So what can I do for making jet airliners? The Center of Thrust is always too low.

[SlabGizor117]

I heard something about moving the center of lift further back to make it nose heavy to counter the way a low center of thrust pushes the nose up, but is there anything else I can do about it? Because sometimes this idea makes aesthetics kinda hard. How do Boeing and Lockheed do it on theirs?

Thanks for the help!

-Slab

[eddiew]

Maybe tilt the wings back a degree?

There's also something to be said for a plane that wants to nose up a little, since it's less likely to come down by accident xD Been trying a triple-engine spaceplane than has strong nose up tendencies when it goes to rockets; but it stabilises about 30 degrees, which is a rather handy feature all told!

[Randazzo]

You could raise the wings themselves (I'm assuming you have wing mounted nacelles), but it may not be the aesthetic you're looking for.

http://forum.kerbalspaceprogram.com/threads/29533-What-did-you-do-in-KSP-today?p=1824830&viewfull=1#post1824830

edit: That's 0.90 aero model.

[TheXRuler]

I would think that real commercial airliners are balanced properly, either by slightly angling the engines up or down or by having the CoL so far back that the engine torque simply creates a "neutral pitching moment" (if my terminology memory doesen't fail me) also, in real planes, fuel is stored in the wings, which means that the CoM will be less far back than in stock KSP.

[Wanderfound]

Angle your engines, raise your wings above the centreline, aim the thrust vector through the CoM. It hasn't yet been updated for 1.0.x, but the RCS Build Aid mod makes this much easier to do accurately.

For example:

These are set up to fly without pitching torque regardless of how many of the engines are running.

[SlabGizor117]

Huh, thanks for the replies. So, it looks like this is my only choice, because I don't wanna build a C-130 Maybe airliners just have a low center of mass?

[Shadowmage]

The way I solve thrust differential problems in my RC model airplanes is to

A: Offset the angle of the motor, usually it is pointing a few degrees to the right and downward. (to the right is to counteract the body torque from p-forces, probably not necessary in KSP as these are not prop driven motors, and likely even the prop motors do not model air vortex correctly / at all)

B: Offset the angle of the wings

C: Offset the angle of the rear horizontal stabilizer

D: Trim the dang thing to fly straight at a given speed/attitude and ignore the rest

They all result in more drag (not sure how this is modeled in KSP), but can make your craft much friendlier to fly. Option D is kind of a joke...but also serious one. If the problem is not too bad, or too hard to fix another way, just use the trim settings (ALT + WASD) to trim it to fly level at cruising speed.

[SlabGizor117]

Oh, thanks Wanderfound. I didn't see the post when I replied. I could do that, I noticed that's what the stock plane that came with 1.0 did, not too bad, but I like the 737/747 style. Cool, though!

[Wanderfound]

Huh, thanks for the replies. So, it looks like this is my only choice, because I don't wanna build a C-130 Maybe airliners just have a low center of mass?

All of the fuel, wings, luggage and machinery is down low (including the mass of the engines themselves), the top bit is just the shell over the passenger cabin. So, yeah, low CoM. They also probably don't mind having a bit of constant pitch-up torque from the engines; it eases the load on the elevators.

The tweakable wing mass in FAR makes this easier to do. For stock, have you tried out the new wet wings?

If all else fails, there's always the Lockheed Tristar option; balance thrust against thrust.

[TheXRuler]

I'd definetly expect airliners to have a low CoM, think about it. All the freight and supplies are stored in the belly of the plane.

Edited May 5, 2015 by TheXRuler

[SlabGizor117]

All of the fuel, wings, luggage and machinery is down low (including the mass of the engines themselves), the top bit is just the shell over the passenger cabin. So, yeah, low CoM. They also probably don't mind having a bit of constant pitch-up torque from the engines; it eases the load on the elevators.

The tweakable wing mass in FAR makes this easier to do. For stock, have you tried out the new wet wings?

If all else fails, there's always the Lockheed Tristar option; balance thrust against thrust.

I have tried the wet wings, they moved the center of mass forward more though, I had all my fuel in the tail to give that classic airliner aesthetic with the wings placed far back. The Tristar is interesting, but not my goal aesthetics-wise, once again. Thanks for the help!

