ALPHA VANE MOVEMENT

[spierpoint jones]

Can anyone kindly explain the movement of the alpha vane in relation to the fuselage or some datum during ....

1) takeoff roll
2) climb
3) Level flight
4) Decent
5) At critical AOA just prior to stall

6) And Most importantly its movement ie: up or down with the adding or retraction of TE devices .....

Yes it does sound like an interview question because it was ..and I had no clue ,especially with point 6...
Thank you all .....

jones:ok:

[Golden Parrot]

The angle of attack vane swivels freely so anytime there is a significant airflow over it it will weathervane into it. It hangs limp (!) when the aircraft is sitting on the apron. During the take off roll it gradually tilts parallel to the relative wind where it remains until the aircraft slows down after landing. For all other phases of flight think of the difference between body angle and relative airflow and that's were the vane is pointing.

[Shore Guy]

Always important to check the vanes on a preflight for damage (from Jetway/stairs)…….On most aircraft the stall warning system does not become active until air mode (usually nose gear squat switch). If the vane has been damaged, you might wind up with a shaker on takeoff…..aircraft have rejected for this with some not so pretty outcomes.

This system and airspeed are two of the systems whose operation you can really not check until the aircraft is moving.

[Piltdown Man]

Lets have a go!
1. Decreases to zero and then tail goes further up on rotation.
2. At constant RAS, remains with a constant tail up. Decrease speed, tail up and vice versa.
3. At per the climb, depends on airspeed.
4. Do.
5. Tail way up.
6. On flap extension, initially it will stay tail up until the airspeed is reduced. But addition of any TE high lift device will allow a greater AofA and thus higher nose attitude and more tail upon on the vane. However, LE high lift devices allow the nose to be lower for a given AoA, thus airspeed. Thus tail will be lower on extension.

Hower does it all work? I don't know but I think that there is some silicon involved.

[Oktas8]

6. On flap extension, initially it will stay tail up until the airspeed is reduced. But addition of any TE high lift device will allow a greater AofA and thus higher nose attitude and more tail upon on the vane. However, LE high lift devices allow the nose to be lower for a given AoA, thus airspeed. Thus tail will be lower on extension.
Ummm. Permission to disagree?

Instantaneously upon extension of any TE high lift device, assuming no speed change at first, aircraft nose must go down to maintain the same flightpath (for example, to maintain level flight). Therefore tail down on alpha vane.
After TE device extended, speed probably reduces, so vane goes more tail up to maintain the same flight path.
LE device - has the same immediate effect on the alpha vane as TE devices.

On a more general P of F note:
- Addition of any TE device - increases wing CL, therefore allows lower speed for any flight path. However, because it changes wing shape, the addition of a typical TE high lift device will allow a lesser critical AoA and a lower nose attitude at that AoA. On the alpha vane at the stall, tail will be not so far down with TE devices extended, than if they were retracted.
- Addition of any LE high lift device - permits a greater critical AoA and a higher nose attitude at that AoA. Alpha vane tail will be a long way down at the stall!

Hope this is not completely useless, ;)
O8

[Piltdown Man]

Permission granted.

I have to agree you with regarding TE devices but surely LE devices move the chord line in the opposite direction, thus the AoA vane will have to move in a different direction.

Permission hereby granted to disgree again. :ok:

[Oktas8]

Cheers Piltdown Man,

I wonder if the real answer is "it depends on the type of LE device, and the angle of attack at the time the device is extended." Or perhaps it's as simple as "you're right"! :}

Relevant accident reports - NTSB/AAR-93/07 (effect on lift unspecified), NTSB-AAR-81-8 (deployed slats resulted in decrease in lift) and perhaps some others I couldn't find.

Cheers,
O8

[TruBlu351]

Just stick your hand out the window next time you go flying and you'll get a good feel for how it works :ok:

If you have an "alpha" readout inside your cockpit (from the AoA vane), it's easy to see what's going on behind the scenes. But there are other ways to observe what's happening, other than looking at the vane on the wing it you can see it!

When in "level" flight, look at the AI (or just remember what you do) and note the difference between the waterline and horizon (attitude in degrees)......this is your AoA, or what the vane is reading (hi/low AoA). Then play with various flap settings/speeds and note the attitude and you'll see if you need to decrease or increase your AoA to maint level....as mentioned by the others.

On approach with.........

a TEF failure (LEF only if fitted or flapless) you'll need a higher AoA to create more lift and/or more speed. Remember your higher attitude flapless approaches whilst maintaining the same rwy aspect.

a LEF failure (TEF only) you'll have a lower AoA on approach and a fairly higher airspeed to have the same lift at the reduced AoA.