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The woes of building a space-plane.


NoSuperman10
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So it seems aerodynamics is a hell of a lot harder than rocketry. My efforts of building a space rocket mostly involve power-to-weight ratios and making sure I have enough fuel. But building a jet fighter is proving to be significantly tougher. I\'m currently on the Mk6 of my current run of space-planes and I barely managed to get the Mk5 off the ground.

My troubles seem to stem from keeping the thing balanced. Mk1 through Mk4 had their engines at the back and so would spin violently end-over-end the moment the SAS was switched off. Mk5 had this problem also but I managed to alleviate it somewhat by altering the weighting and changing where the lift was concentrated.

As of currently the Mk6 has the engines placed in the middle, this has stopped the problem of weight imbalance but now I can\'t get the thing off the runway, it gets to roughly 140m/s and then spins out. I honestly don\'t know what\'s causing it so I\'m coming here to get some profssional advice. If you want I can provide screenshots of my craft so you can get a better idea of what I\'m building

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If you can position the landing gear so the plane is facing into the ground, you\'ll have a much better takeoff. My Swallow takes off at 90 m/s (probably sooner if I pulled up hard, but it would break off the tail).

edit: re-examining your pictures, I think your problem is the wings you\'re using. Personally I have never had success using those wings for anything. Additionally, you will want your wings to be near your center of mass, not at the front and back of your plane.

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I profoundly suck at planes. How do you guys do it? If it has trouble getting off the ground I add an engine, but that screws with the balance so I shift the wing, but then it runs out of gas too quickly so I add a tank, but then it yaws like an anvil, etc. After struggling with it for days, I have one simple little design that both a) gets into the air, and B) is relatively controllable once there.

Question: I make a steering change, the plane rotates a bit, but then it stops and I have to let it sort of get used to that situation for a few seconds before I can get any response out of the controls again. Does that happen to everybody or am I doing something wrong?

As for NoSuperman10\'s problem of getting airborne, I find that putting canards at the nose helps wrench the nose up for the initial translation. I\'m not sure if that\'s a good idea or not, but it seemed to help me.

Behold my patheticness:

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NoSuperman10: Having a major wing in front of most of the mass (Mk 2-6) is adding to your stability issues. Moving it closer to the center of mass should improve things. And it does look like the liftoff issues on the Mk 6 are caused by the aft landing gear being so far back.

Vanamonde: I\'m only okayish. I mainly try to stick with simpler designs, putting most of the wings at/near the center of mass, and adding control surfaces until it can be reasonably maneuvered. That looks like a pretty reasonable plane, though.

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Couple of tips. First is of course the engines. You want them so they form symetry around the centre line of the plane otherwize the trust will want to cause a rotation. Also heavy weight excessively back will cause the nose to want to raise. If you pitch up too much, it will cause a spinout. Also not enough lift can cause problems too, try more wing area based around centre of gravity.

Lastly for takeoff, if you have trouble pitching up, try moving rear landing gears a bit closer to centre of gravity, this will allow you to tip the plain instead of trying to force lift off the horizontal plane.

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Despite the official weights listed for the parts, I find that the cockpit behaves as if it\'s quite a bit heavier than anything else. Put it at the nose, for example, and the whole plane seems to try to behave like a lawn dart, no matter how the rest of the weight and lift are distributed. So I try to put it at the middle and balance the engines around it.

try moving rear landing gears a bit closer to centre of gravity
I\'m annoyed that I didn\'t think of that. Wheels far back would act as a fulcrum, with the whole weight of the plane acting over the lever of its length, mashing the nose down. But then how do you keep the tail from scraping when you translate for liftoff? That\'s why I put the wheels back there in the first place.
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Despite the official weights listed for the parts, I find that the cockpit behaves as if it\'s quite a bit heavier than anything else. Put it at the nose, for example, and the whole plane seems to try to behave like a lawn dart, no matter how the rest of the weight and lift are distributed. So I try to put it at the middle and balance the engines around it.

I\'m annoyed that I didn\'t think of that. Wheels far back would act as a fulcrum, with the whole weight of the plane acting over the lever of its length, mashing the nose down. But then how do you keep the tail from scraping when you translate for liftoff? That\'s why I put the wheels back there in the first place.

That\'s actually a good thing. Haven\'t you ever built paper planes or gliders? You often find that putting something heavy at the nose will increase performance significantly.

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I was thinking it would be a good thing for something that\'s made to descend, like the spaceshuttle. For something that needs to bootstrap itself up there first? Not so much.

I am not sure I understand you.

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I believe that Vanamonde is referring to the fact that when the center of gravity is put further forward, the plane can\'t pitch up as much. And while he\'s right, it can\'t pitch up as much with a far-forward center of gravity, it makes the plane much more stable in pitch, so long as the center of gravity isn\'t too far forward.

Vanamonde, try placing the engines on your plane below the main fuselage and the center of gravity. The way you have them now, the engine thrust makes the plane want to pitch downwards, whereas if they are below they will give it more of a pitch up tendency. As it happens, they also tend to be good places to put the main landing gear, since it should give the plane a lot of room to pitch up on the runway.

If that doesn\'t help pitch the plane upwards, you can try using shift-WASDQE keys in the SPH to angle your lifting surfaces to try and give it more of a pitch up tendency.

And as a final thing, if you still have problems with the tail crashing into the runway, put a single landing gear on it. It doesn\'t need to actually support the plane at rest, just keep the tail from exploding.

