AH FM flaw or miss-conception?

[Pepe]

AFAIK, the profile of the horizontal stab is a lift generator one. If you remove this, the lack of lift, if everything remains the same, the tail will fall.

Just an uneducated €0.02

[tofri]

The biggest problem I see in this discussion, is the question "what means shooting of the tail?".

If it means, shooting a lot of holes into the elevator disabling the lift, then the nose will go up, as it is modelled.

If it means, cutting away half the fuselage , the center of gracity will move forward drastically (wherever the lift may be at the moment)and the nose will go down.

It simply depends on how much mass is lost on the tail.

[SFRT - Frenchy]

Bump for Fester.

[hitech]

Bump For Pitch Up question

[Straiga]

Look at it this way lets take a full loaded Me-109G and balance it on the head of a pin so its level this is the center of gravity. This could even be the aft CG limit. Then put the landing gear down and the CG moves forward and the nose points down. Now cut the enpennage off about about 4 ft. ahead of the vertical stabilizer now what happens the plane noses way over and way out of balance. Now how does this happen?

I like the helicopter bit were the engines pointing straight up and hovering there. LOL

So how did the main wing pitch up anyhow? By the horizontal stabilizer creating tail down force pitching the nose up in a high angle of attack and so you shoot the tail off completly, about a foot behind the canopy and now what happens to the nose, it will pitch over in a hard way nose down.

The horizontal stabilizer is an up side down airfoil creating a down ward force or lift, it also puts the main wing into different angles of attack. To change the angle of attack then change the elevator position up or down which decrease or increases lift on the horizontal stabilizer.

The prop wash also impacks on the leading edge of the main wing and also pitches the nose down.

Read for real pilots in A & V theres good sights and good reading.

Frenchy dont back down your right.
And the same for you to Grunherz, why does the nose point up, has it turned into an AH Helicopter, but were is the tail rotor? It wouldnt supprise me.

Straiga

[Badboy]

Was browsing and found this:



This extract shows an example of stability (achieved with longitudinal dihedral) with CG aft of lift, and in this case there can be no doubt that the nose would pitch up if the tail went away.

Badboy

[hitech]

Thanks for the find badboy

HiTech

[ALF]

Originally posted by NUKE
Just get a balsa glider and test the theory by flying it normally, then remove the tail and fly it.

damn Nuke!

I was scrolling down to suggest that very thing!:aok

[Crumpp]

Just looking it over and learning myself.  Straigia does seem correct on this one.

Look at the force of lift down the tail is providing.  

The fuselage connecting them acts as a  lever and the CG a fulcrum. The tail pushes down keeping the wing from pitching forward and reducing the angle of attack.

Remove that downward force created by the tail and the wing will pitch up with the nose coming forward.

What does the value "M" represent in this diagram?

Also the CG is in a different location for level flight, Correct?  So won't it shift when the downward lift of the tail is radically removed along with the weight of the tail?  I would think it would have to shift forward to compensate and violently pitch the nose forward.

Crumpp

[Badboy]

Originally posted by Crumpp
What does the value "M" represent in this diagram?

The M = Moment, which is similar to a torque, in as much as it describes a turning effect, or a force that causes rotation only. It provides a quantitative description of the tendency of the wing to want to rotate and the units are the same as torque either newton.metres or pounds.foot.

Originally posted by Crumpp
Also the CG is in a different location for level flight, Correct?  So won't it shift when the downward lift of the tail is radically removed along with the weight of the tail?  I would think it would have to shift forward to compensate and violently pitch the nose forward.

Yes, if the tail comes off the CG would move forward, however, the main argument for the aircraft pitching nose down has been that the centre of gravity was in front of the aerodynamic centre producing a nose down couple, in this diagram there is stable and balanced flight with a nose up couple, so in this example, if the tail is removed the nose must initially pitch up.  Once the nose pitches up, what’s left of the aircraft isn’t flying anymore, and stability considerations no longer apply. Now, all you really need to consider at this point is how a heavy lump of metal with a propeller attached to it would fall. I think most people can see intuitively, that the heavy lump would fall first, dragging the propeller behind it.

