REFLECTOR: Snake Oil or the Best thing since Orville & Wilbur?

[KeithHallsten]

Congtatulations, Rob!  You have once again posted the definitive word on the
matter under discussion!  Your post makes complete sense; the difference
between "bluff bodies" (like balls) and nicely faired bodies (like wings and
prop blades) accounts for why we dimple golf balls but not wings.
Keith Hallsten

----- Original Message ----- 
From: <robajohnson at comcast.net>
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Sent: Tuesday, July 06, 2004 8:37 AM
Subject: Re: REFLECTOR: Snake Oil or the Best thing since Orville & Wilbur?


> >>Has all of my sanding been counter-productive?<<
> No.
> I first flew in primer. Not even sanded, just shot the primer and flew.
That primer was rough enough that when I tried to wash the plane it would
snag the wash rags and tear little strings off them. It was harder getting
all the little strings off the plane than the dirt! :-)
> Later on we wet sanded the primer and smoothed it out and that alone was
good for 15 mph. I was shocked! I would have sworn all that roughness was in
the boundary layer and therefore irrelevant but it seems I was wrong.
> THEN I remembered an old magazine article where CAFE did a test with a
Mooney a few years ago. They took an old sun faded plane, took it up for a
speed run on a closed course then landed and had a bunch of people wax it
real fast "pit-stop" fashion so that wind and temps would not change then
took it up and reran the same course again. The wax was good for two knots!
> Smoothness does count!
> About dimples- I read a "Technicalities" article by Peter Garrison in
Flying Magazine a while back. He talked about the golf ball dimple thing.
> Wind tunnel testing showed that on objects with blunt trailing edges
dimples help enormously. e.g. a golf ball in flight. Being round creates a
partial vacuum behind the ball. That vacuum translates into drag. The
dimples help to keep the flow attached to the ball all the way around to the
back side thus reducing the vacuum in the wake of the ball. So dimples do
remarkable things for balls.
> However, the whole purpose of streamlining is to eliminate as much of the
vacuum or drag as possible so wind tunnel testing has shown that airfoil
shapes get very little to no drag reduction from dimples.
> Someone else mentioned VG's. VG's and dimples do the same thing, keep flow
attached. VG's really help at high angles off attack and lower your stall
speed but they don't make you any faster. IOW they don't reduce drag in
cruise flight.
> So my first response is "snake oil".
> However, I always enjoy mental experiments- I call them brain farts- so
lets play a game and combine what we know
> Dimples or VG's can reduce your stall speed by keeping flow attached so IF
the "wing" of your prop blade was stalled then dimple tape could help that
stall?
> The only time I can visualize a stall on your prop is if you had a fixed
pitch prop very highly pitched (a speed prop vs. a climb prop). Too much
"bite" at low airspeeds and it's stalling due to the high AOA and greatly
increasing the take off run. e.g. the Super Marine sea plane racer that the
Spitfire evolved from had a VERY high pitch fixed pitch prop. It had a five
MILE take off run!
> So carry that a step further, with dimple tape on the prop you could
conceivable put a faster, higher pitched prop on your plane without
degrading your take off run and that would make you faster in cruise.
> We have to work pretty hard but we can make this dimpled prop thing work
out though huh? :-)
> But just put dimples on the same prop and you go faster? I can't see that
happening unless you had a really, really crappy prop with a big thick
trailing edge to start with.
> With say an MT constant speed, no way are dimples gonna help. It has a
very thin trailing edge and it's not stalled in the take off run.
> IMHO- Snake oil.
> DM Rob
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