Hope this helps. ;)

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when the center of gravity is put further forward, the plane can\'t pitch up as much.
Right, which is a bit of a problem when trying to get off the ground. Sorry I wasn\'t clear.
try placing the engines on your plane below the main fuselage and the center of gravity.
I did, and then it was flying a permanent loop. Is it a matter of degree?
good places to put the main landing gear
Something like a Corsair, with the engines at the angle of the anhedral?
try using shift-WASDQE keys
I have, but it didn\'t seem to do anything. Is that a glitch? What kind of increments is it supposed to shift things?
tail crashing into the runway, put a single landing gear on it
The problem is that when I tried to use tapering tail cones, the weight of the control surfaces tended to snap them off. So my plane is a cylinder all the way, with a big butt that\'s close to hitting the ground on takeoff and landing, to keep the stupid tail attached. I suppose I could prop it up even higher, but the landing gear already seems dangerously high and rickety. Side view:

(Thanks so much for the advice, gentlemen and/or ladies!)

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Here\'s an example of a (fairly large) plane I made that works, since seeing might help:

mongoose2.jpgmongoose1.jpg

By 'permanent loop' do you mean that it just backflips when it gets into the air, or that it starts to do a loop that you can\'t pitch down out of? If it\'s the former, the plane is unstable and you need to move the wings further back. If the latter, you need more control surfaces at the front/back of the plane.

For engine placement, yeah kind of like that. The engines don\'t have to be too far below, but it helps a lot pitch wise.

What I mean by the WASDQE keys is that if you have a part selected in the SPH (guess I should have been clearer :P) before you place it you can change its orientation using those keys. If you don\'t hold shift it varies by 90 degree increments, if you do hold shift it varies orientation by 5 degree increments. Very good for innovative designs.

Based on the picture you posted, I\'d guess that the plane can\'t fly until it goes off the edge of the runway.

You have a little too much fuel up front. Makes it a lot nose-heavier than I thought it was. Move them back a little. If those vertical surfaces near the front are completely vertical, get rid of them. They make the plane more yaw unstable and more likely to spin out. It also looks like you have a little too much wing near the front. Makes me think it\'s unstable. The picture was taken with SAS on, correct? ;)

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The 'loop' I was refering to was a nice, big aerobatic show loop, which I couldn\'t entirely stop even with full down-pitch. But I just put a little anhedral on that inner wing panel like you suggested. Holy. Freaking. Cow. The thing practically leaps off the runway now. I guess I just overdid it the other time I tried moving the engines down, when I had them on hardpoints under the wings, like a jetliner. Fantastic tip! Thank you.

It\'s not as heavy as it looks, though. Only the 4 cylinders in the engines are 2.5 mass fuel tanks. The others are all the 0.4 mass 'structural fuselage' pieces. In fact, it\'s a little nose-light right now.

The canards on the wings were because that big tail was, for some reason, giving me almost no rudder, and they were the only way I could get the thing to turn. I replaced that vertical stabilizer with 2 canted canards, and it seems to be working better.

I couldn\'t figure out what everybody was talking about with the shift-placing 5 degree thing, because it never worked for me. Turns out that it no longer works if you map pitch/yaw functions to other keys. ::) I put it back and now I can tweak stuff. Cool.

Thanks so much for your help! The thing can almost pass for a working airplane now!

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those long wings in the front may cause it to fishtail in mid-air and flip forward and backward in a crazy manner
Perhaps so, but actually, that\'s one problem I haven\'t had with this one. I was trying to build something as stable as possible so it would stay in the air long enough for me to practice flying. It goes in a straight line like a dream. Get it in the air, leave the avionics SAS in charge, and mild corrections every few minutes will keep it flying until it runs out of fuel. Then once out of gas, it glides about 40% as far as it flew under power (is it even possible to stall under the current flight simulation?). Lands pretty well, too, if you don\'t care what piece of flat ground you come down on. Changing heading is the thing it struggles with. It wallows and balks and pitches a hissyfit then.
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Call me crazy, but I\'ve actually found that an inordinate amount of engines seems to help with controllability. For example, my Ulan-E4 has 10 engines on a tiny chassis, arranged in a fat rectangle with one at the nose to keep the beast from parking on its tail on the runway. As stable as I could ever hope for, and I\'ve clocked the thing going at somewhere around 964 m/s at altitude. ;D

On a related note, if you\'re ever gunning to go fast in a turbojet aircraft, keep your altitude around the second white line in the second blue section (sorry, my altitude indicator dies on ascent, that\'s all I\'ve got). Any higher and the engines suffocate, any lower... Well, your speed gets capped at ~700 m/s at the edge of the lower atmosphere, and you can start roasting weenies on your innermost engine nacelles. Try taking the E4 for a spin, you might have some fun! And you\'ll probably beat my speed record on your first flight because I\'m terrible at piloting.

Edit: Oh, and canards. I haven\'t flown a successful jet yet without the wings in the back and canards as far up as I can push them.

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Railsmith, it is probably flying properly not because of the thrust, but because of the weight. When it comes to short and stubby craft, you need to balance your wings and weight accordingly to prevent it from pitching out all the time, which in this case seems to be that you have accidentally created a perfect balance between the two.

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...seems to be that you have accidentally created a perfect balance between the two.

Yeah, judging by my ~50% success/swirly-twirly-explosion ratio, it probably IS more luck than design. The only reasons I thought it was a thrust thing were the way it goes apeshit after the fuel runs out (which is probably just because it\'s in thin air, herpaderp) and because a five-engined earlier model wasn\'t as well balanced as the E4 over there is. Hooray for lucky accidents!

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