Author Topic: physics flaw in AH? (Read 2372 times)

Swoop · « **Reply #15 on:** August 29, 2003, 12:49:38 AM »

I always thought the stabilizers on the tail provided DOWN pressure to counteract the UP pressure (um..ok, lift) from the wing. Without a tail a plane would continuously loop.......however if the tail suddenly disappears while in level flight then the massive amount of lift being generated by the wing suddenly has nothing to counteract it and so the plane noses up sharply into an immediate stall.

However, why the nose STAYS skyward after the stall is at it's most pronounced and the plane falls tail first is beyond my knowledge. I mean, after the stall has happened the wing is no longer generating any lift.......one would think that the plane would simply tumble and eventually stabilize nose down. Although if it did then the wing would begin to generate lift again and force the nose higher. Hmmm.

Skorpyon · « **Reply #16 on:** August 29, 2003, 01:00:59 AM »

Swoop,
By this time, the nose being initially pointed upward and the engine usually being in full throttle position, your aircraft has undergone an amazing transformation... it is now trying to be a helicopter!

I guess the test would probably be to immediately cut throttle and see if what's left of the plane flops back over. Hmmm... next time I get my bellybutton end shot off, I gotta remember to try this. On second thought, I think I'll remain pointing up under full throttle and try and take out a few of the idiotic "assist grabbers" who invariably dive on a totally crippled, flaming, non-combatant aircraft that is plummeting to the earth, just to get a quick fraction of a point.

Ozark · « **Reply #17 on:** August 29, 2003, 01:06:17 AM »

Now think about the shape of the propeller. (Airfoil)
If that don’t work… think of the airfoil of a helicopter.
Lift one way… auto-rotation (drag) the other way.

Ozark · « **Reply #18 on:** August 29, 2003, 01:08:59 AM »

Skorpyon

Beat to to it.

GScholz · « **Reply #19 on:** August 29, 2003, 01:10:31 AM »

There are some fishy elements in AH's FM. There may have been some aircraft where the tailplane provided positive lift, large aircraft most likely, but on small planes it most certainly provided negative lift. The strange part is that AH does model this. As your plane slows down you need more up trim to stay in level flight, i.e. your CG is forward of your CL, however when you lower flaps CL should move aft, dropping your nose down rather than lifting it up like in AH. If a plane like a WWII fighter loses its tail the nose should drop quickly and the plane should pancake into the wind inverted, and then flip-flop down.

EDIT: Actually I remember in the BoB movie they blew the tail section off of an RC 109 model during one of the dogfights, and as expected it nosed over and flip-flopped while burning.

Ozark · « **Reply #20 on:** August 29, 2003, 01:29:27 AM »

Quote

EDIT: Actually I remember in the BoB movie they blew the tail section off of an RC 109 model during one of the dogfights, and as expected it nosed over and flip-flopped while burning.

Now you added a new and fun parameter to the mix… Momentum!

GScholz · « **Reply #21 on:** August 29, 2003, 02:09:52 AM »

This small plane went into a nose dive after losing its tail.

---

At this point, another climber, Craig "Crash" Bennett came forward, to thank Dawson for the flight. There was a bang and the plane went into a nose-dive. It was Moline and his canopy hitting the left horizontal stabiliser on the tail assembly.

"I didn't see it, but I heard it, and I felt it," says Dawson, "and I just knew that someone was over the tail. I didn't know it was Simon. I was just hoping they could get off, and trying to bring the aircraft up and level and from going down. We were still buffeting around."

Dawson shouted for everybody to get out.

Meanwhile, cameraman Chaberka was falling, with his eyes turned upward and locked on to Moline and the plane's tail. Chaberka saw the parachute canopy go past him and over the tail. He was a little below Moline as Moline was dragged from Boyle's and Drinkwater's hands by the chute. "I saw Simon's canopy get wrapped around the tail and then the tail twisting, 45 degrees to the right, before it snapped off. Simon was still hanging off the tail."

---

http://www.theage.com.au/articles/2002/04/27/1019441314240.html

funkedup · « **Reply #22 on:** August 29, 2003, 02:42:56 AM »

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Originally posted by RDSaustinTX
That's at least partially wrong. Tails provide NO lift in a stable a-c.

Mullah:cool:

Sorry that's just wrong.

funkedup · « **Reply #23 on:** August 29, 2003, 02:44:36 AM »

Guys who are contradicting me, get a flight mechanics textbook, read and learn the section on longitudinal stability, then get back to me.

funkedup · « **Reply #24 on:** August 29, 2003, 02:46:14 AM »

Quote

Originally posted by Citabria
but its a function of CG a tail heavy aircraft will go nose up perhaps

but a plane with no tail NO TAIL and therefor a forward cg on the remaining aircraft should fall nose down

Good point. Losing the mass of the tail could move the CG forward. I wonder if that loss of mass is in the model?

Furious · « **Reply #25 on:** August 29, 2003, 03:30:41 AM »

But would it really matter Funked? As long as the the engine is running and the prop is turning its thrust will over come the tendancy for the nose to drop.

Ozark · « **Reply #26 on:** August 29, 2003, 03:52:49 AM »

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Originally posted by funkedup
Guys who are contradicting me, get a flight mechanics textbook, read and learn the section on longitudinal stability, then get back to me.

Stop looking at the picture and read the text.

You want equilibrium. Airflow that creates equal forces up and down.

If the horizontal stabilizer created lift, then you would need trim the nose up to force the horizontal stabilizer into equilibrium.

Ozark · « **Reply #27 on:** August 29, 2003, 03:57:21 AM »

Quote

This small plane went into a nose dive after losing its tail.

---

http://www.theage.com.au/articles/2002/04/27/1019441314240.html

As I stated before, Now you added a new and fun parameter to the mix… Momentum!

Entanglement of the horizontal stabilizer pitched the aircraft on a downward attitude and momentum completed the process. The scenario in the report is not like getting your tail shot off.

“During the investigation it was calculated that on opening after entanglement, the reserve parachute would have applied a load of approximately 1.6 tonnes to the horizontal stabiliser. That load exceeded the aircraft design limits.”

Full accident report link follows:
http://www.atsb.gov.au/aviation/occurs/occurs_detail.cfm?ID=389#factual

Ozark · « **Reply #28 on:** August 29, 2003, 04:08:33 AM »

Hey Guys!

~~Thanks for the fun. This thread kept me awake during my (un)scheduled night shift at the airport. Time to look busy before folks start arriving.~~

GScholz · « **Reply #29 on:** August 29, 2003, 08:53:05 AM »

Quote

Originally posted by funkedup
Guys who are contradicting me, get a flight mechanics textbook, read and learn the section on longitudinal stability, then get back to me.

I have.

"The movie below shows a main wing and stabilizer in equilibrium. Equilibrium means that the nose down moment created by the wing is exactly balanced by an equal nose up moment created by the stabilizer. As a result the pitching moment is zero. It is important to remember that the pitching moment must be zero at almost all times during flight. A positive or negative pitching moment is only required for a short moment to start the aircraft pitching, or stop it from pitching."

http://142.26.194.131/aerodynamics1/Stability/Page9.html

Btw. this is an excellent site for leaning the basics.

Interesting to read their conclusions in this chapter: Velocity changes do not produce pitch changes directly. In AH they do.