Hi Michael,
In my position as Chief Engineering Test Pilot for a Major GA
manufacturer we tried several ways to lower stall speed. This can be
very important beyond what may come to mind. The stall speed controls
the maximum useful load that a multiengine plane can handle. This is a
restriction in the regulations that limits the maximum Gross weight
(and useful load indirectly) by specifying a minimum Rate of climb at
5000 ft MSL. This rate of climb is obtained by a complex formula based
on stall speed. i.e. higher stall speed lower gross wt.
Understanding the relationship of stall and load carrying capacity
shows the best economic advantage of vg's wing cuffs and etc.
That being said, we tried many various attempts to lower stall
speed without adding additional wing and found the some were effective
in lowering stall speed BUT all of these caused a reduction in cruise
speed in twin engine airplanes of between 8 to 15 mph. The only system
that did not reduce speed but required additional HP was active BLC
(actively sucking down the Boundary layer through small holes in the
wing). If you took the power from the engine and used BLC in cruise
there was a trade off of how much drag reduction and how much power
used. Too much power to BLC and too little drag reduction equals speed
reduction.
But after all of those words I guess that you must realize that
vg's will reduce your cruise speed. Looking at this in a practical way
you know if you lower your landing gear the airplane slows down or
anything extended into the airflow over the airplane will slow it
down. VG's are really very little speed brakes.
Recently I flew a C310 several times before and after the addition
of vg's and it is slower by about 10 kts, but it will carry more load.
If you don's have a flight test boom and very accurately calibrated
airspeed and static system, the best way to compare changes is to do
passing times. This is to take a base airplane that you do not change
and compare the changed airplane to this standard. This can be a
friends airplane. You fly side by side and with a stop watch measure
the time for the fastest or slower airplane to pass at MCP with props
synced by looking through your prop and syncing it with the other
planes prop. This produces identical RPM's for both airplanes and with
clean airfilters the power should be the same. With a turbo charged
airplane you must use a very accurate MP gage and carefully check the
induction system for leaks.
By using this system you can measure very small changes and see
easily the speed increases or decreases. You can easily see 1/2 kt
changes (very long passing times).
The bottom line is there is not anything for nothing. Passive BLC
is your best bet. i.e. removing bugs and clean airplane as possible for
speed. vg's, slots, slats, and etc. are for slow speed.
Jack Webb
L 360, L IV
ATP, CFI Airplanes & Instruments, Multi,
& Sea
Many Type Ratings
Experienced Experimental Test Pilot both multi and single engine
Former Chief Engineering Test Pilot for a Major Manufacturer
AeroSpace Engineer BSAE
Ohio
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