strange with leading control surfaces - and planes in particular often have an aesthetic purpose in KSP.  On the whole, I find it's usually simpler to just find the appropriate amount of main wing incidence and area in the first place, so my takeoff and landing speeds are reasonable to begin with.  And make sure the lift center is very near the mass center so it's easy to rotate and gain lift.  RCS Build Aid is invaluable in the design process, to make sure your center of mass doesn't move relative to the wings. That lets you get away with very close lift centers without risking instability, or large changes in flight dynamics.

Edited by fourfa
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The problem is not in the flap parts, it's in the aerodynamic system. In real life, aerodynamic behavior is governed by the shape of the airfoil, while in KSP its the sum if the behaviors of individual wing parts. So in KSP, deploying flaps increases lift and drag at the location of that flap - but in real life deployment of flaps instead alters the characteristics of the airfoil as a whole, which has effects along the entire chord.

This is even more pronounced if instead of flaps you try to do slats (which deploy down on the leading edge). In reality these can be tremendously effective on large wings, but in KSP I challenge you to make slats that look like they deploy in the correct direction without decreasing lift.

As noted, in FAR it works better, but the best solution would be some kind of procedural wing that can recognize changes in wing geometry, rather than just a bunch of parts.

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12 hours ago, Daniel Prates said:

2 - i saw absolutelly no discermible change in stall speed when flaps are deployed.

I did.  In testing the plane below (using the folding wings from SXT with integrated flaps), I found stall speed to be 16 m/sec with flaps, and 20 without.  Only 4 m/sec difference, but it's a 20% decrease with flaps extended due to the starting number being so small.

jkabsoh.png

 

Edited by RoboRay
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Flaps in KSP do absolutely nothing. I remember when I once tried to use flaps on the wings and because they were behind the CoM the plane just nose-dived into the ground.

Now, I don't think KSP needs this level of aerodynamics simply because it's (and should be) more about spaceflight. However, and I know I've stated it many times, if devs were ever to create a new game about aircraft only and with proper aerodynamics I would fully support it.

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1 hour ago, Veeltch said:

Flaps in KSP do absolutely nothing

Quite wrong. It lowers the AoA somewhat. Check the video in the spoiler for proof. My speed and altitude is locked by AtmosphereAutopilot. As the flaps deploy you can see the AoA change in the GUI to the left. If I were to lower my speed even more, the effect will be more pronouced.

Spoiler

 

 

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I made a "helicopter" which was essentially a rotating wing with engines pointing in opposite directions at either end. I kept the wing flat/horizontal, and with the engines on at full blast, I only span around really fast; I did not take off. When I added ailerons, I could deploy them to angle them "up", and this caused the craft to lift (even at very low engine power). When I retract the  ailerons, the craft would gradually sink. So I think that they do generate lift like flaps would.

Alternatively, you can just have the rotating wings angled like deployed flaps, and the craft will gain height consistently. If you reduce power and slow your rotating down, the rotating wing "stalls" and you fall (rather than sink) because, well, you've stalled the wings.

flaps/ angled wings have also been useful when making boats (in previous versions of KSP), being able to lift the nose out the water with angled wings.

Edited by Stewcumber
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On 11/02/2017 at 10:39 PM, RoboRay said:

I did.  In testing the plane below (using the folding wings from SXT with integrated flaps), I found stall speed to be 16 m/sec with flaps, and 20 without.  Only 4 m/sec difference, but it's a 20% decrease with flaps extended due to the starting number being so small.

jkabsoh.png

 

Hum.... you are using some mod I don't know about... i do not recognize that wing there. What are you using?

To be fair, I forgot to mention that I noticed no lift changes when deploying while using only KAX and AIRPLANE PLUS. Maybe you are using something else that tweaks lift. Some members reported above that FAR does that. 

 

On 12/02/2017 at 1:02 PM, SkyKaptn said:

Quite wrong. It lowers the AoA somewhat. Check the video in the spoiler for proof. My speed and altitude is locked by AtmosphereAutopilot. As the flaps deploy you can see the AoA change in the GUI to the left. If I were to lower my speed even more, the effect will be more pronouced.

  Reveal hidden contents

 

 

That is correct, I have observed that too. But that is because of the changing vectors of the control surfaces (the deployed aileron remains being an aileron and generates the proper response, when angled differently). I can think of several uses for. For instance, in floatplanes. When you are landed on whater, the water attriction makes it more difficult to gef off the ground than it is on a runway (not to mention you needing more thrust because of the corresponding effect on speed). Having your deployed ailerons tilting upwards may help getting it off the water. I sure will try! 

