Folks,
graham@gflight.f9.co.u wrote:
>
> Mike will have a much more valid opinion than me but I believe the
> sensible way to monitor airspeed is by angle of attack, the instrument
> tells that that your airspeed is going to change, you can correct the
> changed AoA instantly and speed won't change The ASI tells you at least
> 20 seconds after it's changed which means you then have to regain
> momentum which will take another 20 seconds or more. Another advantage
> that correct AoA is not dependant on weight, so the figure for stall,
> cruise max range etc all remain constant.
> my 2 cents
> Graham
>
There is much good information in Graham's message but I feel that this last
paragraph
is misleading. Unless your ASI is particularly sluggish, it will indicate
changes in airspeed almost instantly (i.e. not 20 seconds later). What takes
the time is actually changing the speed of the aircraft relative to the
surrounding
air. Fitting an AoA gauge does not make your aircraft accelerate any
quicker!
Let's imagine the situation where you are on approach and your airspeed is a
little
below the optimum speed (the AoA is higher than desired). Assuming you are
not too low, you lower the nose to increase the speed (and reduce the AoA).
As soon as the nose is lowered, the AoA will decrease and if you have an AoA
gauge
that will be indicated immediately. The airspeed, however, does not change
straightaway (the ASI lag!!!) After a while, your airspeed will settle at the
new increased value. (perhaps a kind aerodynamics person can provide us with
a succinct description of what causes the airspeed to increase when the AoA is
reduced).
Moving the stick forward to reduce the AoA is the crucial action. Having moved
the stick, the pilot then has to wait until the speed stabilises before sampling
the ASI again (all gliding instructors will remember giving this demo). If
the speed doesn't have to change by very much (say, < 5 kts) then the pilot
doesn't
have to wait very long (certainly less than 20S).
The rational behind the talking ASI is that if the airspeed on approach is not
far from the desired speed, then the pitch/throttle adjustments required to
return
to that speed are small and the time lag between making an adjustment and
achieving the desired speed is small. The pilot is "nagged" into maintaining
the correct airspeed through small changes in pitch/power.
By nagging the pilot to maintain a sensible speed, the talking ASI not only
protects
against stalling, it helps you obtain the stabilised approach that Mike
Parkin enthused about and is so important when operating out of small fields.
I understand the benefits of flying by AoA and if someone produced a low cost,
reliable, easy to fit and calibrate, AoA instrument that didn't require you to
have your eyes in the cockpit on approach, I would be tempted to have one.
Mark
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http://forums.matronics.com/viewtopic.php?p=51717#51717
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