Static RPM and DA

[PilotAlan]

OK, here's a question I have not been able to find an answer to anywhere.

What is the effect of DA on static RPM? I know drag decreases as altitude increases and at least partially offset the loss of horsepower. But still, when you get to 7000-10,000ft density altitudes, what happens?

My new digital tach shows my engine is not quite making static RPM (about 75rpm low). But DA today was 7500ft at the runup pad.

 

[Geico266]

Thats a good question. Since this is a new tach what did/does your old one say? Do they match?

When you say static RPM are you talking about what the POH says you should on the ground as part of your preflight runup or what you have had in the past?

 

[Dan Thomas]

OK, here's a question I have not been able to find an answer to anywhere.

What is the effect of DA on static RPM? I know drag decreases as altitude increases and at least partially offset the loss of horsepower. But still, when you get to 7000-10,000ft density altitudes, what happens?

My new digital tach shows my engine is not quite making static RPM (about 75rpm low). But DA today was 7500ft at the runup pad.

Did you lean for max RPM? At 7500 feet, full rich is going to cost you a lot of power.

Dan

 

[poadeleted20]

There is a conversion table out there somewhere to correct for DA, but I don't know where to find it. Perhaps Ted knows.

 

[gismo]

My recollection is that the loss of HP due to DA is exactly equal to the reduction in the prop load for the same DA increase. I suspect that the mixture and wind have a more significant effect.

 

[poadeleted20]

My recollection is that the loss of HP due to DA is exactly equal to the reduction in the prop load for the same DA increase. I suspect that the mixture and wind have a more significant effect.

I'm pretty sure that's not true, since RPM for 75% power goes up with DA in my Tiger, and I do remember seeing a DA conversion table for static RPM.

 

[PilotAlan]

Thats a good question. Since this is a new tach what did/does your old one say? Do they match?
When you say static RPM are you talking about what the POH says you should on the ground as part of your preflight runup or what you have had in the past?

My old tach was reading low, and I knew it. It also had the waggles. So did they match, no. With my new tach I can actually tell what my RPMs actually are.
And yes, I am referring to static RPM on the ground, max power runup, trying to meet TCDS static rpm.

Did you lean for max RPM? At 7500 feet, full rich is going to cost you a lot of power.

I know you had to ask, but yes. Out in Colorado you lean or die. But yes, I leaned for max RPM.

I'm pretty sure that's not true, since RPM for 75% power goes up with DA in my Tiger, and I do remember seeing a DA conversion table for static RPM.

Yeah, I can't imagine that the power loss and drag reduction are exactly the offsetting. I was barely producing 75% power on the ground yesterday.

Ron, maybe that's the solution? Figure out max power for DA, then find the RPM for that power at that DA, then check against static RPM at runup?
{EDIT} Or maybe not. RPMs increase as you start moving..... I can vaguely see a way to figure it out, but can't quite seem to get it clear in my head.

 

[poadeleted20]

Ron, maybe that's the solution? Figure out max power for DA, then find the RPM for that power at that DA, then check against static RPM at runup?
{EDIT} Or maybe not. RPMs increase as you start moving..... I can vaguely see a way to figure it out, but can't quite seem to get it clear in my head.

Your edit has it right -- forward speed reduces prop load. In any event, wait for Ted -- if he doesn't have the answer, I'll bet one of his engineers does.

 

[Ted]

I'm pretty sure that's not true, since RPM for 75% power goes up with DA in my Tiger, and I do remember seeing a DA conversion table for static RPM.

Well, that makes sense. Think about it like pulling carb heat - that's increasing your DA. I'm not sure how that conflicts with Lance's statement.

As to how it should impact static RPM, I don't know. But I could see mixture having a significant impact, specifically if you're at high altitude and it's too rich.

 

[gismo]

I'm pretty sure that's not true, since RPM for 75% power goes up with DA in my Tiger, and I do remember seeing a DA conversion table for static RPM.

Ron, I agree that there will be less load (and higher RPM with fixed pitch) at higher DA with the same power, but what I said was the load is affected by DA in the same proportion as power is affected. And while I'm not dead certain of this I'm pretty sure it's true, at least theoretically, especially with an equally rich mixture at any DA you're checking. When the engine is running rich of peak EGT, the power output for a given RPM is limited by the number of oxygen molecules ingested on each intake stroke and that number is directly proportional to the air density which also affects the drag on the prop in the same proportion.

In the case you stated, you were producing the same 75% power at a higher DA so you only changed one side of the equation resulting in an imbalance (less drag, same HP = higher RPM).