100LL

I'm a bit confused on setting manifold pressure and propeller RPM and the expected performance im supposed to get on both the Piper Arrow and Seminole. First with the Arrow, im looking at the power setting chart and it says if I cruise at 6000 ft and operate at 65% power I need 24" and 2200 or 22.1" and 2500 rpm. I understand the differences in manifold pressures but what I dont understand is which one to choose? Does 2200 or 2500 rpm provide any performance benefit? Why choose one vs the other? Same thing with the Seminole only now the chart ranges from 2100 to 2400 rpm. From what I can tell, I apparently I can get the same TAS say at 55% for either 2100 or 2400 rpm. So why the difference? Why choose one over the other?

UAL T38 Phlyer

100LL:

Here's some ideas:

1. Noise. Lower RPM is usually quieter than higher.

2. Temperature. Hot day; engine temps are on the high side. Run high RPM to reduce engine temp; the converse in the winter.

3. Gross weight. A lightly-loaded airplane generally cruises faster than a heavy one for the same power (more apparent in jets, but generally true, even in General Aviation planes). I would think a lightly-loaded, faster cruise-capable airplane would favor a lower RPM (ie, a coarser pitch) than one cruising slower (finer pitch). I would think of it as using 4th or 5th gear, depending on how fast you are driving.

4. Fuel Efficiency. I'm guessing that the lower RPM has slightly better fuel specifics.

100LL

All makes sense, thanks.

pokey9554

There has to be a formula for piston aircraft, but I'll give you an example of turboprop logic:

SHP= Torque x Np x .00019

Therefore, the higher the torque (or MP) and the higher the Np (prop RPM), the higher horsepower output of the engine. There would also be different combinations that could result in the same horsepower output.
Example:
SHP= 1000 x 2200 x .00019 = 418SHP
SHP= 1100 x 2000 x .00019 = 418SHP
SHP= 1222 x 1800 x .00019 = 418SHP

As UALT38 said, it's all about noise, fuel consumption, and temperature. What I've shown here, is there are different ways to get equal performance, but each way has a different drawback.

100LL

Oh ok, so noise, fuel consumption and temperature. As for the fuel consumption yeah I can see that in the Arrow POH how changes in prop setting adjusts fuel consumption most noticeably at the 55% power range which makes sense cause lower setting means lower fuel consumption. Looking in the Seminole's POH though, it only offers two fuel consumptions for each percent power setting; a best economy and best power rate for all RPM selections 2100 to 2400. Would it be safe to believe that fuel consumption change is negligible in this situation for any given RPM setting?
Also a toss up question, why wouldn't there be a 2200 RPM MP option for the 75% power setting on the Arrow while there is one in the Seminole?

TPROP4ever

My guess would be that while they were doing all the calculations for the charts, that in that range some engine limit was succeptable to being exceeded, (remember they always build in a safety margin). Id have to look back at the POH to see, its been a number of years since I flew the Arrow. Or it could be just a misprint in the manual...

Kasserine06

There is no reason to use anything higher than the lowest RPM for your power setting unless you cannot get the required manifold pressure or need more power.

It makes a large difference in fuel consumption. That is the main reason to use lower RPMs. Engine wear is also important too.

The reason the Arrow POH shows you the difference in fuel consumption at the various RPM settings and the Seminole doesn’t is really just a lack of standardization in the Piper POHs. There is a difference in fuel burn when you use the lower RPM settings in the Seminole. I just assume that the fuel burn in the POH for the Seminole is only achievable with the lowest RPM settings (I actually get about 2.1 gph less than published sometimes, but I always use the POH values or greater).

Also, the difference in power settings between the Arrow and the Seminole is because they have different engines.

If you want a simple way to set power, use units of power instead of only using the limited power settings published in the POH. Simply add up the RPM divided by 100 and the manifold pressure. For example, to produce 75% power at 3000 ft in the Seminole, the POH suggests a power setting of 25.6” and 2200 RPM. Simply add 25.6 and 22 to get a total power of 47.6 units of power. 100 RPM is equivalent to 1” of manifold. You can then use any combination of manifold and rpm settings as long as they add up to 47.6 to get 75% power with only one limitation. There is a limit of how low RPM you can go/how high manifold you can have. For the Seminole and Arrow, as long as the manifold pressure is no more than 4.5 units above the RPM settings, you will not damage the engine.

You want to operate at the highest manifold and lowest RPM you can to get the most efficient engine performance. In this case, a power setting of 26” and 2150 RPM is the best (26+21.5=47.5 units AND, 26-21.5=4.5 units).

Twin Wasp

What, you don't have a BMEP guage in your airplane?

BHP = (BMEP*the length of piston stroke*area of cylinder cross section*RPM*number of cylinders)/792,000

Since the stroke, piston head area and number of cylinders remains the same for a given engine, they can be boiled down to a constant (K) for that engine and you get something like

BHP=BMEP*RPM/282 in a R2800.

All you folks got these fancy aviation degrees and they don't teach this?. Actually, Charles Lindbergh was hired as a consultant for the Army Air Corp in 42 and taught pilots to extend the range of their fighters by pulling the rpm back.