How is this plane even flying? (or what's wrong with the COL marker?)

[katateochi]

I'm building a LearJet inspired aircraft (just with the larger engines) and I was having real trouble getting the right COM/COL balance.  
With the COL set just a little behind the COM (where I usually aim to have it) the craft handled really badly (ie it had an unhealthy fascination with the ground).

I felt that I needed to angle the tail wing down a bit, but when I tried to angle it even a bit, the COL was shunted far aft and put at funny angles (in some cases pointing downwards).  So that deterred me from angling the tail wing and I continued to try other things to improve its balance.  Eventually I came back to adjusting the tail wing as during flight tests all I needed to do was angle the tail down a bit to have level flight.  

This is now the COL/COM setup I have, and this craft flies beautifully!!

It lifts off nicely with just the main flaps being extended, maintains a fairly level flight without SAS being on (just needs two taps to the pitch trim) and lands better than any aircraft I've built (touches down on just rear wheels and remains pitched up for a while before the nose comes down).

It even flies well under water! (wait... what?)

Spoiler

 

Actually, to take off from the water with it, best way is to dive a bit and then pitch up steeply and leap out of the sea! Something (several things) feel rather wrong about this!

 

I did wonder if the slightly odd engine setup I have was partly causing this, but taking the engines off doesn't change the COL (only causes a very slight shift in COM).  Without the engines it's really not a complex craft, it doesn't have any sneaky hidden wings inside other parts.  The slightly only non-standard things are the "Swept Wings" have been rotated forward (no longer swept, but that doesn't change the COL angle), and the two sets of wings on the tail which are angled downwards.  Both sets of tail wings apply about the same angle of change in the COL (~45 degrees), but while the top tails wings are vital, the lower set are just aesthetic (and losing them on hard water landings doesn't really affect it's flight much).

So my question is; what's going on with the COL marker!? From looking at it in the SPH I would not have said this craft would fly.  Can you explain what's going on here, or is there a bug?
I used to think that the COL marker was a fairly true indicator of lift but maybe is has issues? Do you more advanced aircraft builders disregard it and go by what looks right or pure trial and error instead?

 

 

[Stone Blue]

Hmmm... I built several subs over the past week in the SPH, and I am having HUGE issues getting them balanced and my CoM is also seemingly acting very strangely, and not where it should be...

[Snark]

Just eyeballing the screenshot from the SPH, it's pretty obvious that the CoL marker is badly out of whack.  I mean, just look at it:  it's shown as about midway between the main wing and the tail wing, maybe even slightly closer to the tail wing.  If you believe that, it would mean that the tail wing has an equal (or even greater) amount of lift than the main wing!

Which is clearly ridiculous.  The main wing is much larger, and also has ailerons.  It obviously has much higher lift than the tail.  The CoL marker should be located much farther forward-- this one is wrong.

Here's a little experiment to try:  Put the horizontal elevators on the tail directly on the fuselage, and remove the vertical stabilizer.  Does the CoL marker leap forwards?

I'm wondering whether the code has a logic flaw that says "oh look, here's this big vertical stabilizer, it has a large lift rating, I'll move the CoL marker back"-- never mind that the stabilizer is vertical and therefore doesn't generate lift when flying right-side-up.

[katateochi]

5 hours ago, Snark said:

Just eyeballing the screenshot from the SPH, it's pretty obvious that the CoL marker is badly out of whack.  I mean, just look at it:  it's shown as about midway between the main wing and the tail wing, maybe even slightly closer to the tail wing.  If you believe that, it would mean that the tail wing has an equal (or even greater) amount of lift than the main wing

Not only is it shifted way aft, it's also rotated a bit over 90 degrees so it point's backwards and slightly down! COL is also parallel with thrust! eh?

 

5 hours ago, Snark said:

Here's a little experiment to try:  Put the horizontal elevators on the tail directly on the fuselage, and remove the vertical stabilizer.  Does the CoL marker leap forwards?

It's not the vertical stab that causes the shift back and change in angle.  The main culprit is the way the two tail wings are angled forwards. 

If I first reset the rotation on the lower small tail wing the COL shifts a bit forward and rotates up about 45 degs;

If I then reset rotation on the main tail wing the COL moves further forward an is now pointing straight up like normal

If I do kinda the reverse of what you suggested and take off both tail wings leaving just the vertical stab then the COL is almost in the right place;

 

I think the calculations for COL can't deal with wings that are rotated forwards and backwards, even slightly. So I guess this is now more of a bug report, I wonder if a kind moderator would shunt this into stock support.

