Yah all be careful out there! Actually flutter can rear it's

ugly

head

at a

lot
lower speeds than 300. A Duster sailplane suffered a nasty crash

after

losing the vertical at less than 90mph. Also, if you lose a

blade at

virtually
any time, you do not have enough time to shut down the engine

before

it

is ripped from it's mounts. The good side to this is that well

designed

composite construction has better flutter resistance than almost

all

other systems,

Speaking of 6000 rpm props, I promised to look at the difference

between

constant speed and fixed props in this range, and the results

should

be

below. this was for a near stock Mazda with 160 bhp at 6000 rpm

for

a 200mph capable plane on that power. The fixed prop was 34 inch
dia with about 42 in pitch. The constant speed was simulated by

the

same blade area with the pitch changed to match speed. This is

for a

clean installation, with free flow - interference from the

fuselage

can severely degrade these predictions. The results are not half

bad

for this little prop, and the gain from varible pitch appears

less

than

what one would expect at a lower disk loading.
Vance J

Vance,

Thanks for looking at this problem. The real issue to me is whether

the

engine produces enough torque to spin the fixed pitch prop up to

speed

at

low forward speed. You would have to look at the power required vs.

power

available to find the operating point of the engine. Even if the

engine

CAN

produce 160 hp at 6000 rpm, it may never get there because it does

not

have

the torque to spin the prop to that rpm. As the forward speed

increases,

the

operating point moves up the curve and you eventually get there. If

you

took

this into account, then it looks like a small fixed pitch prop is

definitely

the preferable simple solution.

Monty

Nix that. Brain fade. That only works for big props. I looked at some

prelim

designs. The biggest problem is that the prop winds up being stalled

over a

large percentage (like all) of the blade at static conditions. There is
plenty of power to spin it up. It just stalls and stirs up the air. The
thing doesn't "hook" up until at least 100 mph. That doesn't give me the
warm fuzzies. Maybe I am looking at this wrong, but my preliminary

numbers

don't look promising for a fixed pitch version. Large portions of the

blade

would be stalled at climb speed. What's the secret? How did you arrive

at

the graph? Give us a good design treatise here Vance.

Monty

Do I have the source code to this program Vance?

PRPLRIT3.BAS
Prplrit3.bas
PRPLRIT5.BAS
prplrit5.bas
Prplritf.bas
Prpsiz.bas


Paul Lamar

Yes - but I do not remember why there is a 3 and a 5 the version that I have
been using is just plain PRPLRIT and it has some engine curves in it
I think I gave you truncated vesions to make them more self explanitary
the prpsiz is for a first rough cut sizing and the iterative programs step
through rpm and flight speed , predicting thrust and power required
The constant speed is not handled automatically - you just keep
running narrow speed ranges with pitch changes at fixed blade
widths, and smooth out the steps - the theory behind this
program is in my web page vjaqua.tripod.com
Vance

Monty, can you handle BASIC?

Paul Lamar

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