Constant speed or constant attitude?

On 17 Aug 2003 17:12:58 GMT, Ian Cant
wrote:

Martin,=20

I just can't resist a good argument.

Sorry, but your vector diagram as described gives AOA with sign =
reversed.

I must respectfully disagree. If you fly into a rising airmass from a
neutral or sinking one your instantaneous sinking speed is reduced by
the vertical velocity of the new air mass - hence the reduced AOA.

On encountering a rising airmass from stabilized flight, the =
instantaneous effect is an increase in AOA - the relative wind that was =
flowing from straight ahead is now flowing from ahead and below. This =
increase in AOA gives added lift at the wing and reduced pushdown at the =
tail. Since the CG is ahead of both CL and tail, both give a nosedown =
moment. The aircraft attitude is disturbed and it pitches nosedown.

Errrm. The CG may be ahead if the wing's CL, but it certainly is not
ahead of the CL of the entire aircraft: if that was the case you'd
soon be flying vertically downward!

An aircraft operating at its trimmed speed in still air has no
pitching moment about its CG. However, the CG is certainly ahead of
the NP (Neutral Point [1]) by the MSS (Margin of Static Stability) and
this must be so for stable flight. The size of the MSS controls the
rate of oscillation after disturbance: too small MSS and recovery
tends to deadbeat or no recovery (unstable) and to large MSS can cause
oscillation and divergent behaviour.

This is true for both aircraft with download on the tail (most manned
aircraft) and free flight duration models, which usually have a
lifting tail and rearward CG. A lifting tail may be unsuitable for a
manned plane but as a way of getting minimum sink from a fixed speed,
fixed trim unmanned aircraft its the best. However, the same NP
stability calculations apply to both of these - and to canards and
flying wings.

Now the inherent stability starts to work through a series of =
transients. The pitch down reduces the AOA and the aircraft accelerates =
because the gravity vector is closer to where the nose is now pointing. =
The reduced AOA and the increased drag gradually restore the aircraft to =
its original stable attitude [maybe after a few oscillations, it is far =
from a deadbeat system] and sink rate through the airmass. The final =
result is an aircraft flying at exactly the same attitude, speed, L/D =
etc in the new airmass, but with a sink rate relative to the ground =
equal to the old value less the upward velocity of the new airmass.

Agree with your conclusion, just can't help nitpicking the argument..

Going back to the original question, whether to fly constant speed or =
constant attitude, both are difficult to achieve when hit by a gust. =
But if you are skilful enough to stop the nose drop then your attitude =
will remain constant and your speed will also be constant if the gust is =
vertical. If the gust is horizontal, there wil be relatively little =
change in AOA but an increase/decrease in total energy will be reflected =
in the vario and your airspeed will change. Holding attitude will let =
the aircraft stabilize itself, but trying to regain airspeed is a better =
bet. If the gust gives a speed increase, a little nose up will turn =
your inertia into more altitude sooner, and if the gust gives a speed =
decrease you might want to nose down a little to avoid a wind-shear =
stall situation.

In my case, gusts are always some unknown combination of vertical and =
horizontal, and my reactions are too slow to hold anything truly =
constant...

Agreed, but we can work on it. :-)

One of the neatest demonstrations I've seen of vertical vector
addition is when, running a cloud street, you meet a region where the
vertical air velocity increases steadily over a relatively long
distance and you can continue to cruise for a surprisingly long time
while climbing with nose-high attitude and constant airspeed:
certainly long enough to look round and see your tailplane is on the
horizon - in a Pegasus that means at least 5 degrees of pitch up.
Ain't gliding fun!



[1] The NP is Lw * Mw - Lt * Mt = 0 at the current trim condition
where
Lw = lift from the wing
Mw = moment from wing CL to the NP
Lt = lift from tail
Mt = moment from tail CL to the NP

Free Flight model designers tend to think of the NP as a fixed point
because they always design for the minimum sinking speed (FF is a pure
duration contest, remember) but it isn't - in reality its trim
dependent and so will move with the trimmed airspeed.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

On Sun, 17 Aug 2003 21:34:19 +0100, Martin Gregorie
wrote:

On 17 Aug 2003 17:12:58 GMT, Ian Cant
wrote:

Martin,=20

I just can't resist a good argument.

Sorry, but your vector diagram as described gives AOA with sign =
reversed.

I must respectfully disagree. If you fly into a rising airmass from a
neutral or sinking one your instantaneous sinking speed is reduced by
the vertical velocity of the new air mass - hence the reduced AOA.


Not so at the instant the airmass is entered.
After some time - about 0.75 to 2.5 seconds the glider will be close
enough to equilibrium in the new airmass.


On encountering a rising airmass from stabilized flight, the =
instantaneous effect is an increase in AOA - the relative wind that was =
flowing from straight ahead is now flowing from ahead and below. This =
increase in AOA gives added lift at the wing and reduced pushdown at the =
tail. Since the CG is ahead of both CL and tail, both give a nosedown =
moment. The aircraft attitude is disturbed and it pitches nosedown.


If like most sailplanes the longitudinal stability isn't excessive
this effect may not be all that large but yes the glider will tend to
align itself with the local airflow but this effect only lasts for a
short time. This can also be masked by mass balancing or lack thereof
in the elevator control system, not just the elevator and how hard you
are holding on to the stick. In any case you are probably going to try
to hold the attitude constant unless you are cruising at high speed
and decide to take the thermal.
I seem to remember the SB13 flying wing had a problem with this when
first flown at forward C of G. The low moment of inertia in pitch
resulted in unpleasant characteristics as the glider flew through lift
and sink.

Mike Borgelt