I messed up my good plane - help me figure this out?

[JoeSchmuckatelli]

I built a mid-level 'good' plane that flew easy, and frankly was fun.  Any Kebal could take off, fly a mission and land it.  It could weave the spires of the mountains west of the Spaceport at any speed.

… So of course I messed with it, trying to 'improve' it.  And now it's a brick.

Sadly I can't post a pic b/c I don't have any of the file sharing accounts allowed by the forums.

The original plane was a supersonic capable Mk2 based plane with 2 Panthers and 4 radial intakes.  Its a very traditional design, swept wing with separate tail and two rudders (not a flying wing or delta - it looks reminiscent of an F-15).  The only thing I did not like about it was the slow turn speed (note not rotate speed, that works great - its the turning by using the w to lift the nose part - i.e the ailerons seem good, the rudder seems good - its an elevator problem .

 So I tried changing out the Elevon 1s on the tail for Elevon 3s, figuring the greater surface area would give me more turn ability.  I also pushed the wings forward to get the COL within, but still behind the COM, figuring that would help it.  Everything works great - as long as the nose isn't pointed to the ground... When it is, the 'getting the nose up' part is very difficult now.  FWIW - holding the 'w' shows the wing ailerons/elevators going down, while the tail elevators go up.

-- Original design had the COL just kissing the back of the COM.  COT is balanced on either side of and inline with the COM.  It handles flat flying great.  It can turn.  It hates lifting the nose up.

 

I've gone over all the tutorials again, and I can't quite figure this out; the plane should fly better than it did.

It doesn't.

 

Suggestions?

[bewing]

Put canards on the nose.

And imgur.com does not require any login or account in order to host pix.

 

[Spricigo]

Sorry to be blunt, but without a picture we hardly have any chance to even start to figure out what may be wrong with your plane.

And as @bewing said 1s ago(!!), no account required to post on imgur

[AHHans]

An image would help!

But the short answer is: if your plane doesn't want to point its nose into the wind (i.e. prograde) then it is aerodynamically unstable.

The CoL display in the editor shows only the lift, not all the the aerodynamic forces. And it only shows the lift for small angles-of-attack. So with significant drag (and the MK2 parts are (in)famous for high drag at nonzero AoA) and or high AoA the position of the center-of-pressure (where the sum of the aerodynamic forces actually attack) can be at quite a different position.

And when you get to building spaceplanes: at supersonic (or even hypersonic) speeds high in the atmosphere the lift (relative to the drag) gets smaller and the drag starts dominating the CoP. So a craft that flies perfectly fine at slow speeds may suddenly flip out when you try to get to orbit.

[Edit:] P.S. A RL example of a plane that gets less stable at high AoA is the 737 MAX.

Edited May 26, 2020 by AHHans

[JoeSchmuckatelli]

Thanks guys - I'll look into Imigur later this afternoon when I get back on the computer.  In the meantime, while I like the 'canards on the nose' suggestion, are there known problems with some of the elevons? 

Oh - and @AHHans, the plane used to fly great for me - no problem keeping the nose in the wind.. If anything its getting it to come out of it that I was fighting (slow turns before became slower to non existant after fiddling) - but what was weird about it was that level flight acted different from angled flight.  Ex: I could flick A or D to get the plane to roll right or left and then pull the turn with S just fine as long as those maneuvers were in the horizontal plane.  But if I angled down, S no longer wanted to pull the nose up.-which I'm guessing means my drag was greater than the elevons could handle 

...although why this happened idk

 

Edited May 26, 2020 by JoeSchmuckatelli

[AHHans]

Sorry, I have a hard time picturing your problem.

But no, there are no known problems with any elevons. They all perform as well as can be expected.

[Vanamonde]

You have learned the bitter lesson: save your working plane design and only attempt alterations to the design on a copy. 

[JoeSchmuckatelli]

2 hours ago, Vanamonde said:

You have learned the bitter lesson: save your working plane design and only attempt alterations to the design on a copy. 

Truer words...

 

I gave up trying to tweak it.  Made an abhorrent looking plane based off of a Mk-1 cockpit that does everything one might ask for...

...except look like it should fly.