Why does my space plane use every inch of runway?

[Brainlord Mesomorph]

When I design a small jet, it can take off from the middle of the runway at about 80 m/s.

As soon as I try to design anything bigger, a space plane, especially one designed to carry payload, I can get up to 130 m/s and no matter how much I pull back on the stick (press “sâ€Â) it goes straight down the runway and off the end.

Good thing it’s downhill from there. That’s the only thing that puts me in the air.

Sometimes, I even bounce off the lawn before I get the gear up. If there was a fence around this spaceport, I’d hit it. But once I’m airborne, everything’s fine.

Is it not enough lift?

TWR?

Edited December 10, 2014 by Brainlord Mesomorph

[capi3101]

Nope - it's your back gear placement.

Imagine your aircraft is a big lever. Your wheels are the focal point, your control surfaces are providing the force and your plane's mass is the load you're trying to move. Now, like most levers, the closer the fulcrum is to the load, the less force that needs to be provided to lift that load. Conversely, the further away the fulcrum is from the load, the more effort you're going to need to make the load move. Your plane works in the same way - the closer the wheels are to the center of mass, the easier it will be for your control surfaces to lift up the nose, and then it's simply a matter of providing enough thrust for the plane to get off the ground.

You have two options here:

1) Move the rear gear forward (or turn them around) such that the wheels are positioned under the center of mass and slightly behind. This is the easiest option, but if you get them too far forward your plane will have a tendency to nose up while it's standing still with the end result being a tail strike. Gear placement is tricky business - too far forward and it tilts, too far back and it won't take off. I've occasionally combated this phenomenon with a "kicker" gear whose sole purpose is to hold the tail end of the plane up until the plane has picked up enough speed that it's no longer necessary. Those designs generally don't work too well as a rule though.

2) Add more pitching control surfaces - dedicated elevators and canards. This works, though you have to be careful about what their addition will do to your center of lift - you don't want it to shift ahead of the center of mass during flight unless you're in to unplanned aerobatics.

Hopefully that's helpful.

Edited December 10, 2014 by capi3101

[LethalDose]

Yeah, what capi's saying. Move your gear closer to your CoM so you need less pitch to lift the nose for take-off. It's a common problem in aircraft design.

[MalfunctionM1Ke]

Read this http://forum.kerbalspaceprogram.com/threads/52080-Basic-Aircraft-Design-Explained-Simply-With-Pictures

[Cranium]

Try putting your rear landing gear closer to your CoM. That way, the elevators on your plane can push down on the tail, which will pull the nose up. Otherwise, you can put canards on the front of your plane to pull the nose up.

[Tex_NL]

The gear suggestions are most likely correct. But without SPH pics (CoM, CoL and landing gear visible) nothing can be said for sure.

[Brainlord Mesomorph]

Oh...

But I put them back there because if I don't my engines it the runway as I take off.

.... because I need more clearance ... Hmmm

Back to the SPH

Thanks guys!

Edited December 10, 2014 by Brainlord Mesomorph

[Tex_NL]

To avoid a tail strike you should raise your ground clearance. Either by using a raised tail or a longer landing gear.

[phunk]

To avoid a tail strike you should raise your ground clearance. Either by using a raised tail or a longer landing gear.

Or take off very slowly.

[capi3101]

Or - and this is just a thought - go ahead and let the plane use the whole runway to take off. It does pretty effectively end the risk of tail strike - so long as the plane doesn't fly right into the drink (which is often how flights in my own early spaceplane program usually wound up; it's amazing to me that I use the things for as much as I do these days).

You might try that "kicker" design I mentioned in the edited post if it's really giving you fits. Use an action group to toggle that single wheel, and then on take-off raise it when your craft gets above 20 m/s or so. For landing, you'll want to lower your gear, use the action group to raise the kicker, land as normal and then lower the kicker once you're down. It does work, but it's definitely not an elegant solution by any means.

The rear gear placement problem is one I still face an awful lot; definitely been there, done that my own self.

