Winter / Focus / Prop Drag

Mon Dec 8 13:27:48 AKST 2003

Larry,

I agree that it would be interesting data to have - probably just enough to
validate the aerodynamic theory.  Whether or not it is necessary to have the
data - well, I've done a lot of qualitative testing and have developed a
very good feel for what works the best (it's the 3 blade on the front of my
plane <G>).

One of the best ways I found to test props -
- straight and level, "cruise power" - pull vertical, go to full throttle,
start rolling ("normal rate" - not fast).  While it may be hard to judge the
speed of the plane, the roll rate is easy to judge and will give you an idea
how much (if any) the plane is slowing down.  I usually do about 5 rolls
up - and then a huge half loop (using the throttle as if heading for a
downline in a sequence) and establish a vertical downline - and start
rolling again - and again, watch for changes in the roll rate.  You can also
compare rolls up/down to get an idea of how "constant" the prop is.  You may
not see much difference between a 16.5-12W and a 17-12, but I guarantee you
will see the difference between a 17-12N and an 18-10 (and a huge difference
if you run either the 15.5-12  4 blade or a depitched 15.75-13 3 blade).

>From my testing with 2Cs - I would say downline braking on a 16-13 at 2000
RPM is the same as downline braking on a 16-14 at 1700-1800 RPM.  A 15.75-11
3 blade offers slightly better braking than a 18-10 2 blade (more difficult
comparison because planform of the blades and airfoil thickness is more
varied).  I suspect the math would support these ideas - as long as enough
of the variables (engine characteristics, prop airfoil, prop planform,
airframe specifics, etc) could be plugged into the formulas.

So far as anyone doing any comprehensive field testing - I believe that is
actually a course requirement for graduation from the Mike Klein School of
.................     Troy???

Regards,

Dave Lockhart
DaveL322 at comcast.net

----- Original Message -----
From: "Doug Cronkhite" <seefo at san.rr.com>
To: "NSRCA" <discussion at nsrca.org>
Sent: Monday, December 08, 2003 5:43 PM
Subject: Re: Winter / Focus / Prop Drag

>
>
> None of this is really necessary. Good solid aerodynamic theory is all
> that is required. I have a paper written by George Hicks that will explain
> a lot of this if you're mathematically inclined. I'll send it when I get
> home. The short version is a 10 pound pattern aircraft turning a 17" prop
> at max drag rpm will be about 15mph slower than a stopped prop. There is a
> margin for error depending upon parasitic drag of the airframe itself as
> well. What that max drag rpm actually is however is unknown (to me
> anyway).
>
> More info when I get home.
>
> -Doug
>
>
> On Mon, 8 Dec 2003, Larry Diamond wrote:
>
> > This has been a very interesting thread on Prop Drag Vs Idle..
> >
> > Couple of thoughts...
> >
> > 1) From a non-engineering mind or non-aerodynamics mind, Stop Vs Idle
seems
> > to be about the same as a motor vehicle. The vehicle slows better when
the
> > tires are not in a skid.
> >
> > 2) If there is enough interest there is a way, not only to prove
> > this theory, but show it's predictability with Prop Disc Size, Idle
speed, #
> > Blades, and to a limit that nature allows, Temperature and Humidity.
Also to
> > show the relationships on how one effects the other or how a subset
effect a
> > single item, or etc...
> >
> > The question is, "Who is willing to take this on?"; and, is it of any
real
> > value
> > for us to understand. For my skill level the answer is "no". For the Top
> > Guns and Designers of Pattern Aircraft, this may be some interesting
data to
> > have.
> >
> > Here is what I think the experiment would take.
> >
> > A) A committed top pilot and caller (Troy N., Dave L., etc...) that
feels
> > this is needed data and is committed to flying the same exact way, with
> > precision, to gather the best data. Precision would mean time Vs
throttle
> > setting, altitude, attitude, etc....
> >
> > B) Flight monitoring system that can down load data to a PC for
collection
> > and archive. Accuracy and repeatability is critical.
> >
> > C) A team of folks that can work together to collect the data and keep
the
> > pilot performing the sequences the exact same way every flight and every
> > attempt.
> >
> > There is a scientific process that (if successful) will not only allow
for a
> > single solution, but will also offer a formula for a given platform by
> > changing a predetermined set of variables. The formula will also be good
for
> > parameters beyond what was tested.
> >
> > The key point here is, "This is a long tedious process and the level of
> > detail
> > required, not only to task but also documentation, is very intimidating
to
> > most." However, if done correctly...It works with very predictable
> > solutions...
> >
> > The down side...
> >
> > The formula will not be the same for every plane, even if it's the same
> > model
> > with the same set up built by the same person with the same jigs and
tools
> > and using the same fuel. However, the relationships should hold true for
> > similar airframes.
> >
> > Upside...
> >
> > We would know that it's a scientific based conclusion and can offer
> > what has the most effect, # Blades, Idle Speed, etc...
> >
> > Depending on the desired output, it can take from days to literally
months.
> > The more complex, the longer it takes.
> >
> > I would be willing to offer some assistance and guidance, but would not
> > tackle on my own.
> >
> > If you think you might be interested in this project, let me know off
the
> > list.
> >
> > Larry Diamond
> > NSRCA 3083
> > AMA 5024
> >
> >
> > ----- Original Message -----
> > From: "Del Rykert" <drykert at rochester.rr.com>
> > To: <discussion at nsrca.org>
> > Sent: Thursday, December 04, 2003 7:59 PM
> > Subject: Re: Winter / Focus / Prop Drag
> >
> >
> > > If you are looking for irrefutable proof go up in a full scale or for
R/C
> > > take a clean design with low idling engine up and do some downlines.
Bump
> > > you idle trim up on some and leave it as low as you can on others.
Will be
> > > more pronounced with 4 cycles turning big side blade props.
> > >
> > > For more proof..  Ever see what happens to a model that kicks back up
in
> > > the
> > > air but the prop stays on windmilling...  You can't fly it any more
except
> > > like a ton of bricks.. If that doesn't convince you nothing will. I
flew a
> > > friends model that it happened to. It only broke the prop on landing
in
> > > weeds as it landed on its nose.
> > >
> > >      Del K. Rykert
> > >      AMA - 8928
> > >      NSRCA - 473
> > >      Kb2joi - General
> > >
> > =====================================
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> > #
> >
> >
>
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