Hope that helps…

Badboy

PS
Seeker had some video footage of a bomber losing its tail due to a collision at an air show, it showed the aircraft lose its tail and go nose up initially, eventually going into a dive. I’ve tried to find it without any luck

[Despair]

Noob's point of view: when you get your tail shot off your plane briefly turnes into a helicopter

[Crumpp]

Yes, if the tail comes off the CG would move forward,

Thanks Badboy for the explaination.  Then that opens some more questions.  If the CG moves forward in proportion to the speed at which the force shifts from the tail, won't it it end up ahead of the Center of lift pitching the nose down as the bomber does in the crash video?  

Certainly an engine and prop in freefall will fall with the prop up.  Will an engine with half a fuselage and a wing attached do the same?

I thought M stood for momentum.  That also adds to the forces pushing the nose down correct?

Crumpp

[hitech]

Crump CG isnt effected by any force. It is just the center of gravity i.e. where the plane always balances at. In some ways you can think of it as a fulcrum.

HiTech

[Badboy]

Originally posted by Crumpp
Thanks Badboy for the explaination.  Then that opens some more questions.  If the CG moves forward in proportion to the speed at which the force shifts from the tail, won't it it end up ahead of the Center of lift pitching the nose down as the bomber does in the crash video?

Yes that bomber did end up nose down eventually, and it didn’t pitch up all the way either, but it was also a very heavy bomber with a large moment of inertia, (which is a measure of an objects resistance to rotation) which means it wouldn’t have rotated quickly longitudinally anyway.

Also, in the situation shown in that diagram, I’m speculating that if the tail comes off of a fighter with that configurartion, there would be an initial pitch up, possibly to a large degree, after that, the CG would move forward (because the mass and distribution of mass has now changed and that changes the position of the centre of gravity, forward in this case) but it would be too late already, by that time the aircraft would be nothing more than a lump of falling debris with a propeller attached to it.

Originally posted by Crumpp
Certainly an engine and prop in freefall will fall with the prop up.  Will an engine with half a fuselage and a wing attached do the same?

Why not? They are both just ballistic at that point, a wing is just a lump of metal without the elevator to hold it at an angle to the airflow in order to create lift.

Originally posted by Crumpp
I thought M stood for momentum.  That also adds to the forces pushing the nose down correct?

Not in this case, the M does stand for a Moment. Momentum is different, it involves the product of mass and speed and if the nose were already rotating up or down at the time the tail came off, momentum would tend to make it continue in whichever direction it was going at the time, but I don’t think that would be enough to change anything, other than perhaps just the rate at which it happened. So, some rotational momentum might speed up, or slow down what ever nose pitching tendency was going to happen, but I don’t think it would be strong enough to change it.

Hope that helps…

Badboy

[Crumpp]

Also, in the situation shown in that diagram, I’m speculating that if the tail comes off of a fighter with that configurartion, there would be an initial pitch up, possibly to a large degree, after that, the CG would move forward (because the mass and distribution of mass has now changed and that changes the position of the centre of gravity, forward in this case) but it would be too late already, by that time the aircraft would be nothing more than a lump of falling debris with a propeller attached to it.

Thanks again for the reply, Badboy.  Very informative.

Why in a fighter would it be slower?

Seems to me it would be faster.  Less mass means less resistance to change.    It is already moving that way because the CG is shifting correct?

Thanks Hitech for your reply as well.  If the CG can be thought of as a fulcrum will it not move instantly to balance?  Since it can no longer balance without the tail won't it shift forward?

Why not? They are both just ballistic at that point, a wing is just a lump of metal without the elevator to hold it at an angle to the airflow in order to create lift.

Yes but the wing is still a large hunk of drag rather like a sail or a streamer, right?  
When the CG shifts foward the largest amount of drag would fall last.  The prop is still trying to pull the engine, correct?  Thrust out front and drag in the rear?  

A good illustration is your legs in freefall.  If you invert and fall feet first you must minimize the drag of your legs by putting them together and pointing your toes to the earth.  Have them apart or your toes not pointed to the earth and you will flip.  Falling head first to the earth.  It is tough to master but necessary if you want to skysurf.

Does that make sense?

Crumpp