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On a real airplane, flaps are only a small % of the wing area, but they increase the lift generated by the entire wing, by increasing the extent to which it deflects air downwards.

In KSP , deploying the flap only affects the lift generated by the flap surface itself, and it   does that by increasing its angle of attack.  

Which leads to the second problem - maximum lift is produced at 30 degrees AoA for all lift surfaces.  Go beyond that, lift declines, though drag continues to increase (a stall).     When your plane is flying slowly, its wings are already going to be at a significant angle of attack eg. 10 degrees.    If you deploy the flaps at the default 30 degrees that surface ends up at a total of +40, which is significantly stalled and may not produce any more lift than it did at +10 (retracted).   You can fine tune this by using the limit authority tweakable, but even then you'd get more lift by just keeping flaps retracted and pitching the whole airplane to +30, so every surface is producing its max lift.

Of course, many airplanes can't reach +30 AoA due to excessive stability/limited pitch authority/risk of tailstrike on takeoff/landing, so this is a case where flaps theoretically might benefit.  Except that there is very little extra lift to be gained going from +15AoA to +30AoA anyway,  a few m/s at best.  Not worth the risk of a stall.

 

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On 11/02/2017 at 3:50 PM, fourfa said:

I'd argue that you can get flaps to work in stock they way we want them to work.

First off, know that lifting surfaces in stock KSP can stall.  Wings in KSP generate lift roughly proportional to angle of incidence - up to a point.  Past that point, they become more and more drag and less and less lift.  Turn on the F-12 aero markers, play with your flap deploy limits, and you can just watch it.  There's a lot of documentation late in this thread with the details (where I recently had a similar question about flaps).  TL;DR: past ~7 degrees of AoI you are starting to stall.  

The main problem here is that the control surfaces at their default 100% tend to move a lot more than 7 degrees!  And if you crank that up to 150% thinking you'll get more lift - you don't.  You just get an airbrake.  So for a functioning flap, you should limit deflection to ~7 degrees if your wing is starting out flat.  If your wing starts at 5 degrees, no more than 2 extra degrees on the flap. It's not trivial to measure that - I use Part Angle Display to view relative angles accurately, and Editor Extensions Redux to control them.  Even then, you have to place a reference wing part next to the flap, set it to the desired angle and gizmo it right next to the flap, set deploy on the flap and tweak until the angle matches, then delete the reference part. Not trivial. EDIT - Of course there's an easier way, the parts' right click tooltips show max deploy angles, just do the math on the correct percentage from there.

Problem 2 is about pitch moment.  If you have a typical control surface on the trailing edge of the main wing, when deployed it will act to pitch the nose down - PREVENTING the earlier rotation and liftoff you're looking for.  Sometime you should try a flap on the leading edge that deploys UP - that can make a dramatic difference in rotation speed.  Perhaps ideally, deploy them in pairs - an 'up' surface on the leading edge, 'down' on the trailing edge, and adjust them so there's no overall change in pitch trim (and observe the limits above so it's not just an airbrake).  By this time it's complex enough that you'll want action groups - I like to bind the flap deploy to the gear group.

So it can work.  Extra parts, extra testing, might look rather strange with leading control surfaces - and planes in particular often have an aesthetic purpose in KSP.  On the whole, I find it's usually simpler to just find the appropriate amount of main wing incidence and area in the first place, so my takeoff and landing speeds are reasonable to begin with.  And make sure the lift center is very near the mass center so it's easy to rotate and gain lift.  RCS Build Aid is invaluable in the design process, to make sure your center of mass doesn't move relative to the wings. That lets you get away with very close lift centers without risking instability, or large changes in flight dynamics.

Great info! 

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15 hours ago, AeroGav said:

Which leads to the second problem - maximum lift is produced at 30 degrees AoA for all lift surfaces.

Really? That's a pretty poor imitation of reality.  In my aircraft performance & design class we typically see Lmax around 15 degrees AoA.

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I think it is important to note, Lift in KSP is a function of lifting surface area and angle of attack.  A lifting surface with 0 AOA to direction of travel generates no lift.  What the deploy flaps does allows for more lift generation at a shallower angle of attack for the main wing.  What I have found is that well placed flaps actually help get aircraft off the runway by generating lift when the main wings themselves may have only a very small AoA, especially for aircraft intolerant of significant rotation on the runway or approaches at significant angles of attack. 

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On 2/14/2017 at 7:24 AM, Daniel Prates said:

Hum.... you are using some mod I don't know about... i do not recognize that wing there. What are you using?

Sorry, I've been unable to get on the forums for a few days...  As noted already, it's a set of small wing parts from SXT Continued (formerly Lack's Stock Extensions).

