tripower222

My Feedback: (5)

Was looking for a chart or formula or online calculator to get a horsepower ratingfrom agiven prop diameter and pitch at a given rpm.

Any one ran across anthing like that?</p>

NikolayTT

Yes, there we go:
http://www.rcuniverse.com/forum/m_92..._1/key_/tm.htm

pe reivers

or just http://www.mvvs.nl/prop-power-calculator.xls

tripower222

My Feedback: (5)

very cool thanks

RTK

My Feedback: (1)

Pe's is the most accurate I have found to date.

Tired Old Man

My Feedback: (1)

Even Pe's has a fudge factor built in, and if the prop used does not already have a factor noted on the chart you're on your own.

Which reminds me...

Pe,

h
How did you come up with the prop factor results? I have a couple of props that are not listed and would like to determine the efficiency factors.

pe reivers

I started out with basic prop calculations, and through the years compared props I had good data for, at the same time tweaking the constants to adapt the outcome to real world results.
If you have a known engine that you can mount the unknown prop on, you can adapt the prop factor until results match measurements. Calculations will improve if you can test a handful of the same size props.
An electric motor is a great tool, as most parameters are known.

tripower222

My Feedback: (5)

It may not be 100% accurate due to prop factors, butI would think it is veryvery close when doing thrust and required hp comparisons with the same prop from one givenrmp to another on a % basis. would you agree PE? Either way I think it a great tool.

I noticed that when you plug in Zenoah's and some others benchmark props/rpm its right on with their hp ratings.

pe reivers

I am happy with less than 5% inaccuracy. This was the accuracy limit imposed on us in engineering exams when using a slide rule instead of electronic calculators.

RTK

My Feedback: (1)

Now you are showing your age Pe

NikolayTT

What is wrong with the Slide Rule ? I have one in my pocket ... just beside my Nokia E90 and
works faster and better than Calculators build in the Mobile Phones and gives you kind of Vision,
not only some borry numbers ... and does not need a ....battery...

In fact 5% precision is too good. I wander if anyone of us gets the RPM measured that precisely in
practice, regardles what the "super precise" RPM metters claim; those metters have error and I
wander if that is even less than 5%. Also, even with the same engine and etc. circumstances no
one can be sure he will be able to repear with +/- 2.5% (in total 5% !!! span) the measurements
for any engine especially glow ones and even gas I believe.

For propellers which are not in the table of Pe's Calculator one way, maybe not the best but still works for me, is
to use the Fish-scale and "play" a bit with the number of the Propeller Constant so to get the static thurst the
same(almost) from the Calculator and Fish-scale. But as mentioned by tripower222 the relative estimates
one gets from Pe's calculator even without knowing the exact propeller constant are very(!) useful things,
even if they were "off-set" by the incorrect propeller constant.
SUGGESTION: - Lets send to Pe our measurement data, as soon as we get new, with all details and then
let Pe think and select what he consider reliable to use for further "tuning" the calculator.
EVEN IF we send date with some errors, the accumulated database will show the correct direction of
further tuning of the calculator constants, if that is at all needed. But at least the LIST of the propellers
could be enlarged, for example with the Graupner's G-Sonic; they are comming as a massive "heavy
weight" competitor to APCs. Check this LINK please:

http://www.rcuniverse.com/forum/m_92...tm.htm#9253447

In any case, so far I haven't seen any other calculator which comes near to the Comfort & Precision
we get from Pe's. Have anyone seen better calculator or at least a competitive enough ? - if so, please
post the link.
Nick

pe reivers

That's a good idea. I welcome data, even if I can't use them all. Be sure to include plane fuselage shape, because the plane drag reduces the fish scale reading.

pe reivers

No need to hide the fact that I'm a warchild. I did my first (of many) engineering exam in 1961. I stillfind my sliderule a nice tool when I need quick fixed ratio references and three-set calculations.

BTW,
For those interested, NACA did a lot of good static prop research in the 1920ies. It makes nice reading.

tripower222

My Feedback: (5)

In reply to rpm measurement accuracy. You may think I am crazy but but I believe my method of rpm measurement is near 100% accurate.</p>

I do it with my laptop and Wavepad recording the exhaust pulses across time.</p>

here is a screen shot of Wavepad exhaust recording <a href="http://classiccorvettes.com/fox/15x6prop.jpg">

</a>

If you look you will see I have 0.1 seconds selected showing 11.7 ehxaust pulses. Thats 117 pulses per second if you want more accuracy count the entire second, multply by 60 seconds 7020 exhaust pulses per minute = 7020 rpm. Divide by 2 for a four stroke engine.</p>

pe reivers

if there's a 20 mph wind blowing, you'll be off about 3%

tripower222

My Feedback: (5)

not sure i understand how wind comes into play in the accuracy of the rpm reading

Super08

My Feedback: (2)

A slide rule, wow. I haven't used one of those for at least 30 years. I wonder if a highschool student would even know what one is today.

pe reivers

The soundwaves upwind are denser than the soundwaves downwind, due to Doppler effect.

tripower222

My Feedback: (5)

I still dont see how when all the pulses in the tenth of a second sample would be exposed to the same wind. and the amount of possable wind change in a sample period would be negligible.


</p>

tripower222

My Feedback: (5)

...

pe reivers

what happens to the plane sound when it moves towards you, and then it moves away from you? Does the sount pitch (=rpm) stay the same?
Wind has the same effect.

tripower222

My Feedback: (5)

Yes the sound doesseem to change but that is the audio frequency. Themeasurement between sound wave amplitude pulses shouldnt change just the frequency of the audio.

NikolayTT

To avoid the Doppler you can put on Pressure Nipple a piezo sensor (microphone) or use the
magnet Hall-sensor if it is gaz engine, BUT the error(S) in ordinary hand-held RPM metters
which rely on the optical sensor being signalized by the propeller's rotation are not so small.
There you get threshold level error(quantazation-error) and although those metters are called
"digital" nowadays, still the pattern of the signal is not Square as it is also seen on the picture.
Anyway the Oscillograms on the Laptop of tripower222 are nice things, Congratulations ! and
surely more precise than hand-held optical rpm-metter. Still 100% accuracy is the always
the "dream-never-comes-true" for any engineer; there is always some "bug" in the system.
Surely it is good idea to keep the Size and Number of those "bugs" as low as posible but 100%
clean room doesn't come at low cost...
ALSO, allmost all (just to be polite and not to say only ALL) Matematical Derivations about
any Physics Phenomena are APPROXIMATE because are based on a number of Assumptions
and Simplifications. I believe Pe can list many of those behind the formula used.
Still there isn't much need to go so precise at least in glow-engines, even 10% precision
should be good enough
in other words I do not care much for +/- 5$ if in addition to the 100$ is possible to
get 20$ (+/- 5$), i.e. all those measurements (for me at least) are Relative, not
Absolute numbers. Of course if the errors gets in the range of +/-15$ out 20$
gain then is getting funny even for a hobby...[sm=spinnyeyes.gif]