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Leonard_Smith_nz

Active Member
All,

Just daydreaming a little (at work!). I've chosen to equip my TMX-O360 RV6 with a Sensenich FP prop with 83" of pitch. I'm expecting an empty weight of around 1080 lbs.

Basic math tells me that I could expect a theoretical max speed (in knots) of 6.87 x RPM (in hundreds) e.g.

6.87 x 27 = 185 kts

Any aviation 'buffs' out there that can tell me the real world efficiency of propellers when mated with the RV frame. I appreciate that this is not an exact science, but then again, dreaming allows us a little latitude...

Regards,
Leonard
RV6 in New Zealand
 
Constant explained

All,

I've had a question as how I came up with the 'magic' multiplier in my previous post. No idea if this is correct, but my logic went as follows:

1. Prop pitch (meters) = Prop pitch (in inches) x 25.4/1000
abbreviated: Pm = Pn x 25.4/1000

2. Engine rpm = H (in hundreds) x 100
e.g. if H = 27 then engine rpm is 2700

3. Engine rps (per sec) = H x 100/60

In an ideal world every rotation of the prop will propel the aircraft forward by a value equivalent to the pitch, therefore:

Speed (in m/s) = (H x 100/60) x (Pn x 25.4/1000)
= H x Pn/23.622

to convert to knots: 1 (m/s) = 1.94384449 knots

========================

Speed (in knots) = H x Pn/12.15

where:
H = engine rpm in hundreds
Pn = prop pitch in inches

========================

for my RV6 with 83" of pitch running at 27 hundred rpm:

Speed = 83 x 27/12.15
= 184.4 kts

This is obviously an idealized situation. Any RV pilot with a FP prop should be able to tell me how far this figure deviates from the 'real' world. This should give us an (very rough) indication of prop efficiency as applicable to the RV airframe.

Regards,
Leonard
RV6 in New Zealand
 
Leonard

I used your math and came up with 167.75 knots. That's a 79" prop at 2580 rpm. At that speed I can get around 163-164 KTAS. So with your math my prop is running at around 97%.

I'm not sure what prop you are thinking of running but some of the Sensenich props are limited to 2600 rpm.

What part of NZ are you living? I started my flight lessons in Wanaka in 1999. I ended up moving to Wanaka a couple years later. Sure wish I was back there with my RV. No more spiralling to get out of Milford Sound like in the 172.

Cam
 
New Zealand

Hi Cam,

I'm on the Kapiti Coast, about 60 north of Wellington. Yes, NZ is a great place for the RVs. From where I am I can just about fly to every airport in New Zealand (with the possible exception of Steward Island) non-stop.

It's for that reason, and the fact that NZ is very mountainous, that I've chosen to go with the 83" as opposed to the Van's recommended 85". Now I'm trying to figure out how much the fewer 2" will cost me in cruise.

Your numbers look a bit high. I recall reading somewhere that the efficiency is in the order of 85% (but my memory is vague on that one).

Be sure to give me a shout if you come to NZ again...

Regards,
Leonard
RV6 in New Zealand
 
Leonard_Smith_nz said:
Hi Cam,

I'm on the Kapiti Coast, about 60 north of Wellington. Yes, NZ is a great place for the RVs. From where I am I can just about fly to every airport in New Zealand (with the possible exception of Steward Island) non-stop.

It's for that reason, and the fact that NZ is very mountainous, that I've chosen to go with the 83" as opposed to the Van's recommended 85". Now I'm trying to figure out how much the fewer 2" will cost me in cruise.

Your numbers look a bit high. I recall reading somewhere that the efficiency is in the order of 85% (but my memory is vague on that one).

Be sure to give me a shout if you come to NZ again...

Regards,
Leonard
RV6 in New Zealand
Click to expand...
Leonard,
This is somewhat more direct than your approach but if you are interested in only the difference in cruise speeds and assume similar prop efficiencies between the 83" and 85" pitch, then cruise speed with your 83" will be equal to the cruise speed of the 85" times 83/85.

Or a reduction in speed of about 2.4%.

If the coarse pitched plane is clipping along at 165kts, you could expect ~161 kts for the same RPM.

-Mike
 
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