Tracy Crook wrote:

I used the RV4 drawings that I borrowed to draw up
an RV4 motor mount.

Boy that was a tight squeeze. Without those cowl cheeks
on there I don't know how I would get a rad under the cowl
and still get the air out from behind it in a reasonably
low drag way.

The other weird thing about this is the way Van took the
aft bending loads from the landing gear. You need to run two
tubes from the tops of the landing gear out to each
rubber bushing tube cluster to take these loads.

One of the things taking up so much room in there is that
foot box for tall people. If you are short you might be
able to get the engine in there with out extending the
cowl as shown. Yes the PSRU is the 14.5 inch version.
Perhaps you can notch the foot box in the center
to clear part of the engine.

The muffler will be a tight squeeze too.

Welcome to the real world of putting this stuff in actual airplanes!  :-)
The job is even tougher than your drawing indicates because the cowl tapers
rather quickly toward the prop hub.

I'll throw this out for what it's worth.  I think the importance of
eliminating the cowl cheeks  is highly overrated in terms of drag reduction.
I say this for two reasons:

1.  They don't really add much (if anything) to the frontal area of the
airplane.  The cowl spreads out to meet the fuselage anyway.   The reduction
in parasitic skin area drag from eliminating the cheeks is insignificant at
200 mph.

2.  This one is more complex but it was sumed up in a conversation I had
with an aerodynamics engineer.  He said "It is not so much how you  *meet*
the air that is important, it is how you leave it".    Those cheeks are
close to the theoretical optimum teardrop shape which leaves the air in the
best possible way (relatively undisturbed).  One example of this principal I
read about was a study on drag reduction for semi trailer trucks.  The
bottom line was that you could streamline the front end of the truck to look
like the Concord but it would make little difference in aerodynamic drag.

I was sitting in Dr. Paul Macready's office back about the early 1970's
or 
so and he and Dr. Peter Lissiaman(s?) were having that same argument. 
Front verses back on trucks. :-) 

The truth is they both matter. I'll bet Dr. Lissiaman's name is on that
study Tracy. It is not for nothing they streamline the front of trucks
these days. It pays off in MPG. There is very little practical 
possibilty of streamling the back of trucks or they would do 
that too.

 

Note that the wider  RV-6 pays very little penalty in top speed (compared to
the -4) even though its frontal area is significantly higher.

As for those who object to the cheeks for esthetic reasons, well, beauty is
in the eye of the beholder, so cut'em off if you like.   The Powersport
custom aluminum cowls were beautiful but I think Everett said it took 9
months to get them built.

BTW, Alan (Tolle), I'm definitely not an expert on aerodynamics so I'd like
to hear your thoughts on why eliminating the cheeks would help
significantly.  I'm always happy to be proven wrong because that is the main
way to learn something new.

On another subject,  I've been working hard on the gear drive for the past 6
weeks and have made a lot of progress.  RFQs for tooling & castings are
finally out for bids. I'll write up a summary on it and post it here.   My
website is also under construction and will be online soon.

Tracy Crook
 rws@altavista.net

The only historical data we have on cowl cheeks suggests that the prop 
works slightly better without them. I completely agree with you on your 
RV4 frontal area argument. They add little to the overall drag.

As far as RV6 verses RV4 frontal area I would say that is worth close
to 10 to 15 MPH.

Given the difference in engines, Tracy Saylor's Rv6 is going 230 MPH or
so
while Dave Anders Rv4 is going about 250 MPH. Both have almost identical
air cooling systems and Dave has a fast back which he claims did not
help all that much. The rest of the aero tricks are the same.
Dave Anders has around 245 HP and I would guess, Tracy close to 200.
That HP difference alone should be worth around 10 MPH.

BTW while I am on this subject I was thinking of why race?
Less drag translates to speed which translates to MPG which 
translates to range.... all other things being equal.

Paul Lamar
 
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