Engine Performance

The engine performance program eperf.exe quantifies the relationship between prop size / engine rpm / displacement / etc. The data has come from RC Reports Engine Tests Book 1 (RCR). I entered in the data for many of the engines listed and plotted the results in various ways using SAS statistical analysis software written by the SAS Institute.

I must admit that this is the first time I have actually looked at engine performance with any attempt at real analysis. A lot of us use tachs to measure engine performance but do we really know what a particular RPM means relative to the prop diameter, pitch, engine displacement? How many times has someone bragged to you about his Gonzo 108 turning a 16x10 at 8400 rpm. Usually I nod politely and agree "Yep that sure is a powerful engine." In reality I have no idea whether or not he has a dog or a champ. I just don’t have a good feel for it. I’m trying to quantify the real output of these engines and see if it can be translated into a layman’s "feel".

I have been calculating Horsepower from the RPM and thrust measurements from RCR. I do it by calculating the velocity of the airflow from the prop pitch and RPM and multiplying it times the thrust measurement. I throw in a 80% efficiency prop factor since I know this is the real world. So far I am very happy with the results. I’m sure there are people that would argue that I’m not getting an exact measurement of HP but it definitely gives a Work/Time measurement that is eye opening.

Check this out. Here is the table of Prop RPM and Thrust for the Fox Eagle III from RCR

Prop	RPM	Thrust (oz)
11x5	13,500	131
11x6	13,400	125
11x7	12,400	115
11x8	11,500	108
12x4	13,400	141
12x5	12,300	142
12x6	11,500	136
13x5	11,200	151
13x6	10,800	152
14x4	10,300	157
14x5	9,500	149
14x6	8,500	140

Now tell me, based on the above chart, where is the engine really performing? What prop do I use?

Now try this. Same table, except I included my calculated data:

Prop	RPM	Thrust (oz)	Velocity (ft/sec)	Horsepower
11x5	13,500	131	75	1.12
11x6	13,400	125	89	1.27
11x7	12,400	115	96	1.26
11x8	11,500	108	102	1.25
12x4	13,400	141	59	.95
12x5	12,300	142	68	1.10
12x6	11,500	136	76	1.18
13x5	11,200	151	62	1.07
13x6	10,800	152	72	1.24
14x4	10,300	157	45	.82
14x5	9,500	149	52	.89
14x6	8,500	140	57	.90

Look how strong this engine is with a 13x6 prop. It definitely has another power peek at this RPM. I never would have guessed that from just the original table.

I think that the HP calculations are very realistic. The values seem lower then manufacturers published numbers but remember this calculation is the power measured from the slipstream of the prop. Sort of like measuring the HP of your car off the back wheels and not from the engine mounted on a test stand hooked to a dynamometer

Observations:

RPM vs Thrust seems to be a relatively fixed relationship for a given prop diameter. It seems that pitch doesn’t affect the curve very much. That’s good news. A RPM/Thrust curve will hold true for a given diameter prop no matter if the prop is an old Top Flight nylon junker or an APC. It’s not perfect, but pretty close.
The most important element that I have determined is the one most ignored by most engine enthusiasts: prop performance. You can back out the horsepower for any engine based on the prop specs. I don't know why someone doesn't go out and buy one of every prop on the market and then run each prop at various speeds on an electric motor. I'd do it except, 1) I'm not retired, 2) I live in Saudi Arabia, 3) I'm not independently wealthy. Thrust and horsepower curves could then be accurately determined and when you put that prop on a real engine you would know exactly what the horsepower and thrust numbers would be with just measuring the RPM. Also, that kind of endeavor would be like a Consumer Reports for props. We would know how much power is required to turn a certain prop at a certain speed (we normally measure the opposite) and would then know which prop produced the best thrust/velocity for a given power input.
This program will at least give you an idea of what a particular prop/engine combination is doing. How many times have you seen prop/rpm specs in some magazine and wondered which prop is the best one for your engine? You can't tell anything from those specs because we don't know how much thrust/speed a prop is developing at any given rpm. All we see is a meaningless table. Sometimes an engine like the Fox 60 will show a nice torque peak at a lower rpm.

Try this program out an let me know what you think.

Download Eperf

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