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Date: Thu, 04 Mar 2010 15:00:58 -0500
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Vance Jaqua and Propellers
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Bryan Winberry wrote:
>
> Ed,
>
> You=92re losing me here. If I understand you correctly, you are taking =

> off with lower prop RPM now compared to the original prop? But the=20
> RD-1C allows the engine to produce more HP(via more rpm). Isn=92t the=20
> goal to turn the prop just short of its=92 max RPM for T/O? Come to=20
> think of it, maybe that=92s your point. Do you know the prop rpm=92s fo=
r=20
> the two scenario=92s?
>
> Bryan
>
Bryan, wouldn't it be great if you were only trying to optimize two=20
variables.

Hp is tied to RPM. The faster you turn, the more Hp you get. The more Hp =

you get, the faster you turn.

Blade pitch is tied to AoA (angle of attack). Add more pitch, the blade=20
has higher AoA and tries to gulp more air.

AoA is tied to RPM. Take little bites of air, and you can take them=20
faster. Try to take to big of a bite, and you don't get any (the blade=20
stalls).

Blade length is tied to Hp. It takes power to move that cylinder of air. =

The bigger the cylinder, the more power required.

dV (acceleration of the air) is tied to AoA. If you're taking bigger=20
bites, the air has to move through faster.

AoA is tied to the airplane's speed. As the plane accelerates, the prop=20
blades AoA flattens out.


Now, let's go through the optimized blade design process.

You want a climb prop. You want maximum Hp right off the line, so pick a =

high RPM for the static tests. Pick the longest blade that will fit on=20
your plane, and pitch it to absorb that Hp that is produced at that RPM. =

Now the blade will have much to little AoA at the chosen cruise RPM and=20
speed. So, you shorten the blade and increase the pitch. You end up=20
absorbing the same Hp, but the second blade absorbs more at a faster spee=
d.

You want a cruise prop. Maybe start with the target RPM for the engine=20
on the takeoff role. This will translate to a certain Hp, that goes=20
through your reduction gear of choice to give a prop RPM. I would pick a =

blade pitch such that the prop would be just barely past stalled. I=20
would know this, because using a short field technique I would apply=20
full breaks at the end of the runway until the engine came up to speed.=20
When the ASI comes alive I'll note that the the RPM begins to drop off a =

little (the forward speed drops the blades AoA lets it move out of=20
stall). At cruise speed, the blade will be in a drag bucket (most=20
efficient) at my cruise RPM. I'll modify the blade length to absorb=20
whatever Hp the engine is producing at takeoff.

If you follow this thought process for real, and bolt the result to the=20
front of your real plane, it might be the last thing you do.
If you spend $50 on a copy of Xplane, you can play with it all day long=20
(minus potty breaks) to see the effects. In any case, there isn't *A*=20
goal. There are a bunch of goal*S*, with each one starting from a=20
slightly different initial set of circumstances. I think someone smart=20
once said, "It depends." 8*)