 

[Val]

10 hours ago, katateochi said:

... I felt that I needed to angle the tail wing down a bit...

@katateochi Instead of angling the tail wing down, angle the main wing up. That will make it fly even better.

You can use the Rotate gizmo in Snap and Absolute mode (toggle with F), to align the wing tanks level with the fuselage again after adjusting the wing angle. You can also use Absolute mode to make sure the landing gear and tail wing is truly on straight.

I suspect your original issue was caused either by:

• attaching the tail wings to the Tail Cone B, which always causes them to be out of alignment with the fuselage.
• you accidentally angled the main wings slightly down when you rotated them forwards.

I recreated your craft, without the cool engine pylons you did.

Here's a picture of how it looks when I used Absolute mode to straighten all the wing pieces after attaching them.

And here's a picture after I angled the main wing up 2° and straightened the wing tanks.

It flies really well, but is hard to land. Too heavy, I think.

[Val]

I ended continuing developing my recreation, because I liked the concept. This is how it ended up.

I went for smaller engines and had to move the wings forward to compensate. I also removed the little tail fins as they were in danger of hitting the runway during too slow landings.

Edited December 6, 2015 by Val
Description

[katateochi]

1 hour ago, Val said:

@katateochi Instead of angling the tail wing down, angle the main wing up. That will make it fly even better.

You can use the Rotate gizmo in Snap and Absolute mode (toggle with F), to align the wing tanks level with the fuselage again after adjusting the wing angle. You can also use Absolute mode to make sure the landing gear and tail wing is truly on straight.

I suspect your original issue was caused either by:

• attaching the tail wings to the Tail Cone B, which always causes them to be out of alignment with the fuselage.
• you accidentally angled the main wings slightly down when you rotated them forwards.

I recreated your craft, without the cool engine pylons you did.

Here's a picture of how it looks when I used Absolute mode to straighten all the wing pieces after attaching them.

And here's a picture after I angled the main wing up 2° and straightened the wing tanks.

It flies really well, but is hard to land. Too heavy, I think.

nice, I 'll try fiddling a bit with the angle of the main wings.  Although I've now got it working really nicely as it is (even with the strange COL marker position).  It's actually the first aircraft I've built that lands like a real plane; touches down on the rear wheels and stays on just the rear wheels for quite a while before bring the nose down.  Don't know why but I find that very satisfying! (as opposed to most of my previous aircraft which land with all the grace of a fish flopping onto a boat deck!).  

hmmm the tail cone, that could be it.  Not at my KSP machine atm so can't check, but that could be it.  That part is rotated down slightly to better mimic the lear's shape, and with the tail attached to it that may be what's causing the COL to freak out!

Yeah absolute snap (is that a drink? ), used that for wing/wheel/fuel tank setup.  Everything is rotated 8 (or was it 6) notches from default pos so wheel alignment hasn't been an issue in this design. 

I've got some more tinkering to do with it and then I'll post it on KerbalX (I +1'd yours!).

[katateochi]

@Val I've just posted the craft on KerbalX - http://kerbalx.com/katateochi/LearJet

[Val]

39 minutes ago, katateochi said:

@Val I've just posted the craft on KerbalX - http://kerbalx.com/katateochi/LearJet

It looks awesome 

Did you re-angle the wings or keep them as is?

Hmm. Negative flaps? Forcing people to land nose high and faster speeds? Just to look cool? 

You can do the same with regular flaps. Just need to fly slower.

[katateochi]

1 hour ago, Val said:

It looks awesome 

Did you re-angle the wings or keep them as is?

Hmm. Negative flaps? Forcing people to land nose high and faster speeds? Just to look cool? 

You can do the same with regular flaps. Just need to fly slower.

Thanks! 
Yeah in the end I lept the main wings level, just tweaked the tail a bit, so the COL marker is still weird, but it flies ok.  
Yep, all in the name of landing a certain way!  Also, I don't believe in slow landings, I like landing at ~200m/s, it's more fun
I did play around with a more conventional flap setup which does what flaps are supposed to do, increase lift and drag.  The problem is that even just 1 set of flaps down creates a lot of lift so landing then becomes harder, that and the drag slowed the craft down a lot, end up touching down well under 100m/s which is kinda dull.