Flaps vs Elevators

[Zeldenthuis]

My planes keep wheelbarrowing, and failing to take off with flaps.  At the same time I can use elevators and take off fine.  This is the opposite of what I expect and confusing to me.  I would like a good explanation of what I am missing when flying a plane.

I am using Realism Overhaul and a version of FAR that goes with it.  The version of KSP is 1.7.3 and the modules are setup according to the Realism Overhaul website.  The plane is a simple propeller driven plane without any technology researched.

Scenario 1

The flaps are placed down and we race down the runway.  The flaps provide additional lift.  This drives the nose and the front wheel down into the runway.  The back wheels lift off, and the plane starts bouncing. The final result is a crash.

Crash speed 80+ m/s

https://www.youtube.com/watch?v=EyzKOWyJke0

Scenario 2

The elevators are placed up.  The elevators provide no additional lift and perhaps even force the back wheels down onto the runway.  Eventually the plane lifts of gracefully and we are flying without problems.

Takeoff speed 55 m/s

https://www.youtube.com/watch?v=76dUf27vRAg

Testing Details

All of my planes seem to have this problem.  I started experimenting with jet planes, then tried propeller planes hoping to learn.  My expectation is that flaps will help since that is how I remember my airplane trips going.  Additionally, the science and descriptions all talk about how flaps will help takeoff.

The plane's weight and design can be seen in the picture.  FAR was used to diagnose a variety of issues and I would be happy to know how I could use it to determine flap deflection or setup the plane properly.

Edited April 18, 2020 by Zeldenthuis
fixing links

[HvP]

Hello @Zeldenthuis and welcome to the KSP forums!

The simple answer to this complex question is this: Since your flaps are behind your center of mass, when they are deployed downwards they will act like a lever pushing upwards on the rear of the plane - just like the elevators in your tail do.

In real life the situation is far more complex, but trailing edge mounted flaps do often create a nose-down impulse just like that. There are other factors such as the eddies from the wash coming off of the flaps that then impinge upon the rear elevators; ground effects; surface adhesion; spoilers; canards; and automated trim settings that can all be designed to compensate for this typical nosing down on real aircraft.

Ideally, for the purposes of this game, the flaps should be attached as near to the center-of-mass as possible. One way to do this is to just move the wings forward a little. This reduces the torque from the lever effect, but it's often difficult to move a wing forward without messing up all sorts of other mass/lift balance issues. If you can still balance everything well it works like a dream - that is what I would do. There are other options however that are used in real life designs. You could:

• Add spoilers to the leading edge of the wing which deploy upwards to correct the angle of attack.
• Move the flaps to the underside center of the wing in line with the center-of-mass so that they won't have any effect on your pitch.
• Add canards to pull the nose up.
• Increase the control authority of the rear elevators and add some extra trim to them to compensate. Holding ALT then tapping "S" will incrementally add upwards pitch trim (ALT+W = downwards.) Remember to cancel trim with ALT+X when retracting flaps. This simulates the control programs many planes use to automatically adjust trim in different stages of flight.
• Rotate the wings very very slightly to add a positive angle of incidence. In other words, when looking at the side view the leading edge of the wings is up higher than the trailing edge as they slope downwards from front to back. Even one degree can be enough. This allows the wings to generate more lift in level flight without raising the nose at all. It will usually also move the center-of-pressure forward so you'll have to carefully balance their placement and even possibly add some positive incidence to the rear elevators too so that the plane doesn't nose-up all the time. You may find that the increased lift means that you don't even need flaps anymore.

Maybe one or a few of these ideas will be the solution you are looking for. Or, if not, maybe it's given you an idea of something that might work better.

Good luck, and happy flying.

Edited April 18, 2020 by HvP

[OHara]

I agree, you do seem to be doing everything right, and what you see is what I would expect as well.

Moderate flaps (usually about 10°) let aircraft take-off at a slightly lower speed, by giving the wing a bit more lift, if you pull the  stick back (elevator pitching down) as much as you need to lift the nose-wheel.  You can see at https://www.faasafety.gov/files/gslac/courses/content/35/376/Use of Flaps.pdf that the usual case "the nose-down pitching moment" from flaps is taken for granted.  The pilot almost automatically pulls back as needed on the stick to keep the aircraft in the attitude that he wants.

The unusual case is that flaps on some high-wing aircraft have enough down-wash that they can push the tail down, nose up.  Stock KSP does not simulate interactions between wing and tail; FAR tries to, but I am not sure it considers interaction that far away.  It might be interesting to see if we can build a high-wing aircraft that will pitch up with flaps under FAR.

[jimmymcgoochie]

KSP doesn't really do air physics that well- the wings only provide lift if they are angled down and provide zero lift at 0 degrees angle of attack. Building a plane exactly like a real one won't work that well, so you need to make a few alterations:

1) Angle your wings a few degrees upwards, this gives them a slight angle of attack and causes them to generate lift even when then nose is level e.g. on the runway. The angle will vary depending on how fast your plane will go, and for really fast planes you have to balance getting more lift at low speeds with the additional drag you'll get at high speeds. This was done on planes such as the PBY5 Catalina to give them additional lift at low speeds, particularly useful for seaplanes and flying boats.

2) Use leading edge slats or spoilers rather than trailing edge flaps, especially with delta wings. On the leading edge they'll probably be in front of the centre of lift and when tilted up will generate both lift and a bit of nose-up rotation, whereas on the trailing edge they'll provide lift but a nose-down rotation too. For real planes, the slats deploy downwards to extend the leading edge of the wing, but since KSP doesn't do aerofoils and wings simply provide lift based on their angle of attack you should make them deploy upwards.

3) Nose mounted canards- a bit like slats, they'll give you more lift and help get your nose up. Just be careful though, there's a fine line between 'pulling your nose up' and 'pulling an unplanned Pugachev's Cobra at Mach 3 and ripping various wing surfaces off followed by an inexorable nosedive or humiliating parachute to the ground surrounded by a rain of debris'... Not that I've ever done that myself, you understand 

4) Make your rear wheels lower than your front wheels, it also creates more angle of attack and will give you more lift on takeoff. This can make for some interesting landings if you brake too hard and end up planting the nose in the runway and either doing some front flips or just exploding depending on your speed and the sturdiness of the parts.

[Zeldenthuis]

Thank you @HvP!

I have chosen to go with a flap arrangement on the tail.  The flap on the tail goes negative (up) which pushes the back down.  It 1) keeps the theme of advanced programming, 2) does not mess with lift much (using the leverage), and 3) it looks like a plane.

The plane takes off like a breeze at 45 m/s.  It climbs a bit much at the beginning, which I can likely modify.  It lands acceptably with flaps, although the transition to landing position is stressful, again I think I can tweak that by managing the tail flap pitch better.