Cargo Plane Wobbe on pitch-axis

[baldamundo]

Made one of my first big cargo planes. Fairly standard two-engine turboprop thing, looking a bit like one of these or that sort of thing. Flied pretty well on the first test, to be honest. The only issue was that I was getting an awful lot of wobble on the pitch-axis whenever I manually pitched up - especially while turning. Is this just an inevitable feature of flying a big plane without using a joystick (I do realise there are on-screen joystick mods) or are there some features and design tricks I'm missing for increased pitch-axis (longitudinal ?) stability?

I can upload some screenshots of the plane later, but it basically looks like a C-160, or any two-engine turboprop cargo plane, but with a relatively small and probably poorly designed tail.

[Champ]

I would guess you need more rigidity. Did you try Kerbal Joint Reinforcement ?

[Kobymaru]

Please do upload a screenshot.

Do you mean by "wobble" that the craft behaves like a noodle, so it bends and twists? Or does it stay rigid, but the nose keeps oscillating down and forth?

If it's the former, yes, either you strut it to hell (beware of increased drag!) or you use KJR.

If it's the latter... Well, that's hard

The problem is this: Control surfaces need about 1 second to full deflection. If your craft is heavy, it only starts turning after this 1 second. But then it has built up momentum! And you overshoot your desired pitch. If you pull back, it takes a while for the control surfaces to react, then a while for your plane to start turning again...

So you end up oscillating back and forth.

Stock SAS is especially guilty on this. You can try one of the Pilot Assistant mods, but I did not have success with this. You can tweak the control surface deflection if you have the Stock Bugfix modules (add more control surfaces, but *less* deflection)

But the only Tip I really have for you until they fix SAS is: fly carefully

[pincushionman]

The OP didn't mention SAS, Koby - but since you did, I will as well:

Don't.

I have yet to have any success with SAS and airplanes. Make aerodynamics work for you for stability tasks (dihedral, pendulum effect, well-sized tailplanes, etc.) and USE TRIM.

The issue you're getting sounds like either lack of stiffness or poorly-sized tailplanes/elevators. Or both. As we say everytime in this subforum: Pics would help.

Also, joystick >> keys when it comes to planes. I've seen planes flown well on keys, but why torture yourself?

Edited October 7, 2015 by pincushionman

[Randazzo]

The OP didn't mention SAS, Koby - but since you did, I will as well:

Don't.

SAS can be tricky, but unless you're flying with a stick or constantly rebalancing your fuel, you will eventually need it even when using trim. I find that if you have a wobbly plane during control input, it sometimes helps to tap the control key as opposed to holding it down. You need to get the timing in sync with the control lag of the vehicle so that it's not bouncing up and down/left to right, but that the overall control stays constant but not pushing too far out of prograde heading all at once. You can also try pressing caps lock for fine control but that usually just results in not having enough control authority for me.

[qromodynmc]

I fly with sas open all the time.

btw did you assigned jobs for control surfaces spesifically? Like no pitch for ailerons, no roll for elevators, no roll and pitch for rudder?

[Vicomt]

SAS can be tricky, but unless you're flying with a stick or constantly rebalancing your fuel, you will eventually need it even when using trim. I find that if you have a wobbly plane during control input, it sometimes helps to tap the control key as opposed to holding it down. You need to get the timing in sync with the control lag of the vehicle so that it's not bouncing up and down/left to right, but that the overall control stays constant but not pushing too far out of prograde heading all at once. You can also try pressing caps lock for fine control but that usually just results in not having enough control authority for me.

This. So much this. I usually end up trying to gently hammer the 'S' key with varying amounts of rapidity until I find a stable rate, then modify as appropriate

I should probably take on-board the comment about more, less powerful control surfaces though.

[sal_vager]

Moved to gameplay questions, also, nice plane Randazzo.

[Skorpychan]

Bigger horizontal stabilisers, or a longer tail. Maybe add some canards at the front?

[baldamundo]

Do you mean by "wobble" that the craft behaves like a noodle, so it bends and twists? Or does it stay rigid, but the nose keeps oscillating down and forth?

Sorry I wasn't clear - very much the latter! Joint Reinforcement seems to be keeping it pretty sturdy.

Bigger horizontal stabilisers, or a longer tail. Maybe add some canards at the front?

Thanks - these are exactly the sorts of tips I needed. For the horizontal stabilisers on the tail, is it just the size of them (both width and length?) that matters? And you say a longer tail - is it specifically the further the tail is from the centre of mass the more stable? Canards I think would look silly on this particular plane though!

This. So much this. I usually end up trying to gently hammer the 'S' key with varying amounts of rapidity until I find a stable rate, then modify as appropriate

I've started experimenting with on-screen joysticks, which were confusing at first, but actually make a huge difference to stability once you start getting used to them.

Re: the SAS stuff, I've been finding Crzyrndm's Pilot Assistant mod absolutely invaluable lately. Especially the functionality that basically lets you enable or disable SAS on individual axes separately, which has been especially helpful actually in making the transition from full SAS flight to on-screen joystick use.

[jaylw314]

I'd vote no on the canards, yes on the bigger tail surface or tail length. Canards are bad for stable flight--once the nose starts pitching up, the canard surface gains angle of attack, which makes more nose lift, so you can get large oscillations. OTOH, when the nose pitches up, the tail surface gains lift which makes the nose drop, which decreases the size of oscillations. A longer tail lever will give you more stability also.