The Downwind turn

[Checkboard]

Not only does the aircraft fly relative to the airmass, but so do your ears! Even this is a "bad" way of expressing what is going on, but will suffice.

Turning in calm air will have exactly the same effect on your balance organs as turning in a steady 60 (or 100, or 200..) knot wind.

[Pub 45]

I agree with you checkboard.

However, I think the optical illusion (that stateandlevel mentions) will still occur due to a change in velocity relative to the ground. This could cause disorientation, because the sensory organs would be indicating something else.

[This message has been edited by Pub 45 (edited 08 February 2000).]

[StrateandLevel]

Because your balance organs act like a pendulum they will detect both the angular acceleration in the turn and the linear acceleration as you turn downwind. In still air they will only detect the angular acceleration.

[Checkboard]

Sorry - still not correct, the whole point is that the acceleration experienced turning from up wind to downwind is exactly the same as the acceleration experienced in still air. For the aircraft, for your body and for your ears.

[StrateandLevel]

If thats the case how does an IN platform measure the change in groundspeed?

[rolling circle]

The original question was - "To all you instructors out there, does an aircraft lose airspeed when it turns downwind?". The answer, as any real instructor knows, is NO! Anyone who thinks different should e-mail me 'cos I've got this great financial opportunity that you will all want to subscribe to!!

Isn't it comforting that there is a never ending supply of suckers??

[Checkboard]

The INS or IRS meaures the change in groundspeed the same way for both situations.

200 knots North, 180° turn, ending up at 200 knots South in calm air, 200 knots TAS the whole time. Change in velocity: 400 knots South.

In a 20 knot Northerly: 180 knots G/S North, 180° turn, ending up at 220 knots G/S South, but still 200 knots TAS the whole way around. Change in velocity: 400 knots South.

The ground speed matters for navigation, but in terms of how it effects the airspeed of the aircraft, or the forces required to move it about, it is about as important as the speed of a car along a freeway below it, or the speed of an aircraft a mile away.

Same forces involved, same accelerations measured by the INS.

[This message has been edited by Checkboard (edited 10 February 2000).]

[StrateandLevel]

Checkboard, you clearly don't understand how an accelerometer works. The IN measures ground speed by integrating the output of three such devices. They consist of a ball and spring. The ball displaces due to the acceleration in exactly the same way as the fluid moves in your balance organs.

As you turn downwind, the fluid moves aft and the pilot percieves a pitch up. The increased groundspeed should induce the optical illusion of pitching down. Does the pilot unwittingly react by continuing the pitch up he is already sensing thereby reducing the airspeed? We know the True airspeed stays constant.

If as you claim everything in the aircraft maintains a constant velocity IN would never work would it ?

[This message has been edited by StrateandLevel (edited 10 February 2000).]

[Checkboard]

I never claimed the aircraft or anything else maintains a constant velocity, only a constant speed.

The "ball & spring" thing is cute.

[BEagle]

This 'Downwind Turn' business is demo'd during 'Low Level Intro' to Bulldog students. It's just illusion leading to insidious pitching attitudes resulting from the perception that the cows should still be going past the window at the same rate!! Other dangers are also highlighted - but there is no aerodynamic reason for loss of speed turning into or out of wind. How does the aircraft know what the wind is doing?? It's a bit like thinking that a Thermos flask somehow knows that it's supposed to keep hot things hot and cold things cold - but not the other way round!!

[StrateandLevel]

I always thought velocity and speed were synonimous.

Of course if something actually deflects then both its ground speed and airspeed change.

Glad you are keeping your eyes on the old cows outside your window Biggles. Saw some nice ones down the pub tonight!!

[4dogs]

strate,

i guess your physics match your spelling...
http://www.pprune.org/ubb/NonCGI/For...ML/000017.html


------------------
Stay Alive,

[email protected]

[rolling circle]

Ah - that explains it. To believe that the downwind turn affects airspeed, you first have to believe that speed and velocity are synonymous.

Inbound in the hold yesterday at 100 knots IAS in 55 knots of headwind. Turned outbound with no variation of airspeed or pitch attitude. Perhaps the advocates of disaster in the downwind turn would like to explain that away.......

[2R]

Question was answered correctly by rolling circle.And someone else mentioned the illusion thing ,where the ground speed increase's and an inexperienced pilot may slow the airplane down and enter a stall in the next turn as the wing is loaded in the turn,made even worse if the turn is skidding.
Attitude And Awareness will prevent most problems.
If the question was "At what angle of bank does the airspeed change ?' the answer would be different.
Aristotle said "Finding the question is harder than the answer".