C90 prop on O-200

GeorgeC

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While chatting at the airport this morning, I learned that some folks run 71 inch C90 props (e.g. 1A90 CM7154) on an O-200 instead of a 69 inch prop (e.g. 1A100 MCM6952 or 1A101 DCM6952), and that in such installations, there should be an RPM limit placarded. What is the reference for the placarded limit?

This is for a C140a with the O-200 STC.
 
While chatting at the airport this morning, I learned that some folks run 71 inch C90 props (e.g. 1A90 CM7154) on an O-200 instead of a 69 inch prop (e.g. 1A100 MCM6952 or 1A101 DCM6952), and that in such installations, there should be an RPM limit placarded. What is the reference for the placarded limit?

This is for a C140a with the O-200 STC.
If this were legal, it would have a STC or field approval on it, the answer to your question would be in that paperwork.
 
Thanks Tom. The logbook entry for the O-200 conversion states that an STC-conforming prop was installed.

A better question would have been, assuming proper paperwork, is a 71 inch prop on an O200 a good idea?
 
Thanks Tom. The logbook entry for the O-200 conversion states that an STC-conforming prop was installed.
Who would know what a "Conforming Prop" would be? I would guess the STC holder.
 
Look up Certified Aeronautical Products, LLC, Moody, TX. Terry Bowden he is a vintage aircraft DER, has done the paperwork on 0-200 installs in L-2 and several different Taylorcraft. He would have a correct answer, wish you the best, gary
 
On an engine with a 2750 rpm redline, a Cessna 140 at close to Never Exceed Speed and a 71 inch prop your tips are supersonic - never a good idea as this can cause fatigue, cracking near the tip and early prop failure. When you are at an airshow and the small aerobatic airplanes pour the coals to it and in a dive you hear the popping sounds - those are sonic booms off of the prop tips. Use the original C-90 engine redline not the 0-200 instead and you will likely be OK. If you do the math the tip should not exceed Mach .82 I believe. You have to calculate the speed of the tip from the RPM, add the horizontal right angle of the advancement through the air. And then calculate the hypotenuse of the two vectors for the distance calculated for each revolution. Adjust the distance by the RPM multiplied by 60 for mph then divide that by the speed of sound at your given altitude. I did this when in Engineering school and when I owned a 1948 C-140. There is a reason why the prop is smaller on C-150's.
 
Dan, isn’t it a factor of air density which is affected by both temperature and altitude?
Air temperature is the factor, and temperature decreases with altitude up to the tropopause, so the speed of sound will generally decrease with altitude since the temperature is decreasing: https://www.weather.gov/epz/wxcalc_speedofsound

In the stratosphere pressure is still decreasing as we climb, but the temperature is relatively constant and so is the speed of sound: http://www.saveconcordegroup.co.uk/the-speed-of-sound/
 
Plane had the right prop but a wrong logbook entry.
 
Air temperature is the factor,

I remember this lecture (vaguely) from university thermodynamics. Was worked out from first principles.

The effect is due to Temperature being a measure of the speed of the moving air molecules and faster molecules get to the next one along in a shorter time which increases the speed of the sound pressure wave.

For some reason the greater distance between molecules as pressure reduces gets cancelled out by some other effect? I forget that one.
 
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