I agree about hp flys the plane. Takeoff roll and climb can be
significantly affected- so long as the proper gear ratio and prop is
selected! An interesting tidbit on the GreatPlains web site
www.greatplainsas.com/ <http://www.greatplainsas.com/>
Using the same basic engine hp (103 takeoff, 70 continuous)
62" prop, direct drive = about 215 lbs thrust
72" prop, 1.6 drive = about 350 lbs thrust
84" prop, 2.0 drive = about 450 lbs thrust
For fixed pitch use the biggest dia prop that will fit!! :D
Dan
I have known Steve Bennett owner of Great Plains for years. One sharp
fellow.
To quote his web site:
"The NEW Series 3 Reduction Drive is available in drive ratios
from 1.6:1 up to 2.5:1. Props used vary from 72 inch x 40 inch
pitch up to 96 inch x 56 inch pitch. It greatly improves the take
off and climb performance of any high drag VW powered aircraft
that needs more thrust."
"On a 2180cc engine with a 1.6:1 drive and a 72 inch prop,
you will see about 350 lbs. of thrust. On a 2.0:1 drive and
an 84 inch prop will pull about 450 lbs. of thrust. You get
the idea! (A direct drive VW with a 62 inch prop will pull
about 215 lbs. of thrust.)"
Static thrust is not the whole answer but you get the idea.
Paul Lamar ...No rotor no motor.
-------------------------------------------------------------------------
Dan,
Are the thrust results above an indication that bigger props create more
thrust?
Is there any relationship between the torque multiplication using a PSRU and
ratio too the results as well?
I would think that going from direct drive to a 2.0 ratio would indicate
that the torque multiplication is contributing substantially to the
increased thrust?
What do you think about this analogy?
This is data from the university testing our single rotor 65HP engine with a
2.5-1 reduction.
Here are the latest test results:
Pitch Thrust GPH Max Max HP
Setting 4000 4000 thrust RPM
6....... 215 ... 2.50...370.....5500...57
7....... 250 ... 2.42...400.....5220...55
8....... 255 ... 2.56...380.....5060...52.5
9....... 250 ... 2.75...350.....4870...51.5
10...... 250 ... 3.30...335.....4630...51
11...... 260 ... 4.00...300.... 4330...49
Notes:
1) The pitch setting is an arbitrary number on a protractor provided by the
manufacturer, and represents neither blade angle nor inches of pitch.
Larger numbers mean more pitch.
2) Thrust 4000 is the thrust at 4000 RPMs, averaged over about a two minute
period.
3) GPH 4000 is the average fuel consumption over the same period, while
holding the speed at 4000 RPM.
4) Max thrust is the maximum thrust possible at this pitch setting.
5) Max RPM is the maximum engine speed possible at this pitch setting.
I think we have learned about as much as we are going to from the current
setup, and I suggest that we next turn our attention to the reduction ratio.
The 2.5 to 1 ratio we are currently using was an educated guess made back
before we had any engine or prop test data, and I am quite sure that we can
do better. I wish I could say that I have a mathematical model that will
predict the effect of changing the ratio, but without test data from other
ratios, this is not easy to do. So, I think the next step is to cut some
additional sprockets and run the tests again with the different ratios.
With more data, I believe we can draw some conclusions with regard to
matching the optimum prop speed with the best compromise in engine power and
fuel consumption. When we designed the reduction drive, we chose sprockets
with a simple semi-circular tooth profile so that we could easily CNC
machine additional sprockets to achieve various ratios. While we are
capable of making sprockets here, I am under the impression that your
machining capabilities are superior to ours, and I wonder if it might be
quicker and easier for you to make the new sprockets. If so, I would be
very happy to send some drawings and suggest some ratios to try.
Again, we are very pleased with these results, and we think with some
additional work, the combination of the RotaMax engine and the Kiev prop is
going to be an excellent power source for light sport aircraft. With the
bugs worked out of our test stand and a group of enthusiastic students
returning to campus in a few weeks, I think we are in an excellent position
to work together with you to complete the work required to take this design
from prototype to product.
Don
Eric Barger
RotaMax Inc.
Just a warning. These thrust numbers are for reference only. Do not optimize
for static thrust.
Paul Lamar
Hello Eric,
You ask very good questions!!
Yes bigger diameter props generate more thrust, all other factors, like
hp, being equal, or so I have learned. I first read of this from a book
by Martin Hollman some years ago and have been doing my best to apply
this principle to pracital flying, which was confirmed by testing via
Great Plains. My latest post about prop diameters and PSRU ratios was
to provoke activity about what we should be thinking about as amature
builders when we are designing our planes. As George (down under) said,
we can sometimes lengthen the landing gear!!
As Paul Lamar has stated several times, prop tip speeds will limit RPM.
My original investigation into prop possibilities indicated RPM well
below supersonic tip speeds was prefered for composite props. (also,
our LSA application is for a pusher and I thought a 3 blade would give
less noise when going past the trailing edge vs. a 2 blade) This
question is ongoing, as several people would like a direct drive engine,
most of which specify a wood prop (ie. 2 blade), and my preferance is
for a 72"+ prop (at 3300RPM).
Your investigations into prop pitch settings vs. RPM is very good
engineering. Do continue to investigate and please publish your
findings! There should be a database available for this information.
As to machining of hubs, for basic investigation you are on the right
track. For higher HP, Gates HTD belts may work better. (just DON"T say
that the application is for an AIRPLANE... umm, it's for an offroad
application) Then, when you get the proper ratios worked out, apply
your info to a belt drive or gear drive that will not be a liability
problem... :D
Regards,
Dan
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