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Milesy

Using flaps to prevent spaceplane flipping

Question

Whenever I design a space plane I almost always find that it'll want to pitch down when full of fuel and pitch up on re-entry and almost empty of fuel. So far my solution has been either have the wings way back which makes the pitch down even worse and take-off's a pain or have too much fuel so that I can move it all forward on landing which is wasteful (and uncontrollable if very low on fuel).  So I've been playing around with a third option, have some flaps which pushes the nose down on reentry/landing (so more like a trim tab).  I've had inconclusive results so far, it does seem to push the nose down but often not enough to stop the plane flipping upwards.  Am I onto something here and just need more/bigger/better positioned "flaps" or is it a wasted effort?

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Do you put canard wings at the front ends of your spaceplanes?

 

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Posted (edited)
3 hours ago, Milesy said:

I've had inconclusive results so far

First place to start is in the SPH with the CoM (Center of Mass) and CoL (Center of Lift) indicators switched on.  Normally, the CoL should be set back from the CoM.  When the CoL is too far back, you won't be able to hold the nose up.  Inside that limit, distance from the CoM gives stability and proximity gives maneuverability.  Too close or in front of the CoM and you approach uncontrollable instability.

Then empty the tanks and see where CoM & CoL move in relation to each other.  Moving fuel tanks, engines and wing surfaces in relation to each other all enable you to tune this equation.  Some people use the Correct CoL mod to assist with this process in the SPH.

(For very heavy cargo transporters, CoM/CoL on the runway at take-off can somewhat be sacrificed to just get airborne and en route, with maneuverability being more important nearing the destination with emptier tanks.  Not very applicable to spaceplanes, perhaps.)

As a not very attractive resort, pumping fuel in flight (watching trim) is an option but bear in mind it is not an option  while you have Full Fuel or Bingo Fuel.

If you're still curious about the effect of your flaps, I think if you extend/retract them in the SPH, you should be able to see how much difference they really make to your CoM/CoL relationship...

Others will have good advice, but you may finally have to post some detailed screenshots of your craft to get more insight from the experts.

Edited by Hotel26

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3 hours ago, Milesy said:

take-off's a pain

Also check that your main landing gear are right behind your CoM.  This will cause them to act as a fulcrum and your plane will pivot up on the wheels, giving you the angle of attack you need for take off.  Be careful though, as the nose comes up, the tail swings down, and you can tail strike, which leads to a bad day quite often.

If you have the wheels farther back behind the CoM, then as you pull up, you have to have enough lift to actually lift the CoM off the ground, as it will be trying to push the wheels down into the runway, instead of using them as a fulcrum. 

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Posted (edited)
4 hours ago, bewing said:

Do you put canard wings at the front ends of your spaceplanes?

 

Funny you mention that I'd just tried putting a canard at the front of my plane to only act as a push nose down flap, I was putting the flap next to my roll aileron at the back of the wing before, plane still flipped on re-entry however my CoL was only slightly behind CoM at takeoff  (was trying to get the best of both worlds)

3 hours ago, Hotel26 said:

If you're still curious about the effect of your flaps, I think if you extend/retract them in the SPH, you should be able to see how much difference they really make to your CoM/CoL relationship...

Didn't know you could do that, would certainly save time test flying and give a rough idea how well it'll work.

2 hours ago, Gargamel said:

Also check that your main landing gear are right behind your CoM.  This will cause them to act as a fulcrum and your plane will pivot up on the wheels, giving you the angle of attack you need for take off.  Be careful though, as the nose comes up, the tail swings down, and you can tail strike, which leads to a bad day quite often.

Scott Manley taught me that one before I ever built my first spaceplane :). The difficulty is more to do with the amount off pitch up SAS has to deal with with the CoL way behind the CoM (wings way back solution), so every time I touch the controls it pitches down a lot and  pitching up is very sluggish.

Edited by Milesy

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3 hours ago, Hotel26 said:

As a not very attractive resort, pumping fuel in flight (watching trim) is an option but bear in mind it is not an option  while you have Full Fuel or Bingo Fuel.

That was my main way of dealing with CoM/CoL imbalance on reentry until I came in with bingo fuel one day only to find out the plane was uncontrollable.  This is what gave me the idea of trying flaps, like I said haven't quite got it to work yet otherwise I'll just have to push the wings back for when the planes on fumes, suppose if the plane can pitch up slightly on takeoff even with CoL way behind CoM then it's a success.

