George Lendich wrote:
Tracy tested my idea for prop cuffs on his new wood propellor
with good results. Faster top speed and better cooling.
If you have an IVO prop these cuffs will make it much better.
They also should strengthen the IVO blades slightly.
Paul Lamar
Paul,
So what is the reasoning behind these prop cuffs - I can't remember
seeing
anything about this before. Another senior moment for me?
George (down under)
The inner area of the IVO prop near the hub is flat. This means it goes
into
a negative angle of attack and generates counter thrust at 150 MPH on up.
The airplane runs into a brick wall somewhere between 180 MPH and 200 MPH
depending on the diameter of the spinner. These cuffs create a high
positive angle of attack near the hubs where it is needed.
Paul Lamar
Old message
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Subject: IVO props
Date: Mon, 03 Apr 2000 07:13:48 -0700
From: Paul <rotaryeng@earthlink.net>
One of the few good (maybe the only) upside to Chuck's crash
is; we now have a broken IVO prop blade to dissect.
This is a cross section drawing about one foot out from the hub.
Looks like a Clark Y more or less. The pitch angle at this point is
only
20 degrees or so which is way far short of what it should be. Most
props in
this region of the blade are 45 to 60 degrees. Consequently it would
take about a three foot diameter spinner to turn this prop into
something
useful and then it would have trouble absorbing 200 HP. You could,
on the other hand, fabricate enormous foam and fiberglass cuffs to
fix the totally inadequate pitch angles near the hub.
If you use this prop on a high speed airplane as is you will run into a
brick wall somewhere around 180 to 190 MPH. Inner portions of the
blades will go into a negative angle of attack absorbing all your
horsepower and canceling what little thrust is being generated
by the outer portions of the blades.
Note this is a variable TWIST prop and not a variable PITCH prop.
Twisting the blades on this prop does little or no good. Only the
outer portions of the blades are twisted and that is not what is
needed. The inner portions of the blades must be drastically
increased in pitch to make this an effective prop!
Paul Lamar
I have looked at these briefly upon occasion and my impression
is the same as Paul's, way too little alpha in the inner 1/2. Another
serious issue with faster planes is that when tip mach # hits about
0.65 to 0.7 for a Clark Y type foil the C sub L max drops
dramatically at the same time the Cd increases dramatically - L/D
rapidly goes all to heck -- more HP but no more thrust. IIRC the
IVO section is way too thick (my memory says about 15%) and
has too much local acceleration of the flow, it hits local critical mach
# way below the speeds that the thinner 16series sections can still
use.
Tip Mach # is the vector sum of circumferential velocity (rpm
and diam) and fwd velocity. Also Mach 1 decreases with
altitude, down about 3% at only 8K MSL. Keeping the tips below
0.82 Mach for even a very thin (6%) blade is important, and thicker
blades need to work at much lower Mach #s.
Just went and looked at the picture that Paul sent and the section
is even thicker than I remembered - 18% !! My tentative design
has this thickness at the half radius point or less.
IIRC these props appear to be best suited to the Kitfox class with a
high
ratio PSRU (moderate prop RPM) so tip vel and fwd vel are low. For
this app, it should probably be a reasonable prop. As both prop RPM
and fwd speed increase, the best prop design shifts away from this type.
No offense to the IVO, it just doesn't seem to be a good match for
fast A/C with high prop RPM and large prop diam. Please correct me
if my memory is faulty.
Bill
Robin Ream wrote:
For those of us who haven't been at this too long, what propellers are
available in the 200 - 300 HP range that the average Joe punching a time
clock can afford, and do the faults listed for the IVO design also apply to
those of us building pusher canards? Does the inner part of the blade
matter as much if it's basically got an entire fuselage acting as a
stationary spinner?
Thanks for the help,
Robin
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