Power setting frustrations

[andrew phillips]

I have been flying for several months and I still don't have a good understanding on why to configure the engine one way vs. another. I can control manifold pressure (throttle), RPM (prop) and mixture. For example, why would I choose to run 21" at 2500RPM over 24" at 2300RPM or 25 squared? What determines if you run over or under square? I would love to hear some rules of thumb for setting these parameters for various flight configurations if there are any. It seems to me that for any given RPM I can run a wide variety of manifold pressures, which one is right?

[hevansrv7a]

OK, I'm talking out of turn because mine is FP. Here goes anyhow.

In general and in theory, slower prop speeds for the same power are more efficient. That's probably not always true, but likely quieter anyhow.

Also in general and in theory, higher MP means less pumping loss which means greater efficiency.

As to the dangers of "over square", read up on the GAMI site.

To further complicate the picture, your power and thus its percentage when ROP is a function of RMP, MAP and altitude. But when LOP, its a function mostly of fuel flow. See other threads and posts in this forum for more on that.

[Steve Barnes]

Andrew, I've been playing with this issue for years. For me personally, it comes down to where my engine runs and feels the best. I used to run my engine 2450 to 2500 because of vibration issues at lower RPM's. I re-clocked my prop backwards 60*, and now my engine runs smoothest and quietest at 2100 to 2250. I don't run over square because I don't need the extra fuel burn and speed. I would have no problem running 2" to 3" over square should the need arise. I could go on and on about this subject, but I think I covered the basics of how I fly my plane. Others will have different views on this and you should pay attention to them too. Running your engine is both a science and an art. Steve

[osxuser]

Typically (but not always) lower RPM means less prop drag. Sometimes, depending on the torque curves of the engine you can get more speed out of running oversquare. Can be done down low in a parallel valve Lyc because the torque flattens out above 2500 rpm. Running 27" and 2500rpm gets us the same or a little more speed than 27 squared. Just do some flight testing, don't get insane with the oversquare, and run on the rich side to avoid detonation.

I like running 2200rpm and whatever MP I can get up higher. Saves gas, and it quiet.

[Norman CYYJ]

Don't be concerned about running over squared. Every fix pitched plane runs over square on take off and no damage is done.

[Ironflight]

I'll put the nail in the coffin on "squared" when it comes to engine operation....it is purely an accident of the units we use! If instead of Revolutions per Minute you use Revolutions per Second, and measured MAP in psi, the numbers wouldn't even be close, and the engine wouldn't know the difference. Forget "over square" (in terms of "25x2500" at least), and set what feels good.

I (and many others) have %HP computed and displayed in real time on the EIS or EFIS. In truth, I pay no attention to MAP at all - I set the RPM I like, then the throttle selects the %HP I want (usually as high as I can get up to 75% in cruise). My engine is very smooth at 2350 rpm, so that's where I cruise it. Higher RPM for more HP on climb, lower if I really want to feel like I am whispering along....

Paul

[f1rocket]

Andrew, great question. I had the same question when I first flew my Rocket as it was my first CS prop airplane for me. What I did was put together a table of various RPMs and MAPs. I think I started out with 2600 and went down to 2100 RPM and for each one of these, I used 24 through 20 inches of MAP. I took all my measurements at my typicall cruise altitude which is either 7500 or 8500. Now that I had my table set, I measured my fuel flow at each table entry for both LOP and ROP.

Once I had my table filled out, I calculated range and time for each combination. This gave me my best range setting and my best time setting.

When I flew cross country, I looked at my chart and determined which was most important and set my MAP and RPM accordingly. BTW, I almost always fly LOP.

When just hopping locally, I tried to stay under 75% power so I could run LOP. Once I got much over 4500, that wasn't an issue as I could run WOT.

Like Paul, I had a GRT EFIS which displays percent power and I used it a lot. I don't totally ignore MAP because I'm running a bigger engine but down low, being able to run LOP was my primary consideration. Above 4500, I'd run WOT and then set the RPM and fuel flow to match either the range or duration I wanted.

Hope this helps somewhat.

[Bob Axsom]

I always run wide open throttle unless I'm landing. In cross country races I go to maximum RPM which usually is around 2720 and lean rich of peak for best speed or around 100+ ROP (usually 1300F EGT on cylinder #4 if I can't find a peak speed - normal situation). For cruising I operate the same way but pull the RPM back to 2450 and if I am worried about the range I lean it out to peak EGT or around 1400F.

Bob Axsom

[hevansrv7a]

I, too, use the GRT, but I think that the % Power calculation is wrong (reads higher than reality) when operating LOP. I have a handwritten table of fuel flows that I refer to if I'm curious. Using BSFC and fuel flow, I get lower HP numbers than the GRT and I've calibrated the GRT per the Superior power charts. This is no big deal, but it is interesting.

[Chino Tom]

I kinda cheated and borrowed a POH from a Cessna 172RG which has a O-360 with a constant speed. I made a chart using the MP/RPM/GPH/%pwr data from that POH (of course the TAS was off a tad ). I had to interpolate some because the RV's induction system can attain higher MP than the Cessna. I used this chart in my six and now in my eight (eh). Of course both have O-360/CS with a carb. Your welcome to the chart (its in a word format). e-mail me and I'll attach a copy. Like others have said, I usually am up high and run WOT and using the MP that results, then adjust the rpm to the % pwr i want (leaning ROP) with the resulting fuel flow.

redtail.rv8a@verizon.net