Cherokee II Performance

update - all number crunching is pretty much complete.
Here's what i have written elsewhere. all plots are available at:

http://picasaweb.google.com/abcondon/CherokeeII#

I have many other plots in my monster excel file. If you're curious about
other performance characteristics, let me know and ill share.

Max L/D came out to 23.5:1 at about 49 mph. First plot is the L/D vs speed
curve. This was pretty easy to get to, just took speed and descent rate
info straight from my flight test.

To gather the data I mounted a digital video camera behind the pilot seat,
trained on the instrument panel. Also had a good view of the outside
world.

Next is the actual descent rate vs. speed. The 'Polar' for us glider
junkies. Minimum sink is about 165 fpm at 42 mph or so. Also direct
measured data. The really cool thing is that this matched up really well
with theoretical data provided by Stan Hall in the design documents.

From there, I started to come up with some numbers. I used the lift
equation (L = 1/2*rho*V^2*S*Cl) to come up with a Coefficient of Lift
value at each speed. Similarly used Drag equation (D = 1/2*rho*V^2*S*Cd)
and the L/D value to find Cd.

Theres an alternative (simplified) equation for Cd.

Cd = Cdo + Cl^2/(pi*e*AR)

e is span efficiency factor (1 for an elliptical wing, a la Spitfire) and
AR is aspect ratio.

Cdo is the coefficient of drag at an angle of attack of 0.

I plotted a huge table of Speed vs. e, finding the Cdo value at each
combination. From flight performance, at minimum sink speed, Cdo should
equal 1/4 Cd. So i generated the next really pretty plot, and found which
e value provided a Cdo that equaled 1/4 Cd for that speed. that e value
turned out to be .905

so i re-calculated Cd using the Cdo and e value that I had found. It
matched up fairly closely.

I also wanted to figure out some angle of attack data. I used a basic
stability equation to do so. That is:

Cl = Clo + dCl/dalpha*alpha + dCl/delev*elev

Clo is Cl at 0 angle of attack. dCl/dalpha is the slope of the Cl vs
alpha line. dCl/delev is the change in Cl with change in elevator angle.
and elev is the elevator angle.

I had meausured the stick position during flight and was able to associate
that with an elevator deflection angle. This gave me the last two terms.
I got the change in alpha from my video, by measuring the change in pitch
attitude between various speeds, and subtracting off the change in flight
path angle from my L/D numbers. I then divided this into the change in Cl
between the speeds to get the dCl/dalpha value. So i knew everything but
alpha and was able to solve for it. Now i could convert speed to alpha.
All of this is 1G level flight of course.

So next is my Cl vs alpha plot, also plotted is the Lockheed plot.
difference between a 3D wing with a fuselage and a 2D code, i suppose.

Also the Cd vs alpha plot, with both methods. I think the measured one
runs a little higher because it includes the drag from the fuselage.
makes sense.



-Tony Condon
Cherokee II N373Y

I got ahold of a digitizer, and brought in the polar for the Cherokee from
"New Soaring by the Numbers" Compared it with the polar that I got and
they match very well. Keep in mind that I was flying at a weight of 600
lbs while Stan Hall provided the data based on a 530 lb flying weight.

http://picasaweb.google.com/lh/photo...6o-nPKhw-4yjtg

Also digitized the L/D vs speed plot that he had in an information packet
that I have. That plotted up very closely to what I found.

http://picasaweb.google.com/lh/photo...OCw5fIOmi4sdew


-Tony Condon
Cherokee II N373Y

Here are my slides for my presentation:

http://www.slideshare.net/tonycondon...t-presentation

Ill let you know how it goes, then I get to start writing a paper about
it!


-Tony Condon
Cherokee II N373Y

Presentation went very well. I managed to cram everything into about 15 or
20 minutes. No one had any questions which probably means everyone was
confused beyond belief. I think there's real promise in expanding the
technique to get simple glide polar measurement to the masses.


-Tony Condon
Cherokee II N373Y