What to do with a back-heavy plane

[Spheniscine]

I am trying to build a low-tech UAV (nothing required beyond Aerodynamics), however, the Stayputnik probe is pretty light, and I end up with a back-heavy plane, whose muzzle starts to climb when it gets low on fuel and becomes extremely hard to control (can't lower altitude except by turning the engines off, and it takes a really long time to glide toward the ground this way). I tried moving the center of lift back, but that's only by bunching the wings up together, and if I move it too far back it makes ascension while nearly full on fuel difficult.

Edited October 19, 2014 by Spheniscine

[Tex_NL]

Pump fuel forward.

In case you can't pump fuel forward because you use a single tank you might want to consider splitting the tank.

Pictures, with CoM and CoL visible, would help with diagnosing your problem.

[Spheniscine]

Pump fuel forward.

In case you can't pump fuel forward because you use a single tank you might want to consider splitting the tank.

Pictures, with CoM and CoL visible, would help with diagnosing your problem.

How do you do that? Do you need fuel lines? (trying to keep it low-tech, and the fuel lines don't seem to connect anyway)

Pictures: full, half (front pair of tanks gone), empty

[MarvinKitFox]

Try making a longer plane?

In your case, the only means of pitch control is *so* close to your CoM, CoL, that they have almost no effect.

Putting your pitch control elevators way forward or way back is the way to go.

Also... your CoM is too close to the CoL, especially as fuel drops. But fix the elevator placement and this may not matter so much.

[Tex_NL]

Quick solution:

Run fuel lines from the rear tank to the front tanks (left to right/right to left is easiest to connect) in order for the engines to use the rear tanks first.

Better solution:

Eliminate the front fuel tanks: your fuel will now sit almost on top of your CoM.

To compensate for the CoM shift backwards from eliminating the two tanks give your plane a tail. This will shift your CoL backwards and gives much better pitch control.

To pump fuel right click the source tank and [alt]+right click the receiving tank. Click the in/Out buttons to start pumping. Please note pumping fuel will shift your CoM and can potentially cause instabilities.

Edited October 19, 2014 by Tex_NL

[sanoj688]

I recommend getting a mod called RCS balancer (or something). I know you aren't doing anything with RCS but the mod also shows your CenterOfMass when your fuel tanks are empty, so you can see in what way the CoM of the craft will change as you run out of fuel.

Next thing is that you probably want to redo your craft in such a way that the starting CoM and the ending CoM (ending= out of fuel) are as close together as possible. This will make sure your craft will fly pretty much the same no matter how much fuel is in it. This will eliminate the need of pumping fuel around, which is not easy to do when you are flying.. especially for the more complex crafts.

Continuing.. try to put your Center of Lift just in front of your CoM, and try and keep it there as you run out of fuel..

Also I dont recommend all those wings you used, better to keep it simple and use less but larger wings. I recommend a redesign

[Spheniscine]

In your case, the only means of pitch control is *so* close to your CoM, CoL, that they have almost no effect.

This is too close? (side view)

Next thing is that you probably want to redo your craft in such a way that the starting CoM and the ending CoM (ending= out of fuel) are as close together as possible. This will make sure your craft will fly pretty much the same no matter how much fuel is in it. This will eliminate the need of pumping fuel around, which is not easy to do when you are flying.. especially for the more complex crafts.

How do I do that? By adding ballast to the front? (by the way I wish there was proper ballast in this game)

Also I dont recommend all those wings you used, better to keep it simple and use less but larger wings.

There aren't that many options for wings in the tech level I'm aiming for.

Edited October 19, 2014 by Spheniscine

[Tex_NL]

Continuing.. try to put your Center of Lift just in front of your CoM, and try and keep it there as you run out of fuel..

You mean keep the CoL just BEHIND the CoM!

CoL in fron of CoM is what's causing his problems in the first place.

[Spheniscine]

Also, fuel lines didn't seem to work; it still eats the front ones first... (yes, I made sure the direction is right)

Looks like I have two options available to me... either pump manually, or use a mod to do so. I guess that's fine, since it only happens when fuel is running low anyway, and you may note that this plane isn't designed to land in one piece .

Edited October 19, 2014 by Spheniscine

[Red Iron Crown]

Another option is to extend the central girder system further aft and mount your pitch and yaw control surfaces back there. That will shift the CoL back without moving the wings, and give better control authority too.

[capi3101]

I recommend getting a mod called RCS balancer (or something).

You're thinking of RCS Build Aid, a nifty little mod that, among other things, shows you your "dry center of mass" (i.e. where the CoM will be when your tanks are dry).

As far as ballasting is concerned, you might consider sticking a Roundified RCS tanks up by the nose. The only tricky bit there is making sure that you've got sufficient thrust and lift with the new design. Might help a bit. The other thing I might suggest (assuming you have Advanced Flight Control, also on Tier 4 with Aerodynamics) is swapping out the Stayputnik with an OKTO probe core. It'll add an extra half-tonne forward. Other than that there's not much I can suggest; I haven't attempted to build a low-tech craft my own self. Probably oughta give it a go...

[UmbralRaptor]

Given the craft's limited mass, I would be tempted to go with a single engine on the centerline (fed by fuel lines if needed) instead of 2. This would shift the CoM forward a bit.

[LethalDose]

I feel like the term "back heavy aircraft" is really relative. When designing aircraft, I build the fuselage first, then place the wings/lifting surfaces as needed to balance the CoM and CoL when the tanks are full and empty. Then I place the canards and elevators as needed for pitch authority and to better balance CoL.

Advice more directly applicable to this situation: Try locking the fuel flow in tanks as needed. This is done by r-clicking the tank and then clicking the green arrow to change it to a red no symbol. This can also be done in the editor as well with tweakables. You can also use tweakables to empty tanks before flight and simply use them as structural parts.

Overall, though, small planes with multiple tanks will always be problematic because of fuel flow rules. A better solution for small drones may be to use a single tank at the CoM, and build the plane around the tank so the CoL is also on the CoM. In this configuration, fuel drainage won't affect CoM or where it lies in relation to the CoL.

[herbal space program]

Just run a fuel line from the back tank to the front and you should be A-O good.

[Blind Dead McJones]

When I have that problem and shifting the wings back doesn't work, I will usually try to move around the other parts of the plane to compensate. Move your landing gear, batteries, etc. It might be a big difference or it might not, depending on the size of your plane.

[KBluey]

Ideally, mount your fuel tanks so every fuel tank's center of mass is level with the plane's COM. That way your plane will always be perfectly balanced through its entire flight. This may mean mounting some fuel tanks alongside each other rather than inline.

[bakanando]

The strucutural fuselage is on the same technology level as the jet fuel, try using it to extend the fuselage one section behind so the center of mass shifts forward. This will have the problem that the mass will shift a lot towards the back as fuel is used. Try adding some dead weight to the front, a RCS tank, Science Jr., Mystery goo, etc. to shift it to the center further.

[The_Rocketeer]

Alternatively, try building your UAV with only one in-line jet engine, as this will more evenly distribute the mass and make locating your wings a little easier. It probably won't solve the problem, but it will make it easier to work around.

[Captain Vlad]

Another option is to extend the central girder system further aft and mount your pitch and yaw control surfaces back there. That will shift the CoL back without moving the wings, and give better control authority too.

This would actually be my suggestion as well, if you're married to the two-engine set up. If you're not, a single engine on something that size is more than enough.

Have you just turned off the fuel feed from the front tanks so it'll use the rear fuel first?