Help designing this aircraft for take off stability & altitude

[paul23]

Well I'm trying to build a simple lifter for ksp. In the end I wish it to become a ssto, or nearly that.

My aircraft:

Simple design right? I'm using 4 wheesley's to give power, a lot of wing surface to provide lifting of heavy body. And (right now) a swivel engine in the middle to do the "final" stage. This middle engine is to be updated once I've unlocked the mainsail/skipper.

all 4(! I also tried with just 2) vertical control surfaces are positioned far from center. This is done on purpose and I wish to keep that, it's so I can add a payload on top of this aircraft. (Think about the Antanov 225).

Now the problem I'm experiences first is that I can't seem to stay in a stable linear line with this aircraft. It first of all has a constant tendency to go to the left, more than sas can fix so I always end op trying to fix this. Then around 70-90m/s I experience a very slight turn and it suddenly goes out of control: quickly turning off the runway and crashing. This happens either to the left if I don't fix the turning manually, or to the right if I do fix it.

Just after the turn:

Another thing: in the few cases I did manage to lift off I notice I can't seem to go above a speed of 400 m/s, or an altitude of 8 km (well I can, but 8km seems the altitude ceiling for a level cruise flight, at about 250 m/s). Should I just put such a design out of my head until I unlock a better engine? Or is there something else I should do?

[*Asterix*]

You should probably consider less wings.

[Scarecrow88]

The Weasley engine is not useful for high altitude/high speed flight.

[paul23]

You should probably consider less wings.

I wish to lift heavy bodies in the end (1-2 tonnes payload). So a lot of lift is required.

Well I managed to fix the unstability by putting the nosewheel slightly "higher", which meant it points down and forces itself on the ground until I actually pull up the nose.

@scarecrow: are you simply stating that until I get two tiers further in the tree I shouldn't do any airbreathing engines (all contracts are now in space, I never receive a check a place on kerbin anymore)?

[mabarry3]

That engine won't get you reliably into the upper atmosphere (it's incredibly hard to get 15k+ if I remember). It doesn't have enough thrust (the Whiplash has something like 3x the thrust) and you get better aerodynamic surfaces and intakes by the time you get the Whiplash anyway. Before I unlocked the Whiplash, I just took a tiny rocket and (very carefully) aimed it for the target location, attempting to read it on the way up. You don't even really need a recovery option for it since it'd be unmanned, and could just aim and fire and retry til you get it.

So, I would agree with scarecrow that you should wait another 2 tiers to do more airbreathing engines if you're trying to get that high. A small unmanned rocket will do the trick usually.

[paul23]

That engine won't get you reliably into the upper atmosphere (it's incredibly hard to get 15k+ if I remember). It doesn't have enough thrust (the Whiplash has something like 3x the thrust) and you get better aerodynamic surfaces and intakes by the time you get the Whiplash anyway. Before I unlocked the Whiplash, I just took a tiny rocket and (very carefully) aimed it for the target location, attempting to read it on the way up. You don't even really need a recovery option for it since it'd be unmanned, and could just aim and fire and retry til you get it.

So, I would agree with scarecrow that you should wait another 2 tiers to do more airbreathing engines if you're trying to get that high. A small unmanned rocket will do the trick usually.

Well I just installed KCT - so reusability is a pre nowadays ;P

[CrashTestDanny]

When ground tracking is an issue, the problem is [almost] always landing gear placement. Been there, got the T-shirt, as they say... I wish I had learned this a lot sooner than I did, but in 1989, I spent several months building a tractor canard, of all things for my senior project. The thing that proved fatal for it was lack of design work on the landing gear. Since I can't see your LG in the pics, I'll have to make a couple assumptions, but they're probably pretty spot on... The yellow arrows on your rudders show the torque that they are applying to try to correct the unwanted yaw, which looks correct for an aircraft in that situation. Your vertical surfaces will not affect this as long as you have enough and they are not rotated about the yaw axis.

1 - I'm guessing your nose gear is way far forward of the CG, expecting that to improve your command authority over the unwanted torque. Move it back. I would put it just forward of the aft end of the forward FL-T800 tank. If it's already there, then leave it - the mains are the real problem.

2 - oh wait, I do see your mains now. I was going to guess they were just inside the outside pair of engines at nearly the same station as CL. You would think (as I did in 1989) that this would give you lots of stability, and it's fine for sitting on the runway. However, when moving, any unbalanced force hitting one of those wheels produces a lot more torque about your yaw axis because of the distance. You have to find a balance. I would have the wheels hitting the ground no further out than the outside edge of the inner pair of engines. Should still be plenty stable on the ground.

Next thing you need to do is check the limits of your CG. In the SPH, use the right-click menu to drop the fuel and ox level down to 0 (hopefully you already have ox at 0 in all tanks, since you are only running air breathing engines. Then slowly drain the next tank back until the CG stops going back and begins coming forward again. If you get to 0 before the CG stops moving, then continue to remove all fuel forward of the CG until the CG stops moving aft. This will show you the aft limit of the CG. If it is aft of the CL, then you need to move some wings aft or the aircraft will flip at that point in flight.

