Chris, et al,

 

There is a practical side your determination that it may take minutes before a stable airspeed is reached.  Laminar flow takes a while before it gets itself settled down.  I feel sorry for the 300 series Lancair pilot that levels out at his desired altitude and has his desired power set only to keep trimming the nose down for several minutes to stay at altitude as the final cruise speed is approached.  If you have a pretty good idea what the cruise speed will be it is best to accelerate to that speed (either by a slight dive or leave in climb power until there) and then trim - that way it will only require one more final tweak.  BTW, ram induction air (MAP) is also affected until final cruise speed is reached.

 

Grayhawk 

 

Fred,

Very nice write up.

Fanatic? Yes

What timing!

For unrelated research, N91CZ happens to be loaded up with calibrated instrumentation and data acquisition equipment.  The purpose was to generate complete drag polars for different configurations along with some S&C parameters.

  

Some of the points you made can't be emphasized enough when it comes to collecting this sort of data.

 

-Settling times to reach steady state speed (high speed end of the envelope) is longer than what one might think.  It is very difficult -if not impossible- to discern a 1 knot change in speed over the period of a minute - well perhaps with a digital readout in tenths of a knot.  I found two minutes to be too short.  The data showed three minutes to be an acceptable minimum.  Three minutes is also suggested in some test flight texts.  At low speed (<100 KIAS), where induced drag is high, speed converges much more quickly - not applicable however for these full power flat plate drag runs.

 

-Atmospheric disturbances will scatter your data.  If the autopilot is holding altitude yet the airspeed is fluctuating, the atmosphere is not steady or has some vertical component to its movement. It takes some patience to find the right conditions.

 

-The sensitivity to error is indeed very large.  Aircraft Instruments will have some amount of error. Acceptable for use in aircraft operation perhaps, not so much in drag measurements.  MP is the worst I have seen in terms of percentage error.  Next come altimeter and the ASI. 

 

-Altitude must be held precisely.  Descents or climbs of even 10's of fpm will throw the results off.

 

-Unless all your data is recorded automatically, take another set of eyes with you.

 

Now everyone have some fun collecting data!

 

Chris Zavatson

N91CZ

360std

http://www.n91cz.net/

 

 

 

 

From: Frederick Moreno <frederickmoreno@bigpond.com>
To: lml@lancaironline.net
Sent: Friday, January 25, 2013 5:23 AM
Subject: [LML] How Slick Is Your Airplane? (PDF attached)

Make some careful test flights and follow the instructions in the attached paper, and you can make a reasonably good estimate.   Fred Moreno

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