I'd definetly expect airliners to have a low CoM, think about it. All the freight and supplies are stored in the belly of the plane.

Yeah, that's what I figured. I like your signature XD I don't understand why Squad hasn't fixed that yet.

The way I solve thrust differential problems in my RC model airplanes is to

A: Offset the angle of the motor, usually it is pointing a few degrees to the right and downward. (to the right is to counteract the body torque from p-forces, probably not necessary in KSP as these are not prop driven motors, and likely even the prop motors do not model air vortex correctly / at all)

B: Offset the angle of the wings

C: Offset the angle of the rear horizontal stabilizer

D: Trim the dang thing to fly straight at a given speed/attitude and ignore the rest

They all result in more drag (not sure how this is modeled in KSP), but can make your craft much friendlier to fly. Option D is kind of a joke...but also serious one. If the problem is not too bad, or too hard to fix another way, just use the trim settings (ALT + WASD) to trim it to fly level at cruising speed.

Umm,

right.... Gotcha.

[TheXRuler]

Glad you liked it, I had quite a good laugh when I read the original post the first time

The weirdest thing is you kind of get used to it and expect a certain key to rotate something in a certain way and then it does something completely different. Needless to say I absolutely love the editor gizmos added by squad in 0.25(?) and barely use the qweasd keys to rotate things anymore ^^

Edit: Maybe they will fix it some day (I hope they do) but I guess it's just too much work for too small a problem, as there are much more important things that need working on atm.

Edited May 5, 2015 by TheXRuler

[Juicy]

Huh, thanks for the replies. So, it looks like this is my only choice, because I don't wanna build a C-130 Maybe airliners just have a low center of mass?

When you sit in an airliner, look at how high the wingtips are relative to yourself while the plane is still standing still on the ground.

Next look how high the wingtips are relative to yourself during take off and in flight.

Few aircraft have completely stiff wings, and especially on airliners and gliders, which both have relatively wide wings, it's very noticable that the shape of the wing bends into a slight V shape during flight, but especialy during take-off.

(for fun, if you know the person next to you is afraid of flying, look wide eyed, and exclaim "Omg! They're gonna snap off!")

[SlabGizor117]

Glad you liked it, I had quite a good laugh when I read the original post the first time

The weirdest thing is you kind of get used to it and expect a certain key to rotate something in a certain way and then it does something completely different. Needless to say I absolutely love the editor gizmos added by squad in 0.25(?) and barely use the qweasd keys to rotate things anymore ^^

Edit: Maybe they will fix it some day (I hope they do) but I guess it's just too much work for too small a problem, as there are much more important things that need working on atm.

Yeah. One thing I haven't taught myself to do yet is just place the control surface wherever and use rotation and offset to get it in place.

When you sit in an airliner, look at how high the wingtips are relative to yourself while the plane is still standing still on the ground.

Next look how high the wingtips are relative to yourself during take off and in flight.

Few aircraft have completely stiff wings, and especially on airliners and gliders, which both have relatively wide wings, it's very noticable that the shape of the wing bends into a slight V shape during flight, but especialy during take-off.

(for fun, if you know the person next to you is afraid of flying, look wide eyed, and exclaim "Omg! They're gonna snap off!")

Lol, yeah. That makes sense

[Kuu Lightwing]

Actually, don't they just... stabilize aerodynamicaly IRL? I mean, look at this baby:

The engines are just below everything. They certainly introduce some torque, but plane has an advantage of having aerodynamic control surfaces that can be used to compensate for off-center thrust.

Seaplanes are another example:

This thing has CoT so high that no amount of up/downthurst would completely remove it. So they probably just use control surfaces to compensate.

[SlabGizor117]

Yeah, but it didn't seem to work that way for me. Maybe it was .90 errordynamics, not sure, but originally no amount of control surfaces could keep my 737 from flipping in air right after taking off.

[adinfinitum]

Have you tried lining up your CoT directly below the CoM? The shorter the distance between the two, the less torque there is, and the easier it is to compensate for the nose up tendency.