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Not quite that simple, Andrew.
While more rpm (air mass flow) + more fuel = more power is
basically correct, there is more to matching prop load and engine
than that.
I know of an indivdual who was a very smart
individual when it came to electronics (converted auto CPUs for aircraft usage),
but did not understand the relationship between rpm, prop pitch and
thrust. He tuned his adjustable pitch prop to get maximum rpm - He took
off and discovered (sadly too late) that he had very little thrust. He had
sufficient thrust to get airborne but could not fly out of ground effect -
contacted a catus off the end of the runway and died. Maximum rpm is not
the answer. The answer is maximum thrust (at least for take off).
The key is balance -- if you have too little prop load (too little
pitch for example) , you get great rpm - but little thrust. If you have
too much prop load (too much pitch or diameter or chord), you reduce the rpm the
engine can obtain, therefore your power and thrust. So the key is
balance.
I run an older 13B with the 2.85 and an 74x88x5.25 prop -
I get 6000-6100 rpm static at WOT. However, if I take the prop off (no
load), my engine will turn 5000 rpm at the idle setting. So rpm is not the
sole criteria.
Ed
Sent: Thursday, December 22, 2011 5:39 AM
Subject: [FlyRotary] Re: off topic, propeller carving and
paint
whats with assuming max rpm is 6000, I intend to use a CS prop
with Tracy's 2.85 drive so I can rev the renesis out to 7500rpm when needed and
dial it down when not, correct me if I'm wrong but doesn't more rpm + more fuel
= more power. Ernest's drive should be able to withstand 8500-9000 rpm. Way to
go! up there in Lynn's league.
Andrew
On Thu, Dec 22, 2011 at 4:08 AM, Bill Bradburry <bbradburry@bellsouth.net>
wrote:
That is true, but if I was going to go with something else,
I would try to
find something that will turn my prop close to the 2700 that
most props are
designed for. That means that I would look for
something in the 2.2-2.4
ratio range, not 3.17. That ratio will mean
your prop will never get more
than 2000 rpm with the rotary.
Bill
B
Behalf Of Ernest Christley
Sent: Wednesday, December 21, 2011
2:26 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: off topic, propeller carving and
paint
Bill Bradburry wrote:
> Tracy's PSRU ratio is either 2.17
turning backwards (A,B) or 2.85
> turning forward (C) I
think??
>
>
And it is a good product; but, it's not
the only PSRU in the world.
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