Is level flight a pipe dream?

[LethalDose]

So, I restarted my career mode a couple of days ago (first time in 0.23 due to travel) and as I was working through the career tree, I unlocked the first few controlled flight techs. As I was playing around, I noticed my aircraft had to angle up slightly to maintain altitude, meaning that aerodynamic lift was not fully counteracting the weight of the craft.

"This won't do!" I exclaimed, "My fuel is for thrust, not the lecherous forces of gravity!"

Assuming my craft were just too heavy (I tend to add more than I take away), I worked to pare down my craft to reduce weight.

Every smaller iteration had the same problem: Level flight led to altitude loss.

I got down to this and this.

Even these incredibly small and lift-oriented craft don't produce enough lift in level flight at max speed to maintain altitude! They were all tested at the crafts' maximum speeds at just above sea-level, 5 km, and 10 km in Kerbin's atmosphere.

Is this a physics engine issue (there has to be SOME upward thrust), a game-balance issue (lift values were selected by devs for this reason), or operator error (I am crap pilot and/or engineer)?

Has anyone else noticed this?

[StrandedonEarth]

I noticed it but never tried to correct it to level. I gather the wings don't generate lift from the classic asymmetric airfoil design (Bernoulli principle, IIRC) so much as get the reaction from deflecting air as the angle-of-attack increases. Have you tried perhaps tilting the wing up (leading to trailing edge, not root to tip) relative to the fuselage? Then the fuselage (and thrust) can stay level with the wings tilted up and perhaps get lift that way.

[RocketPilot573]

Last I checked, KSP wings don't produce any lift AT ALL when level, so level flight is unlikely.

[barrenwaste]

Well, haven't tried it, but then wouldn't attaching the wings at a slight angle achieve your purpose?

[How2FoldSoup]

I know it's not quite the answer you're looking for but if it's aerodynamics you're looking for you best just download FAR. I can't answer any questions on stock aerodynamics since I think I only used them for a few days since I've always been a planes kind of guy and FAR is simply amazing.

If I had to guess perhaps your CoL is slightly behind your CoM causing the craft to pitch down...I wager if you shifted it forward of the CoM the craft would tend to pitch up. That is how it would work with FAR anyways.

[jedensuscg]

"level flight" with aircraft is not really what people think it is. Depending on the wings AOA, some aircraft have to maintain a slight nose up pitch in order for the aircraft to maintain level flight, and this is compounded more so at slower speeds. Just pointing the nose at zero degrees pitch and hoping you don't lose altitude only works if you have the correct angle of attack for that speed to maintain the required lift, if you start to lose altitude a slight pitch up will change the aoa enough that more lift is created, until you start to speed up, in which case you will need to pitch down to offset the increased lift from the increased speed.

Strandedonearth mentioned change the aoa of the wings themselves in relation to the fuselage, just not that at high speed you might generate to much lift and make your aircraft want to pitch up hard.

Try Ferram Aerospace Research, is changes the flight models so make them more realistic as well as gives you more control on setting up flight controls and has automated features to counter unwanted yaw pitch and roll issues.

[numerobis]

In stock KSP aerodynamics, the amount of lift is related (in a complicated way) to the sin of the angle of attack. So if you have zero angle of attack, you have zero lift. If you give your wings some tilt in the SPH, using shift-wasdqe, you can get positive angle of attack for the wing while the engines push forward. Oddly, the direction of lift is perpendicular to the velocity, so if you're in level flight, you have no parasitic drag at all.

[LethalDose]

"level flight" with aircraft is not really what people think it is. Depending on the wings AOA, some aircraft have to maintain a slight nose up pitch in order for the aircraft to maintain level flight, and this is compounded more so at slower speeds. Just pointing the nose at zero degrees pitch and hoping you don't lose altitude only works if you have the correct angle of attack for that speed to maintain the required lift, if you start to lose altitude a slight pitch up will change the aoa enough that more lift is created, until you start to speed up, in which case you will need to pitch down to offset the increased lift from the increased speed.

Strandedonearth mentioned change the aoa of the wings themselves in relation to the fuselage, just not that at high speed you might generate to much lift and make your aircraft want to pitch up hard.

Try Ferram Aerospace Research, is changes the flight models so make them more realistic as well as gives you more control on setting up flight controls and has automated features to counter unwanted yaw pitch and roll issues.

Okay, so it sounds like this is physics engine issue, and simply adding more lift won't solve what I'm perceiving as a problem. Editing the pitch of the wings on the craft sounds like a good solution for small craft, but I'm worried about it's application to larger craft with structural wings.

I know it's not quite the answer you're looking for but if it's aerodynamics you're looking for you best just download FAR. I can't answer any questions on stock aerodynamics since I think I only used them for a few days since I've always been a planes kind of guy and FAR is simply amazing.

If I had to guess perhaps your CoL is slightly behind your CoM causing the craft to pitch down...I wager if you shifted it forward of the CoM the craft would tend to pitch up. That is how it would work with FAR anyways.

This isn't a pitch/CoL/CoM issue. When I say I'm flying level, that means I'm flying level, no pitch. I put enough SAS on those craft to control and lock pitch at all but the most extreme climbs. Also moving the CoM to behind the CoL can be reaaaaaaly disastrous in a lot of situations. I learned that in 0.22 working on SSTOs.

You both mentioned FAR, and both did it in a way that was really respectful. I'm really tired of the "LOL Just use mods" responses I usually see, and you guys didn't do that. I appreciate that. Right now, FAR makes me a little nervous because, based on what I've seen, it's going to greatly increase the complexity of my missions. I don't think I'm ready for it yet.