Just to cover all the bases here - can you show us a pic of the craft/share the craft file you're having trouble with? If you can, we can give you better diagnostic information. In general:

-15 tonnes maximum per turbojet, 13 per RAPIER and/or 10 per Basic Jet.

-At least .03 intake area per engine (i.e. no less than three Ram Intakes or the equivalent)

-combined sum of lift coefficients equal to the plane's mass (for example, an eight tonne plane should have a total lift coefficient of roughly 8, the amount provided by two Delta Wings and two Wing Connector A/Bs).

Edited December 10, 2014 by capi3101

[James_Eh]

Or - and this is just a thought - go ahead and let the plane use the whole runway to take off. It does pretty effectively end the risk of tail strike - so long as the plane doesn't fly right into the drink (which is often how flights in my own early spaceplane program usually wound up; it's amazing to me that I use the things for as much as I do these days).

Couldn't agree more. That's why the runway is as long as it is. The runway length (and the dropoff immediately thereafter) are gifts from Squad. Use 'em. Just beware of those stupid wing- and wheel-wrecking thingys at the end of the runway. Hate those jerks.

[Empiro]

Adding more control surfaces helps. I play with FAR, and I've found that having a all-moving tailplane at the back can often provide the extra leverage to take off.

[Brainlord Mesomorph]

Read this http://forum.kerbalspaceprogram.com/threads/52080-Basic-Aircraft-Design-Explained-Simply-With-Pictures

Just read that.

At the start it seems too simple w/ CoG and CoT and CoL, (duh) but then later there is a lot of good stuff about wing angles and wheel placement!

And (predictably) when I build something that looks like a real airplane (and less like the triangular space fighters I drew as a child) it handles more like an airplane (and less like a lawn dart with a bottle rocket up its *#$%.).

Thanks!

[Scarecrow88]

Another option that I don't think has been mentioned yet is to mount the nose gear slightly lower down, if possible. This has the effect of raising the nose slightly which will help get your craft off the ground if you can get the leading edge of the wing higher than the trailing edge.

[GoSlash27]

^ What he said.

In addition to reducing the leverage needed to get the nose to rotate and increasing the force you have available to get the plane to rotate, don't forget to have the nose higher so that the wings aren't planting your aircraft into the runway.

Best,

-Slashy

[Alshain]

You can also angle your wings up toward the front to give them a natural AOI. If you do that the plane will lift itself off the runway.

[Duribun]

Another option I would suggest, if you don't wish to go to the end of the run way, press Cap lock to turn on fine controls for the keyboard, then as you raise your nose to get airborne, just keep the plane balanced until you're in the air, provided you have enough speed going forward, even a 2-3 degrees of pitch will let you take off, an then once you're clear of the runway where you can pull up more sharply to actually gain some altitude, you'll be fine.

The best thing to remember about take off is, you don't need to go straight into your accent climb right from the word go, sometimes just let your plane actually get it's wheels off the ground and away from the runway and THEN start your accent climb.

[Wanderfound]

You can also angle your wings up toward the front to give them a natural AOI. If you do that the plane will lift itself off the runway.

...but be aware that it will complicate landing if you overdo it. Planes that self launch during liftoff are nice, but planes that self launch during landing are a nuisance. It's still possible​ to land, but it's a bit trickier.

[Alshain]

...but be aware that it will complicate landing if you overdo it. Planes that self launch during liftoff are nice, but planes that self launch during landing are a nuisance. It's still possible​ to land, but it's a bit trickier.

I guess it depends on your landing profile. On all but one of my planes I take a long steady approach following the glide slope (using NavUtilities). I usually find the slope as soon as I pass the highest mountain peak. Usually angled wings will help there. If on the other hand you want to take off, do a vertical loop and set straight back down it could be a problem.

Also, in FAR, spoilers will keep you on the ground, but on approach, angled wings may make flaps unnecessary.

In short, you can screw it up, but it can work too if you take care in your design and purpose.