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Static RPM

cubflier

cubflier

MEMBER
Palmer, AK
I recently static tested my cub and it is running 100 rpm slower that the static test we did last summer.

We used a strobe on both tests and the big difference is in temp and altimeter setting.

Questions:

1. What effect will temperature have on a static test?

2. The altimeter was low in the 28.65 to 28.95 range on the recent test that scored 100 rmp lower. How much will the Altimeter setting effect static rpm?

Thanks for any insight out there.

Jerry
 
Static rpm

Jerry,

The static rpm will vary (increase) with density altitude. This is due to the air being thinner and the prop will have to work less to pull the same volume of air. Or, the air that the prop pulls on a hot day weighs less than air on a cold day. Same with altitude. On my 2 stroke fixed pitch prop ( I know its not a "real" engine) the RPM will vary from 6200 down to 57-5800 from summer to winter. ( +3200' to -1500' density altitude)

larry
 
Power

Hi Cubflier. There is a formula to correct for your problem. But it is very unworkable. I have been trying to find an easy one but they just don't exist. Most want a dry and wet ball humidity test. The big guys that I have talked to just have a room or building that they can keep at sea level when running the engines. I just go to 1,000' pressure altitude and do everything there. The temp correction for a 40 degree spread is in the .000. Not worth the correction. A am and pm test with a fast moving front can mess up the best tests. Take Care. Jerry B.
 
Jerry,

So it looks like temperature is not a factor and the only thing that may have changed my test is altimeter setting.

I'm still a bit confused about the temp thing. Over the years in AK I have always noticed my cub did not seem to wrap up as high under real cold conditions. Since many of these cold conditions can be associated with high pressure the opposite should occur.

This is the first time I have ever started testing with a strobe tach so I have no real data to support my observation above. I am starting to see the complexity of some of your pursuits and how difficult it must be to draw an accurate conclusion.

Thanks - Jerry
 
Static rpm is inversely related to density, so anything that impacts density will impact static rpm. That said, I agree with JerryB. For practical purposes just use either pressure or density altitude at your preference, but don't switch between the two.

An example of this, is that Lycoming should theoretically use density altitude for their mass flow cooling requirements. They don't; they use pressure altitude, and it works just fine.

JimC
 
Jerry,
I would think that it has to do with the amount of HP your engine is putting out on that particular day. Cool temp means more dense air which means more engine HP than a hotter day. If the air is thinner (hotter day) than you did the static run last year, your engine is not putting out the same power. Water in the air (humidity) will also have a huge effect on engine performance.
My 2 cents.
PR
 
Cubflier... Correct me if i am wrong, maybe I haven't had enough coffee yet but I believe temperature is one of the main ingredients in density altitude. So it should affect your RPMs quite a bit.
I see the same affect winter Vs. summer up here.
 
It seems there are two contradictory effects from cold.

Dense air will make it harder to turn a prop thus JimC's statement that RMP is inverse to density.

But...

Dense air will also make more horsepower that should help the engine pull the prop through the air and help to balance out the extra work involved in pulling a prop through denser air.

But..

Do these two forces balance each other?

I understand some of the theory but I'm wondering if there is anyone that has tested these theories to see which force wins or is it just a wash and temperature really has no effect on rpm.

Still confused - Jerry
 
bearsnack said:
Cubflier... Correct me if i am wrong, maybe I haven't had enough coffee yet but I believe temperature is one of the main ingredients in density altitude. So it should affect your RPMs quite a bit.
I see the same affect winter Vs. summer up here.
Click to expand...

Bearsnack - Simple question ...does your cub get more or less rpm when it is real cold out?

Mine always seems to get less.

Thanks - Jerry
 
Anytime the pressure or density increase for whatever reason, you will develop more engine power -- but your prop will slow the engine down because of the increased propeller drag. So, you'll get better takeoff performance at high pressures and densities, even though you're turning the engine slower.
 
I definatley turn a lower RPM in the cold temps. on a day that is 0 to 10 above I see at least a 25 to 35 rpm decrease, maybe more. I am pulling numbers out of memory, I haven't flown much this winter and what flying I have done is with a repitched prop, different exhaust and so on. So my numbers are affected this winter by those modifications.
 
There is no question in my mind that colder and denser air will yield lower static RPM results than less dense air.

That is a fact. Doesn't matter that your engine is making more HP, unless that engine is not hooked to a prop, but is hooked to a tire on a drag bike or the load of a dyno.
 
static rpm

Dave Calkins said:
There is no question in my mind that colder and denser air will yield lower static RPM results than less dense air.

That is a fact. Doesn't matter that your engine is making more HP, unless that engine is not hooked to a prop, but is hooked to a tire on a drag bike or the load of a dyno.
Click to expand...
I FELL MOST OF THE QUESTIONS AND SPECULATION HERE COULD BE SETTELED BY BOYLES LAW : T 1 X P 1 T 2 X P 2
----------- = ------------
V 1 V 2
HOWEVER WE HAVE THE VARIABLE OF THE HUMIDITY . THE HIGHER THE HUMIDITY THE MORE OXYGEN WE HAVE IN EACH FIRING OF THE GIVEN CYLINDER THUS A MORE COMPLETE BURN AND THUSLY MORE HORSEPOWER. THIS ALSO HINGES ON THE PROPER MIXTURE OF THE CARBURATOR. IF THE ENGINE IS RUNNING AT STOKAMETRICK THIS COULD LEAVE THE ENGINE RICH AT THE HIGHER TEMPURATURE AND A TAD LEAN AT THE LOWER TEMPURATURES . IT DOES APPEAR TO BE A CRAP SHOOT BUT WITH DILIGENCE IT CAN CERTAINLY BE BROUGHT TO AN UNDERSTANDABLE CONCLUSION .
CUBNUT1000
 
Would a manifold pressure gauge help? DA would be the only determining factor then if my thinking is halfway right.
 
Cubnut, don't you mean less available oxygen at higher humidity? The oxygen in the water molecules isn't available for burning.
 
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