[sal_vager]

Well, haven't tried it, but then wouldn't attaching the wings at a slight angle achieve your purpose?

barrenwaste has it, it's what I do as well, I angle my canards or just my control surfaces slightly so they generate more lift in level flight, it makes a huge difference

[cr055H41rz]

That's what trim is for

shift + arrows

[numerobis]

Why are you screaming? Also, trim is not the problem here; read the thread.

[Armchair Rocket Scientist]

As mentioned, all the wings in KSP are basically symmetrical airfoils, meaning that they produce no lift at 0 angle of attack. Real aircraft almost always use asymmetric airfoils for the main wings, but may use symmetrical airfoils for the control surfaces. They also typically have the front control surface at a higher angle of attack for stability reasons.

The devs do have valid reasons for using symmetrical airfoils in KSP. A given part might be a main wing, a horizontal or vertical stabilizer, or rocket fin. In addition, wings may be mounted backwards (which would cause an asymmetric airfoil to produce negative lift). For an aircraft with a single vertical stabilizer, an asymmetric airfoil would produce sideways lift, making the plane difficult or impossible to control. If used as fins placed around a rocket with rotational symmetry, the lift produced would cause the rocket to roll - okay for spin-stabilized rockets, not okay for trying to do a gravity turn.

Giving wings tweakable airfoil shapes could fix this in theory, but this would require a proper aerodynamic model and still be relatively hard to implement.

"This won't do!" I exclaimed, "My fuel is for thrust, not the lecherous forces of gravity!"

Don't worry about that. Even flying at a 15 degree AoA, only about 3.4% of your thrust is "lost." You get a vertical thrust of 26% of the total thrust, but unless you have small wings and a really high TWR this is probably outweighed by aerodynamic lift.

[Claw]

Okay, so it sounds like this is physics engine issue, and simply adding more lift won't solve what I'm perceiving as a problem. Editing the pitch of the wings on the craft sounds like a good solution for small craft, but I'm worried about it's application to larger craft with structural wings.

This isn't a physics engine issue. All airfoils in this game are (currently) symmetric. You must have Angle of Attack on your airfoil to produce lift. This means there will always be a difference between your prograde vector and the aircraft body pitch (the yellow "V"). This is also how it works for real airplanes.

This isn't a pitch/CoL/CoM issue. When I say I'm flying level, that means I'm flying level, no pitch. I put enough SAS on those craft to control and lock pitch at all but the most extreme climbs. Also moving the CoM to behind the CoL can be reaaaaaaly disastrous in a lot of situations. I learned that in 0.22 working on SSTOs.

You both mentioned FAR, and both did it in a way that was really respectful. I'm really tired of the "LOL Just use mods" responses I usually see, and you guys didn't do that. I appreciate that. Right now, FAR makes me a little nervous because, based on what I've seen, it's going to greatly increase the complexity of my missions. I don't think I'm ready for it yet.

You said "that means I'm flying level, no pitch." In aerodynamics, these are two different things. "Flying level" means you are not gaining or losing altitude. "No pitch" can mean a variety of things, but I'm gathering you mean to say your yellow "V" is level with the horizon (or maybe coincident with your prograde vector?). I haven't used FAR, but if it is an accurate representation of physics, you would still have the same result if you use a symmetric airfoil.

That being said, even if you have an asymmetric airfoil (like camber used on light civil airplanes), that asymmetry is only good for one aircraft weight and speed. That means if you go faster/slower or weigh more/less, you will have some Angle of Attack on the airplane. (Technically it's the Angle of Attack relative to the wing that is important.)

Sooo, if you want your yellow "V" to be in the center of your prograde marker and level to the horizon, then you need to tilt the front end of your lifting surfaces upward so they have angle of attack when flying straight ahead. Then you need to figure out what the airspeed is that corresponds to your aircraft weight to achieve zero pitch. That airspeed will change as you burn fuel (due to weight and Center of Mass changes), and will be different if you add a new component to your airplane.

Also, as far as losing trust to gravity, this is also how it works in real life. You're either going to lose thrust directly to gravity, or you lose thrust to overcome the drag that is produced from creating lift. And creation of lift is required to overcome gravity. Without gravity, you wouldn't need lift to begin with.

[AwesumnisRawr188]

Well, I've got to say, even though I'm not the OP, you guys have taught me a lot about space planes and such, in this thread. I never knew about trim, or rotating pieces, and I have been having this same problem, even with an insanely small aircraft(I think it's pretty small.), it doesn't get any lift at all. I have to control it with the ailerons.

Thanks for the info guys, this forum is really helpful

[Claw]

Well, I've got to say, even though I'm not the OP, you guys have taught me a lot about space planes and such, in this thread. I never knew about trim, or rotating pieces, and I have been having this same problem, even with an insanely small aircraft(I think it's pretty small.), it doesn't get any lift at all. I have to control it with the ailerons.

Thanks for the info guys, this forum is really helpful

Your plane must be getting lift or it wouldn't fly at all. If you're using ailerons to control pitch, then they're actually called elevons. And they are a legitimate design "feature."

There is additional basic aero info in the tutorial section. Keptin has a good article (Basic Aircraft Design) that touches on a lot of basics. Although it doesn't quite cover rotating pieces and trim, it talks about Angle of Attack and a lot of other considerations. I've written a tutorial to go along with Keptin's work.

Edit: There is also this thread, which links to a lot of info. Most of it is not about airplanes, but there is some air/space plane info in there.

Edited January 12, 2014 by Claw