Re: Direct Drive Considerations
- Changed the title to Direct Drive Considerations.
Quoting from Jeron Smith;
"Prop and possible resonance issues are a black art that require very
specific engineering expertise and equipment to isolate. It is
totally different than even large but steady inertial loads."
First, it is not my intent to debate Jeron on this issue, but I see
time and time again the reference to crank and drive torsionals as the
"black art". I sm retired now, but spent nearly 20 years dealing with
drive train torsionals in one form or another. What follows is a brief
discussion of the issue to try to convince you, the reader, that this
is not a black art, but one that can be approached very methodically
by even the home developer with little expense.
Most important is to recognize that there are only two questions to be
answered to provide all the information necessary to determine if a
potential torsional resonance problem may exist.
1. What frequency does the system want to vibrate at?
2. What is going to try to force it to vibrate at that frequency?
Thats it, thats is all we need to determine, and we do NOT need the
engine to be running to determine either answer.
How to answer Question 1.
Remove the drive system from the engine, in our case, the flywheel,
crank, and driven load, ie, the prop. Assemble it, and hang it from a
rope so it is isolated. Now with any device you can imagine, impose a
vibration input on this assembly. Suggest a small variable speed
motor with an eccentric weight. This can be very small, ie, a motor
from a toy car.
Run the motor over the speed range and determine the freqency where
the assembly vibrates the most. Use a hand held tach to determine the
equivalent rpm where that occurs. A vibratach will work well here.
Now we know where in the freqency range the assembly wants to vibrate.
And we have answered question 1.
How to answer Question 2.
This is all done without test, simply determine the engine speeds that
you will be operating at and then determine the prop blade pass
frequency, If you want to run at 3000 rpm the blade pass frequency is
the number of blades time the engine speed.
To get the final result, simply compare the frequency from Question 1
to the speeds of question 2. If they match up, you have a problem.
This is not black art, but s simple methodical approach to solving a
Now, finally, let me state that there will a frequency that the
systems wants to vibrate at, and there will be a speed that will have
increased vibration. That is unavoidable. But, just make sure that
is a speed that you wont be spending much time at. Just that simple.
I like to use my pickup truck as the example here. It has a serious
torsional at 15mph in direct drive. But no one complains about it,
and the truck designers put it there on purpose. They had to put it
somewhere and they knew most users would not drive at 15 mph in top
gear. That points out how most torsionals are dealt with.
--- In Small4-strokeEngines@yahoogroups.com, Jeron Smith
- Thanks Shrinivas, and welcome to the group.
I had a strong interest in the XR-40 engine that Erik of Phantom
Aeronautics, working with the manufacture of the XR-50, had developed
a belted reduction drive version specifically for his Phantom
ultralight and was building up test hours prior to making it
available to customers. Then he had an engine failure and out-landing
due to a bearing problem, and stopped working on it.
There were rumors that the factory was working on a gearbox equipped
aero version of the engine, which would have a lot more applications
and general appeal.
I ran across a blurb that said Desi at www.renntechkarting.com knows
something about it.
At 12:47 AM 10/15/2007, you wrote:
>Hello Jerry Booker ,[Non-text portions of this message have been removed]
>I am Shrinivas Gadre from Pune,India.
>I am a new member at small 4 stroke engine yahoo group.
>Congratulations for your successfull ThunderGull flight.
>I have visited your excellent website.
>A small rotary engine Aixro XR 50, 14 kg,42 hp basically designed
>for gokart application is available for 2945 euros.
>Visit : www.nova-racing.com
>This may be useful to you.
>I will appreciate to receive your comments for it's suitability for
>Thanks and regards ,
>JERRY <j.soar.aero@...> wrote:
>The first and second flights of my ThunderGull powered with the
>direct drive Generac engine have been made. The first was about 6
>minutes, the second 15 minutes.
>The plane climbed well at 3200 rpm and 55 mph, I don't know at what
>rate though. After leveling off at 1300 agl, about 65mph was
>indicated at 2900 rpm.
>The free-air engine ran cool, around 350 degrees cht, oil about 140
>degrees. It all looked fine, a real relief.
>There was some minor shaking at certain rpms, but nothing excessive.
>I think I can work around it, maybe with a flatter prop for more rpm
>On the ground they said it sounded like a real airplane. (Thanks for
>all the support you guys on the ground gave me.)
>Next issues are to get some hours built up to see if it stays
>together, to get some better numbers for a good benchmark to compare
>adjustments, and to get used to the new sounds and vibrations of the
>4 stroke engine.
>Also I will need to get used to going thru a lot less fuel, and no
>2-stroke oil. 8^)