Drag Reduction

[Gary Gustafson]

Thanks Walter for your further explanation of your comparison. I
can see where a 54 or a 56 pitch Sensenich prop is called for
with floats and that the recommendations provided by Sensenich
are for Rebels on wheels. I also agree that 2" can make a big
difference. Thanks!

I agree entirely with Walter that a side-by-side comparison is the
best way to compare performance. However, the following reference
material should give one a good idea as to where to reduce drag
(particularly if you don't want to land shorter) and get better
performance and more efficient operation at the same time.

Drag reduction can produce considerable gains in performance. The Rebel
is a "draggy" in its original design abd with bungee cords. It will
never be a high performance plane. If we wanted a high performance plane
we would have bought a Venture or an RV. Obviously we were looking for
other characteristics and made trade-offs. However, that does not mean
that improvements can not be made. I first became interested when I
read about Steve Sloan and the performance improvements that he claimed
to have achieved after making drag reduction modifications on his
Rebel. (I recall him saying that he saved over 1 gal an hour or
increased his speed by 7 mph) That intrigued me so I started reading
about drag with the following two articles being the most significant.

The subject is discussed in NACA Report No. 485 titled The Drag of
Airplane wheels, wheel fairings, and Landing Gears. It was written in
1935 (all aerodynamics research at that time is entirely applicable to
the Rebel and physics has not changed much since then) and a copy in pdf
format can be obtained at:

http://naca.central.cranfield.ac.uk/rep ... rt-485.pdf

Figures 24 and 25 indicate that the total drag reduction at 100 mph for
the landing gear design similar to that of the Rebel with coil spring
gear can be reduced from 98 lbs to 27 lbs and it provides information as
to the fairings, wheel pants and fillets that enable that reduction.
Remember that drag increases with the square of the velocity or going
From 100 mph to 120 mph one would have to multiply the figures by 1.44
to get the effect of the drag at the higher speed.

The second is a book which can be obtained at www.aerodrag.com for
free. Just click on down load whole book for free. In it, it explains
the various things that cause drag and the impact of that drag. Based
on these two reference items, I was able to determine the following (in
theory but most likely true). This fellow has based his findings on
actual airplanes and spent much of his life studying drag in airplanes.

Total Drag of the Rebel at 120 mph is 375 pounds

Drag associated with the gap between the flaperons is 7 pounds
Drag of coil spring gear is 94 pounds and can be reduced to 35 pounds
for a reduction 59 pounds,
Total drag of the tail wheel and cross bar is 28.8 pounds which can
be reduced to 18.2 pounds for a saving of 10.6 pounds.
Putting an aerodynamic shape on the two tail supports will save another
3 pounds in drag.

So these 4 items will save close to 80 pounds of drag. This is on the
conservative side a 20% reduction in drag (if the airfoil shapes are
exactly right). That means that your engine will be pulling 89 pounds
less drag through the air and that results in increased speed (7 miles
per hour) or lower fuel usage at the same speed (about 1.1 gph). You
can climb out faster (about 7%). You can glide further (sbout 10%).
Sounds interesting.

Now Walter is right. One has to prove it. After I get my plane flying
(shortly) I will get the numbers and then start to install the above
indicated modifications. I have not proved that this is true. However,
Steve Sloan indicates these ranges of improvements. I recommend that
those who are interested read the referenced article and book and
satisfy yourself that this is possible.

It might be interesting to have Steve Sloan chime in as to what he
actually was able to do.

Gary Gustafson Rebel N52GG (242R)

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Drew
Dalgleish
Sent: Tuesday, January 20, 2009 6:56 PM
To: rebel-builders@dcsol.com
Subject: RE: [rebel-builders] Prop selection


Different prop makers use different methods for calculating pitch so it's
very hard to compare props just using those numbers. Interesting drag
calculations I had no idea that the spring gear made that much difference
to total drag. Sounds like using streamlined tube for the gear legs would
make a noticable difference.
Drew






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