Here's a couple of more pictures of the plane, and the .craft file if you want to experiment with it: https://dl.dropboxusercontent.com/u/14176520/Bumblebee.craft

(The only non-stock parts are from SXT.  Some of the stock parts look different because I had Ven's Stock Revamp installed at the time.)

7LngKqh.png

Lf2vVl8.png

It fits inside a Mk3 cargo bay when folded up.  It's not fast, but it's a little faster if you take the floats off.

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7 hours ago, RoboRay said:

Sorry, I've been unable to get on the forums for a few days...  As noted already, it's a set of small wing parts from SXT Continued (formerly Lack's Stock Extensions).

Here's a couple of more pictures of the plane, and the .craft file if you want to experiment with it: https://dl.dropboxusercontent.com/u/14176520/Bumblebee.craft

(The only non-stock parts are from SXT.  Some of the stock parts look different because I had Ven's Stock Revamp installed at the time.)

7LngKqh.png

Lf2vVl8.png

It fits inside a Mk3 cargo bay when folded up.  It's not fast, but it's a little faster if you take the floats off.

I'll check it out, thanks!

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I usually prefer to give my planes split-aileron / split-elevon type airbrakes which are MUCH more effective. Or I give them Spoilerons, which in their deployed state decrease the lift at the normal angle of attack and thus force u to land with the nose raised, still slowing the plane down and ensuring that the main gear touches down first. That is often useful when there's delicate stuff on the front that shouldn't be harmed.

An example of a plane with split-elevon airbrakes:

 

This being said, yes, flaps and spoilers DO kinda work the way they're supposed to, they just do it in a different way than in real life.

On 12.2.2017 at 2:45 PM, Veeltch said:

Flaps in KSP do absolutely nothing. I remember when I once tried to use flaps on the wings and because they were behind the CoM the plane just nose-dived into the ground.

To me that sounds like a bad design on ur part. I can only speak for my pretty old version since I'm somehow unable to update but in order to have a conventional plane fly properly I have to put the CG somewhere inbetween the wings and the elevator, where the center of lift is. In that case, flaps should make the plane want to pitch up instead of down.

I'm just assuming ur not stupid enough to think that flaps on a delta wing or something like that would work, so I'm wondering what that plane looked like.

 

Also, I threw together an example real quick, one that comes closest to a normal private pilot's plane, and look how the CG is almost at the trailing edge of the wings instead at the 1st third of the wing like it should be. The reason why is that the center of lift is there too and I need to keep them close to each other in order to make a plane that works. Granted, when I deploy the flaps on this it does want to pitch down a little due to the CG and CoL being in front of the flaps, but that's nothing that makes the plane dive hard or can't be neutralized by the SAS. I didn't screen cap this coz right now I can't and I don't think it's really necessary right now.

exampleoip7f.png

Edited by DualDesertEagle
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1 hour ago, DualDesertEagle said:

I usually prefer to give my planes split-aileron / split-elevon type airbrakes which are MUCH more effective. Or I give them Spoilerons, which in their deployed state decrease the lift at the normal angle of attack and thus force u to land with the nose raised, still slowing the plane down and ensuring that the main gear touches down first. That is often useful when there's delicate stuff on the front that shouldn't be harmed.

An example of a plane with split-elevon airbrakes:

 

This being said, yes, flaps and spoilers DO kinda work the way they're supposed to, they just do it in a different way than in real life.

To me that sounds like a bad design on ur part. I can only speak for my pretty old version since I'm somehow unable to update but in order to have a conventional plane fly properly I have to put the CG somewhere inbetween the wings and the elevator, where the center of lift is. In that case, flaps should make the plane want to pitch up instead of down.

I'm just assuming ur not stupid enough to think that flaps on a delta wing or something like that would work, so I'm wondering what that plane looked like.

 

Also, I threw together an example real quick, one that comes closest to a normal private pilot's plane, and look how the CG is almost at the trailing edge of the wings instead at the 1st third of the wing like it should be. The reason why is that the center of lift is there too and I need to keep them close to each other in order to make a plane that works. Granted, when I deploy the flaps on this it does want to pitch down a little due to the CG and CoL being in front of the flaps, but that's nothing that makes the plane dive hard or can't be neutralized by the SAS. I didn't screen cap this coz right now I can't and I don't think it's really necessary right now.

exampleoip7f.png

The problem is that if you put these "flaps" in the same axis the CoG is then you just made yourself elevator control surfaces. I've built enough planes in FAR and played IL-2 for long enough to know how flaps should work. And they clearly don't work in KSP because the control surfaces and wings put together don't act as one solid body.

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