2.85 drive takeoff.

Snips...

This would be no problem at all if Tracy went to the 2.85:1

PSRU.

Prop rotates in right direction.

Paul Lamar

This would be a good point for me to get input on interest on

the

2.85 : 1

drive.

It is now apparent that the RX-8 and the Renesis engine are

actually

going

to arrive so I've moved ahead on plans for the 2.85 drive.   I

consider

this

the main reason for using the higher ratio.  I'll be ready to

cut

metal on

the prototype after getting the next batch of 2.18 drives done.

Opinions vary on this but here is mine:  Unless you are swinging

long

prop

(at least 75" or more) or plan to run your engine at 7000 rpm or

higher, I

still think the 2.18 : 1 drive is the best compromise.  This is

especially

true if your airplane is in the 200 mph or more range.  The only

exception

to this may be gyros or other low speed aircraft where very wide

chord or

multi-blade (4 or more) props can be used.

You don't have to commit to buy now but I'd like to get a

reading on

how

much interest there is in this ratio.

Tracy Crook
tcrook@rotaryaviation.com
www.rotaryaviation.com

What ever turns you on Tracy :-)

IMHO the current engine can also benefit from the 2.85:1 ratio with
the current diameter props. There is nothing fundamentally

different

about the structure of a  Renesis engine that somehow makes it
better for 7000 RPM. 9000 RPM no question about it...it's better!

I'm already on the list for Tracy's 2.85 (double check that Tracy).

My

13B

is rigged for those RPM, I figure a 6000 rpm cruise and 8500 full on

with a

three or four blade fixed pitch prop (LAEV when it's ready).  This

should

make for an easy 175 kias cruise in the RV-4 and 210-ish flat out.

Personally I like the idea of an easy cruise turning the prop at

running 160 kts.

Mike McGee, RV-4 N996RV, O320-E2G, Hillsboro, OR
13B in gestation mode

If you use a fixed pitch prop on a slow turning out put shaft you
will go from somewhat stalled prop on your take off roll to
hopelessly stalled. Slow props on fast planes almost beg
for the controllable "high priced spread".

I also will state again - that I have not seen dyno data that supports
increased horsepower at 7000 rpm for a modestly modified
13b engine. Vance J

Yes but it will become unstalled at some point in the takeoff run.
Just because it is stalled does not mean it has no thrust.
Besides, you don't have to use full throttle at the beginning of the
takeoff run. Feed the throttle in slowly.

VJ - Stalled does not mean that you have to reduce power - stalled
means that your thrust per horsepower efficiency is in the pits,
the prop bogs the engine down to limit your produced power
and most of that power is being reduced to wasted turbulance.
At this level it will not unstall on the roll, but if you are lucky
and make it over the trees, it will probably start unstalling at
climb speed.



That sounds plausible to the layman but I don't buy it. When wings
stall they don't lose all lift. When props stall they too
don't lose all thrust. Do you have any numbers to back up
this claim. My guess is a prop blade stalled at 45 degrees has
a lift and drag coef. of one.

Von Doenhoff page 3 says a typical wing stalls at a lift coef 
of 1.7 and 17 degrees. The lift coef is still 1.5 at 24 degrees
while the L/D goes from about 13 to 5. I would think
the engine would bog long before the blade L/D got down to 5.  

If the blade segment that is two foot out on the prop radius
is running at a stalled angle of attack of 24 degrees (pitch angle)
while the airplane is stopped and the prop RPM is 2000 (418 FPS) 
it will run at an angle of attack of about 12 degrees at 60 MPH 
(88 FPS) and at the same prop RPM. In other words about 12 degrees 
angle of attack less than what it had when the aircraft was
stopped and the blade stalled. Check my arithmetic.

No doubt it is inefficient. That is not going to bog the engine
down to a stop. 

When I flew with Ian in Perth with his 200 MPH RV6a and his
2.85 gear box I noticed no decrease in take off performance
compared to Tracy's airplane.

Ian how far do you accelerate down the runway before you
break ground. What is your initial climb rate?

BTW Vance were you not the guy that said there was no advantage 
to the constant speed prop? :-)

More discussion of your longer message later.
One thing at a time.

Paul Lamar
 
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