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Well after a bit of testing I've decided using flaps to counteract weight imbalance on re-entry doesn't work,  not surprising really otherwise it'd be well know trick.  Problem as far as I can tell is that although it helps in level flight as soon the plane's pitched up it's useless and the plane's gonna flip. Never mind pushing the wings back a bit more to compensate for low fuel is fine anyway.

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Try  the RCS Build Aid Continued mod. It provides an extra indicator for dry CoM. Very useful to figure out where the CoM and CoL are going to be when low on fuel during re-entry. 

The other thing is how you design your craft's fuel tank layout. If you arrange your craft to have equal volume of tanks in front of and behind the CoM then it won't shift when dry. 

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On 5/25/2019 at 10:26 AM, Foxster said:

Try  the RCS Build Aid Continued mod. It provides an extra indicator for dry CoM. Very useful to figure out where the CoM and CoL are going to be when low on fuel during re-entry. 

The other thing is how you design your craft's fuel tank layout. If you arrange your craft to have equal volume of tanks in front of and behind the CoM then it won't shift when dry. 

Thanks for pointing out the RCS Build Aid Continued mod, sounds useful, especially as I'm often too lazy to empty all the tanks to find out the dry CoM/CoL.

Never had huge success with arranging parts so it doesn't shift when dry.  Having said that my latest self refueling plane wasn't  that bad, putting heavy mining stuff at the front and middle meant it only needed the wings pushed back a touch for when dry.

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When it comes to spaceplanes, you'll want to design around the craft's centres of lift and mass rather than trying to compensate with aerodynamic control surfaces, which just won't work in the thin upper atmosphere or add too much wing mass for an efficient design.

When building what I do to save headaches is have it so the centre of mass moves as little as possible as the fuel drains, meaning tank placement is always close to the CoM or balanced evenly around it, with fuel priorities set accordingly so uneven drain doesn't upset the arrangement. I also always empty every tank I place and only fill the craft with fuel before I go to test it, again working around the dry CoM and adjusting the wet CoM to match. This means I only have to worry about fiddling with wing placement to adjust the centre of lift rather than requiring major redesigns of the entire structure if it doesn't work as planned.

When the CoL is far behind the CoM you get that "lawn dart" quality where the plane is very stable but only in a nosedive. Since spaceplane re-entry often involves some very wild pitching and rolling out of the airstream to alter trajectory, it's best to aim for manoeuvrability, which means having the CoL much closer to (but still behind) the CoM. Since this is generally the mission phase with little or no fuel left in the plane, you can see where designing around the dry CoM comes in handy here.

So if your CoL is far behind your CoM and you don't want to re-do the whole thing from scratch, you can add leading canards to bring the CoL forwards and empty some noseward fuel tanks to move the mass rearward. Just a guess without seeing your plane but works for most use cases. The lawn dart quality of planes with a wide M/L gap is also what makes them hard to take off, not necessarily landing gear placement (though that helps), since the thick atmosphere at ground level is forcing them to nose down harder as they accelerate.

I easily spend more time on any of my aircraft tweaking the centre of lift than I do on any other aspect of the design, and when it clicks you'll find them taking off, handling and landing buttery smooth every time. Mastery of this part of craft design definitely pays off, and informational tools like RCS build aid's DCoM indicator are a good way to learn some of these quirks, though not strictly necessary.

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On 5/23/2019 at 5:16 AM, Milesy said:

Whenever I design a space plane I almost always find that it'll want to pitch down when full of fuel and pitch up on re-entry and almost empty of fuel. So far my solution has been either have the wings way back which makes the pitch down even worse and take-off's a pain or have too much fuel so that I can move it all forward on landing which is wasteful (and uncontrollable if very low on fuel).  So I've been playing around with a third option, have some flaps which pushes the nose down on reentry/landing (so more like a trim tab).  I've had inconclusive results so far, it does seem to push the nose down but often not enough to stop the plane flipping upwards.  Am I onto something here and just need more/bigger/better positioned "flaps" or is it a wasted effort?

I don't think the game actually models a flap.The flap is more like an extra control surface. I find not using "flap" actually helps taking off. I use both RCS steering jet and control surface. RCS is good for early phase of reentry. When your altitude is low enough, control surface takes over.

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I have been thinking of adding canards at the front but rotating them down 15 degrees to give more control authority when I enter at 35 degrees pitch.  Would want to trim them level or even a bit nose up before takeoff.

