Trouble with plane taking off.

[PTNLemay]

Before people direct me to that one super guide, I have found it and read it:

It was very helpful, but I get the feeling that my design has met all of the requirements, and yet... my plane is absolutely struggling to get off of the runway.  Am I missing something obvious?  I put nose canards to help bring the center of lift forward, I added empty fuel tanks and pushed the existing fuel way back, I even adjusted the wings angle of attack so that by default they will want to pull the plane up.  Even when running off the runway and getting that bit of air which usually helps the plane take off, it just steadily drifts back down and smashes into the ground at the beach.  No aerodynamic mods like ferram, just Kscale2 and some mechjeb.

So... yeah, is there some obvious mistake I'm not seeing?

 

Edited May 23, 2016 by PTNLemay

[Streetwind]

Well, ask yourself this: what do you have onboard this plane that actually makes it pitch up? Because as the CoL/CoM indicates, this is a nose-heavy design that naturally wants to pitch down. Slightly nose-heavy is good for stability, but you need control authority to balance it out.

And in your plane, it looks like you have only the reaction wheels in your cockpit for that job, and they're not enough.

The only two control surfaces you have are small, and almost directly in line with the point around which the plane pivots. This means they have almost no lever arm - and therefore, pretty much no effect on your pitch at all. They're ailerons, only good for roll control.

What you want are elevators. They should be placed far away from the point around which the plane pivots, so they have a long lever arm. Two locations are typical: on the tail, or on the nose (canards). Your plane has neither, because what you mounted as "canards" are not control surfaces, but rather static fins. They don't steer.

 

Edited May 23, 2016 by Streetwind

[FullMetalMachinist]

I would say that you don't have enough pitch authority. 

Either change your basic fin canards up front for one of the controllable kind, or add some control surfaces way at the back for elevators, or both. I like to use the same type that you're already using for your rudder, so just add two more in mirror symmetry at the back. 

That will probably move your CoL further back than you'd like, so maybe move the wings up a bit. And set the control surfaces on the wings to work only for roll, canards and elevators only for pitch, and rudder only for yaw. 

Also, your rear landing gear position makes a big difference. Try to get it just barely behind the CoM. 

You could also add some angle of incidence to the wings. That will give them some lift while the rest of the plane is flat on the ground. 

[PTNLemay]

Well, at my current tech level the only other control surface I have is the elevon 1 http://wiki.kerbalspaceprogram.com/wiki/Elevon_1
But I'm not sure how to snap it onto those angled wings.  I can rotate it, but then won't it be turning in the wrong direction?

Also, it says the deflection range of that part is 0.20, that's practically the same value as the delta-deluxe that I'm using.  Does the lift get influenced more by total surface area than by the numbers given in the specs?

[FullMetalMachinist]

You don't need to add any control surfaces to the main wings, those delta deluxe ones at the end should be enough for roll control. But that's not your problem, and adding more to the wings won't help. 

I would try moving the whole main wing assembly forward a little, and then add two more of the RV-8 winglets (the one you already have as your rudder). Put them horizontal in mirror symmetry mode at the back, basically at the same position as the rudder. 

[bewing]

54 minutes ago, PTNLemay said:

Well, at my current tech level the only other control surface I have is the elevon 1 http://wiki.kerbalspaceprogram.com/wiki/Elevon_1
 

The tailfins are excellent for horizontal control surfaces too. Don't ignore them.

[Geschosskopf]

19 minutes ago, FullMetalMachinist said:

I would try moving the whole main wing assembly forward a little, and then add two more of the RV-8 winglets (the one you already have as your rudder). Put them horizontal in mirror symmetry mode at the back, basically at the same position as the rudder. 

I agree.  It looks to me like the CoL is too far behind the CoM.  With that much space between them, the plane will very likely have a natural tendency to nose down.  I recommend moving the wings forward until the CoL mostly overlaps the CoM, but is still a tad behind it.

HOWEVER, this will also move any control surfaces mounted on the wings.  That's OK for ailerons but might put the elevators too far forward to work well.  So then you might have to add some canards on the back to act as elevators, which will again make the CoL go backwards, which means you'll need canards on the nose to keep the CoL in the right place relative to the CoM.

Edited May 23, 2016 by Geschosskopf

[PTNLemay]

Hmm, I guess I'll have to give up the delta-plane look.  Thanks for all the help, I'll give these suggestions a try.

I swear, I've logged over 400 hours in KSP but it feels like the more I play the worse I become with planes.  With rockets it feels like you've got a huge margin of error, so long as the TWR is higher than 1.1, and so long as you're roughly pointing up... you'll be fine.  With planes it feels like the margin of error is a tiny window 3 inches across, and if you're not spot on you're not going anywhere today.  lol

EDIT:
The lead raven flies!  With extra engines to get that higher atmosphere lift.  Thanks again for the tips guys.  It's embarrassing to ask for help with such basic aspects of the game, but humans really are the best help.

 

Edited May 24, 2016 by PTNLemay

[pincushionman]

12 hours ago, PTNLemay said:

Hmm, I guess I'll have to give up the delta-plane look.  Thanks for all the help, I'll give these suggestions a try.

I swear, I've logged over 400 hours in KSP but it feels like the more I play the worse I become with planes.  With rockets it feels like you've got a huge margin of error, so long as the TWR is higher than 1.1, and so long as you're roughly pointing up... you'll be fine.  With planes it feels like the margin of error is a tiny window 3 inches across, and if you're not spot on you're not going anywhere today.  lol

EDIT:
The lead raven flies!  With extra engines to get that higher atmosphere lift.  Thanks again for the tips guys.  It's embarrassing to ask for help with such basic aspects of the game, but humans really are the best help.

 

Don't sweat it. Planes are legitimately FAR more complicated beasties than are rockets, which are not much more than a tube that has BOOM coming out the back end.

Glad you got the thing to fly, but…threeengines in the back is a lot of weight, so your CoM is gonna be waaaay back, so your elevator authority isn't as good as it could be. And drain your tanks in the SPH to see if your CoM will shift behind your CoL furing flight, which is all kinds of bad.

Also, what happened to your ailerons? Is your tailplane providing both pitch and roll? As long as it works, cool, but a bit unconventional. I just ask because your new design seems radically different from your old one, which really was almost very good.