After you've done that, you can worry about the engine. Someone already mentioned the Wheesley not being the greatest jet around, but you should check it's right-click menu during flight. It will show you how much thrust it is producing and when it flames out and why. I think you might improve your altitude with more intakes, but I would be surprised if you got much increase on the speed unless you switch to ramjets or rapiers.

Danny

[UnusualAttitude]

Well I just installed KCT - so reusability is a pre nowadays ;P

Building a lo-tech spaceplane with a useful payload around just Wheesleys is probably not possible. However, if reusability is essential, it is quite possible to build a suborbital craft that takes off horizontally, flies up to about 10k on the jets and then boosts its apoapsis out of the atmosphere on rockets. Separate your payload, switch to it and circularize, then switch back to the launcher to re-enter and fly back to KSC for full recovery.

This would be more trouble than it's worth in a normal career game. But if full reusability is a requirement and you don't want to wait for the Whiplash, you might consider it. I even built a launcher like this using Mk3 parts (just for the hell of it).

[paul23]

@crashtestdummy: having wider from cog wheels does indeed increase a yawing moment if only one wheel touches the ground. However it does help prevent a -much hard to prevent when landing- rolling moment, where one of the wing tips hits the ground.

I've already identified the problem as lifting off too soon, and thus "bouncing back" on one wheel. I need to have more pitching control to prevent this? Or is there another way to prevent a failed/bouncing lift off - and stick myself to the runway for a time/higher speed.

Putting my wheel at the inner pair of engines means they're actually quite far forward, almost exactly at the COG. - Once the tanks are empty they're even in front of the cog. Well moving the cog forwards isn't really an option, so I might have to put the engine further down....

Edited October 28, 2015 by paul23

[Generalstarwars333]

Have you tried adding more boosters? How about struts?

[CrashTestDanny]

@crashtestdummy: having wider from cog wheels does indeed increase a yawing moment if only one wheel touches the ground. However it does help prevent a -much hard to prevent when landing- rolling moment, where one of the wing tips hits the ground.

I've already identified the problem as lifting off too soon, and thus "bouncing back" on one wheel. I need to have more pitching control to prevent this? Or is there another way to prevent a failed/bouncing lift off - and stick myself to the runway for a time/higher speed.

Putting my wheel at the inner pair of engines means they're actually quite far forward, almost exactly at the COG. - Once the tanks are empty they're even in front of the cog. Well moving the cog forwards isn't really an option, so I might have to put the engine further down....

You could attach them to the fuselage - I was using the engine as a reference, not a suggested attachment point. Your rolling moment is probably not as severe an issue as you expect and your ground track is clearly being adversely affected by the wide track. But hey - more boosters will probably fix it all, right?

Danny

[paul23]

Hmm I just tried that - indeed it fixes the jawing.

But then another problem occurs: the fuselase is "higher" than the engines, now the engines actually touch the ground, even with the wheels to the very low (mid) of the fuselage this happens.

Will putting the engine actually through the wings (clipping) give an adverse affect on the physics/lift?

[CrashTestDanny]

Clipping engines causes them to not thrust in some cases. I would recommend you use the offset and rotation tools to move the mains to the correct location. You could also try using cubic octagonal struts (in structural) or other structural components to build mounts for your main gear.

Danny

- - - Updated - - -

... more boosters would have been more fun though... personally when I have trouble with a space plane, I like to stick a pair of kickbacks on its wingtips and just open her up!

Danny

[Val]

Clipping the engines into the wing will not affect aerodynamics at all, but if the engine exhaust is blocked it affects thrust or can damage parts.

In KSP, the most stable landing gear configuration is to have a pair of load bearing wheels right underneath CoM and a nosewheel as far forward as possible.

If the craft lifts the nosewheel a little before settling back down, when you load it up on the runway, then it's perfectly positioned.

The main gear can can have a wide track or narrow, but attached directly to the main fuselage gives the best yaw stability.

If you're worried about tail strikes put a tail wheel there that doesn't touch the ground until right before the tail strikes.

Another way to reduce risk of tail strikes is to attach wings with angle of incidence, which also has other benefits. More info in the spoiler here.

Wheesleys are poorly suited for SSTOs and may prove very frustrating. I haven't given up on building a Wheesley based SSTO yet, but no luck so far.

Another important design consideration for aircraft and especially SSTOs is to not have the CoM move too much between full and empty tanks. It's very frustrating to find out during re-entry that your craft is uncontrollable because CoM moved behind CoL, or that it's a lawn dart that can't pull out of a dive, especially after a long successful mission.

Edited October 29, 2015 by Val
Typos

[Generalstarwars333]

Clipping engines causes them to not thrust in some cases. I would recommend you use the offset and rotation tools to move the mains to the correct location. You could also try using cubic octagonal struts (in structural) or other structural components to build mounts for your main gear.

Danny

- - - Updated - - -

... more boosters would have been more fun though... personally when I have trouble with a space plane, I like to stick a pair of kickbacks on its wingtips and just open her up!

Danny

I once put a mk1 command pod on a kickback, with the FAT airplane wings on the front and back, with some large landing gear. i called my creation the worst idea ever. it was awesome.