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3 hours ago, Nich said:

I have been thinking of adding canards at the front but rotating them down 15 degrees to give more control authority when I enter at 35 degrees pitch.  Would want to trim them level or even a bit nose up before takeoff.

Interesting idea, I never tried that.  How would you trim the canards on takeoff? by having them deployed?  If so it sounds similar to what I was doing only in reverse so I'd be interested to hear if  you have any success with it.

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Could you share your craft file? I’ve spent many nights perfectly balancing spaceplanes (CoL _just_ behind CoM at all times) and I can re-enter at whatever angle (45°, 90°, etc) and she’ll be fine. 

Though I do fly with the Atmosphere Autopilot mod for the solid FlyByWire. 

Maybe there are some changes you can make to improve handling. 

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One thing that works miracle for nose refusing to pitch up during take off is to modify the angle of attack by rotating the wings a bit. Hit C in the space plane hangar to have fine grained control and rotate the wings very slightly so that the fore side is higher than the aft side.

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On 5/29/2019 at 11:09 AM, Jognt said:

Could you share your craft file?

I don't have anywhere on the internet to save it to.  To be honest though my last mining plane wasn't that bad at all.  And my worst plane for CoM shifting  is an unmanned probe launcher, so doubt much could be done about it as it's practically all fuel tanks with heavy engines at the back.

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5 minutes ago, Milesy said:

I don't have anywhere on the internet to save it to.  To be honest though my last mining plane wasn't that bad at all.  And my worst plane for CoM shifting  is an unmanned probe launcher, so doubt much could be done about it as it's practically all fuel tanks with heavy engines at the back.

If most of the craft is fuel (and the cargo is at CoM) it’s actually easier to balance it. You can pick one tank and empty it out (completely or partially) so the CoM is near dry-CoM

I doubt it’s all LFO, and most LF tanks carry more than you need. Try playing with the placement/filled capacity of LF/O tanks for future planes :).

Remember, the more fuel you bring for “might be handy” cases, the more fuel you have to move even though it’s not helpful. 

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On 5/29/2019 at 12:00 PM, kermand said:

One thing that works miracle for nose refusing to pitch up during take off is to modify the angle of attack by rotating the wings a bit

Would just changing the angle of the 2 front wings be OK.  Before when I've done that I've tried changing all the wings and it was a right pain to get them lined up correctly.

3 minutes ago, Jognt said:

If most of the craft is fuel (and the cargo is at CoM) it’s actually easier to balance it. You can pick one tank and empty it out (completely or partially) so the CoM is near dry-CoM

Thanks for the tip, I'll definitely be trying that out on my probe launcher then.

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Posted (edited)
1 hour ago, Milesy said:

Would just changing the angle of the 2 front wings be OK.  Before when I've done that I've tried changing all the wings and it was a right pain to get them lined up correctly.

I never tried. It might work, or it might increase your drag so much that your plane will turn into a flying brick.

Fine tuning wings is a PITA, no doubt. My solution is to have all my wing parts attached through a common ancestor, say, the foremost wing part, so that I can manipulate them at once. You'll have to use autostrut though, to solidify the rest.

Don't let this deter you from tuning your angle of attack. I know of no other parameter that can so dramatically improve the performances of a space plane.

Edited by kermand

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On 5/28/2019 at 8:02 PM, Milesy said:

Interesting idea, I never tried that.  How would you trim the canards on takeoff? by having them deployed?  If so it sounds similar to what I was doing only in reverse so I'd be interested to hear if  you have any success with it.

Hold ALT while doing wasd will make a permanent adjustment.  So just hold ALT-S until canards look level.  If you have rear pitching surfaces you may not want to do all the way to level.  Forgot how to clear it.

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26 minutes ago, Nich said:

Hold ALT while doing wasd will make a permanent adjustment.  So just hold ALT-S until canards look level.  If you have rear pitching surfaces you may not want to do all the way to level.  Forgot how to clear it.

Alt-X will clear trim.

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I find the mod RCS Build Aid invaluable for spaceplanes.  With that mod, you can build the thing so the CoM doesn't move at all with fuel burn, which means the distance between CoM and CoL never changes, so the thing flies the same full or empty (other than the difference in wing loading and TWR).  Also, if the CoM doesn't move, you can perfectly balance your RCS thrusters should you want to dock the thing in orbit.

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