AH FM flaw or miss-conception?

[funkedup]

Originally posted by hitech
Misconception: You can move the cg behind the cp of the main wing and a plane will still be stable. You can not move it behind the total center of lift generated by both the tail and the wing.

Per your diagram both surfaces are producing up forces, but the tail is producing more torque than the main wing, hence it is still stable.

When the HStab is removed in AH all forces from it are removed from the flight model. Hence why they fly nose up do to physics.

HiTech

FWIW this is exactly how the professors taught it to me.  

I'm sure some planes have a CG forward of the wing's center of lift.  But for best performance you want a CG aft of the wing's center of lift, but forward of the combined center of lift of the wing and the horizontal stabilizer.  In Frenchy's picture, this point (the aerodynamic center of aircraft) would be somewhere between the wing and the tail.

[SFRT - Frenchy]

Originally posted by hitech
Per your diagram both surfaces are producing up forces, but the tail is producing more torque than the main wing, hence it is still stable.

When the HStab is removed in AH all forces from it are removed from the flight model. Hence why they fly nose up do to physics.

HiTech

Heuuu ... Hitech ... are you talking about my diagram? When you had your private pilot licence, what book did you used? If by any chance, you used the comon "Private Pilot Manual" by Jeppesen, the light brown one in color this one  ... you may want to check page 3-29.

I showed your answer to an Aeronautical Engineer this morning, who also happen to own a Cj6 (Chineese Yak52). He said that the second part of your answer didn't really made sense.

Would you mind to clarify it for me please?

[lakc]

There is an old saying in r/c aircraft: A nose heavy plane flies badly, a tail heavy plane flies once.

The tail actually produces downforce to hold the nose up when flying level. It increases this downforce to pitch the aircraft up, and decreases or even provides lift to force the nose down.

You should try and kick this question over to Mary Shafer at sci.aeronautics, she has a gift for these kind of explanations.

[Furious]

Originally posted by GRUNHERZ
Hitech but why does the AH plane keep falling nose high 90 degrees vertical?   The engine is by far the greatest concentracion of weight in the plane - seems to me it should fall nose down after any wing/tail aerodynamic forces are negated by the 90 degree attitude of the falling tailess plane.

It seems to me that if the prop is still spinning and producing thrust, then once the nose pitches up you basically have a helicopter and this would keep the nose up.


F.

[john9001]

this whole thread is silly, when your tail is shot off , your going to crash , does it really make a difference if you hit nose first or tail first?

lets get back to something interesting like "how can you put 125% fuel in a airplane" or "why do i get killed with one shot but the enemy won't die after ( insert large number here ) shots"

[Fishu]

Originally posted by john9001
this whole thread is silly, when your tail is shot off , your going to crash , does it really make a difference if you hit nose first or tail first?
 

If one thing is wrong with damage -> flight model, then it is reasonable to expect that other factors can be wrong as well - with a plane that doesn't go down.

[Seeker]

John, you misunderstand our interest:

No one's suggesting anything's wrong with the FM. As was mentioned in the linked thread, there's enough real life observations (with models) to show AH's FM exhibits the same behaviour as real life.

The point of the question is not to throw doubt on AH's flight modelling, but to ask for some of the gaps in our aerodynamic knowledge to be filled.

I know aircraft have a forward centre of gravity; it was my job to balance all DHL's aircraft traveling through scandanavia for some years; and most of 'em made it.

I know an aircraft that loses it's tail flutters nose up down to the ground, because I've seen enough models do precisely that.

I don't know  why that should be, because with my limited understanding of the physics, they should go down nose first, and that's what I'm interested in understanding.

I've a clip of a Blenheim lose it's tail after a collision at an air show: It goes straight up, then sinks nose high into the ground, along with (presumably) with four men screaming their last. Why didn't it plough nose first into the ground? That's what I'm trying to understand.

<edit >

I've just checked the clip again, It's Beauforts, an MPEG of 2,355 Mb if any one wants it, and it looks exactly  like AH.

[john9001]

sorry , i forgot to put a    in my post

[whels]

Originally posted by GRUNHERZ
That doesnt explain why AH planes fall tail first. The engine is by far the heaviest and densest part of the plane - it would pull the nose down if the balancing tail weight was gone.

in a non power off situation yes the nose should fall if tail is gone, but in a power on , the nose is trying to go up and the tail has to counter that or u would climb all the time.

granted it shouldnt fall from 30k tail down the whole flight,
but  i have seen RL guncam footage of a LW fighter losing its
tail and go nose up.


whels

[GRUNHERZ]

Yep whels, my whole issue was after the plane stopped flying and started falling vertically it doesnt make sense that it would continue descending nose up regardless of prop blast.

[moot]

GRUNHERZ what about the sideways stalls of some planes like P51 and Ta152? Or 109 and spit5, easy to induce?
P51 and Ta152 both also have flat stalls, 51 does it inverted at least, spins sometimes, stays that way to the ground.
152 can do a tail centered backwards >=45deg spin to the ground too.

[GRUNHERZ]

I'm not sure what you are talking about moot.

[hitech]

moot now your into a hole new area, btw center of lift is not a constant on an airfoil, the cp moves with changes in AOA and can change drasticly with post stall AOA's. Also the CP/CG relationship is not the only force movinging the nose, CM also can have a large effect at post stall AOA's.

On the orignal debate I assure everone the physics are correct, all your realy disscussing is to move the CG more forward on the plane, and it would then be nose down with the VStab gone.


Think of the p51 in which everyone has heard the story of instablity with the aft gas tank full. Note the plane still flew, therefore it didn't have the cg behind the "Planes" Center of lift but they were very close. Now when you remove the tail the CP of the entire plane will move forward. Hence nose up.

It's much easyer to think of the Center of lift of "Plane" i.e. both wing and tail combined when trying to understand stablility.

And frenchy, most pilots realy don't know much about the physics of flight. I have had length disccussions with multiple CFI's and most just have a very basic understanding and would draw the same conclusion from your diagram.


HiTech

[funkedup]

Frenchy I think this is what Hitech meant when he made the reference to your diagram.

a.c. = aerodynamic center

As long as the c.g. is forward of the airplane's a.c., the airplane is longitudinally stable.

[funkedup]

You could put the c.g. ahead of the wing's a.c. and the plane would be plenty stable, but the wing would be fighting the tail.  The wing would have to create extra lift to make up for the downforce from the tail.  This would in create more drag and lower the maximum g that the airplane could achieve.  So for high performance aircraft you will always find the c.g. behind the wing's a.c. but ahead of the airplane's a.c.