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Bryan,
The AoA will decrease as the prop is moved thru the air in a forward
direction. This may be a little difficult to visualize but think what would
happen if the plane was moving forward and the prop was not rotating. The
air hitting the front of the blade would have to do something to get around
the prop blade. Some of it would flow across the top of the blade and some
of it would roll around the leading edge and flow across the bottom of the
blade. This would produce a flow of air that was oriented to the direction
the blade would be moving if it were rotating. This same flow happens when
the blade IS rotating and the plane is moving forward. This is what causes
the AoA to be reduced.
Bill B
-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
Behalf Of Bryan Winberry
Sent: Thursday, March 04, 2010 3:28 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Vance Jaqua and Propellers
Thanks Ernest,
Can you splain me one thin. How does the AoA change on a fixed pitch prop?
Bryan
Disregard previous empty reply
-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
Behalf Of Ernest Christley
Sent: Thursday, March 04, 2010 3:01 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Vance Jaqua and Propellers
Bryan Winberry wrote:
>
> Ed,
>
> You're losing me here. If I understand you correctly, you are taking
> off with lower prop RPM now compared to the original prop? But the
> RD-1C allows the engine to produce more HP(via more rpm). Isn't the
> goal to turn the prop just short of its' max RPM for T/O? Come to
> think of it, maybe that's your point. Do you know the prop rpm's for
> the two scenario's?
>
> Bryan
>
Bryan, wouldn't it be great if you were only trying to optimize two
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 has
higher AoA and tries to gulp more air.
AoA is tied to RPM. Take little bites of air, and you can take them faster.
Try to take to big of a bite, and you don't get any (the blade 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 bites,
the air has to move through faster.
AoA is tied to the airplane's speed. As the plane accelerates, the prop
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
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
speed. So, you shorten the blade and increase the pitch. You end up
absorbing the same Hp, but the second blade absorbs more at a faster speed.
You want a cruise prop. Maybe start with the target RPM for the engine
on the takeoff role. This will translate to a certain Hp, that goes
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
would know this, because using a short field technique I would apply
full breaks at the end of the runway until the engine came up to speed.
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
stall). At cruise speed, the blade will be in a drag bucket (most
efficient) at my cruise RPM. I'll modify the blade length to absorb
whatever Hp the engine is producing at takeoff.
If you follow this thought process for real, and bolt the result to the
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
(minus potty breaks) to see the effects. In any case, there isn't *A*
goal. There are a bunch of goal*S*, with each one starting from a
slightly different initial set of circumstances. I think someone smart
once said, "It depends." 8*)
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