Wheel Pants

The following speed comparison of two wheel pant styles was posted to the RV-List

I first flew my RV-6 with one-piece wheel pants, but had to change to two-piece pants because of an interference problem. That gave me an opportunity to evaluate the speed difference of the two styles.

These tests were run by reading TAS off my RMI Microencoder. Before the first set of tests I calibrated this instrument at 6,000 feet by comparing the indication to a two way (upwind and downwind) GPS groundspeed check and found no discernable error.

For my tests yesterday under somewhat bumpy conditions I again checked it at 6,000 feet and found the Microencoder reading, perhaps, 1 knot fast. I did another check at 12,500 and the Microencoder seemed to be reading about 3 knots fast, perhaps because I was running some cabin heat that may have caused the temperature sensor, which is inside the air vent on the left side, to see too high a temperature.

The engine instrumentation is Vision Microsystems VM1000. I have previously written that the MP on this set-up reads in minimum increments of .3 inches. That is the smallest change I have observed when I moved the throttle and was watching it, but I see in my data there are smaller increments than that, so I'm not sure what the minimum increment is. Vision's advertising seems to say .1 inch.

My set-up is RV-6 with a box stock Aerosport Power O-360A1A with a carburetor and fixed pitch Sensenich 72FM8 prop pitched at 83 inches. The gear leg fairings are Van's fiberglass. Everything is completely stock.

For the tests yesterday, my takeoff weight was about 1400 pounds. Since the first tests the airplane has been painted. That and the different wheel pants are the only changes since the first tests. Altimeter setting yesterday was 29.93, I reset to 29.92. MP reading on the ground at 3,270 feet was 26.6 prior to engine start. ASOS temperature was 26C and my Microencoder read 78F (the same within 1 degree). I tried to keep the engine leaned to best power.

I thought yesterday would be a good test day, after work. There were no weather systems nearby and there was very little wind, even at altitude (about 10 knots), but it turned out to be choppy at all altitudes. The 90F high yesterday may have had something to do with that, but I did not take off until about sunset.

Flying these kinds of tests is difficult at best, and very difficult, especially with a fixed pitch prop, when it is not completely smooth. It is like balancing on the head of a pin. The slightest disturbance either lugs the engine down, or over speeds it, which causes a big speed change (easily 5 knots) and it takes a lot of small adjustments and patience for things to stabilize again. Yesterday there were times that there were obvious up and down drafts. I just tried to get the data between those.

One reason my data is a bit odd, is that the prop is pitched such that I am never using full throttle, except for one check at 8,500, because full throttle runs the engine rpm over readline at all altitudes that I have flown at. Accordingly, my procedure is to run the engine at 2700 RPM. Things seems to offset, such that my TAS at that RPM is essentially the same at all normal cruising altitudes. Weird!

Pres Alt	Temp F	MP	RPM	TAS(KTS)	%Pwr
6000	81	22.0	2700	171	75
7000	80	21.7	2700	171	75
8000	76	21.0	2700	172	73
8500	71	20.5	2700	171	71
9500	67	20.3	2700	171	71
10500	62	19.3	2700	171	68
11500	56	18.6	2700	172	65
12500	51	18.1	2700	171	64
13500	47	18.1	2700	170	65
14500	42	17.4	2700	170	63
15500	37	16.7	2700	171	60

Pres Alt	Temp F	MP	RPM	TAS(KTS)	%Pwr	Notes
6500	73	21.7	2700	175	75
7500	71	21.3	2700	175	74
8500	64	20.8	2700	174	73
8500	62	21.8	2780	181	78	(Full Throttle)
9500	58	20.0	2700	176	70
10500	53	19.6	2700	175	70
11500	47	18.9	2700	173	67
12500	42	18.2	2700	174	65

I sure don't understand everything that is going on here, and the absolute numbers are somewhat suspect, but I am confident there is a significant speed difference in the two different style wheel pants. The apparent 3 or 4 knot difference is about what I thought it would be from general flying.

I don't think anyone would want to go around all the time at 2700 RPM, but the numbers are impressive to me and I think they show a good fixed pitch prop is a very viable choice, and a slight underpitch may be a good thing.

It is my practice to fly, normally, at around 165 kts TAS. With my present pants that is usually around 2550 RPM, which is real comfortable for me. I could fly this same cruise speed with no pants at all, but it was kind of a struggle, and fuel consumption was at least .5 GPH higher (9.0 vs 8.5).