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-   -   Speed Question (https://vansairforce.net/community/showthread.php?t=180898)

ebackscheider 03-30-2020 10:38 AM

Speed Question
 
Hey All, I'm new to an RV6 and I have a FP prop with a O360 power plant. I don't know the specs on the prop, but I will get that when I go to the airport next. I also have the old wheel pants and landing gear fairings.

At 2300 rpm I'm getting about 142kts indicated. I hear stories of 160+ kts in a 6, and talked to a guy yesterday that is getting 155 with an O320.

What do I start with to find where I'm bleeding speed from?

Thanks in advance for your thoughts!
Eric
eric at StopFlyingScared dot com

RV8JD 03-30-2020 10:55 AM

Here are the Van's performance numbers for the -6/-6A. Note that they were measured with the Hartzell 2-blade CS prop. All speeds are in MPH TAS.

https://www.vansaircraft.com/rv-6/#aircraft-details-2

sjhurlbut 03-30-2020 10:59 AM

To measure cruise speed you need to specify altitude. Do you mean 142 kts true or indicated?

2300 rpm cruise for a FP is very low. I cruise at about 2550 rpm.

You're bleeding speed by pulling throttle back too far.



Quote:

Originally Posted by ebackscheider (Post 1418238)
Hey All, I'm new to an RV6 and I have a FP prop with a O360 power plant. I don't know the specs on the prop, but I will get that when I go to the airport next. I also have the old wheel pants and landing gear fairings.

At 2300 rpm I'm getting about 142kts indicated. I hear stories of 160+ kts in a 6, and talked to a guy yesterday that is getting 155 with an O320.

What do I start with to find where I'm bleeding speed from?

Thanks in advance for your thoughts!
Eric
eric at StopFlyingScared dot com


WingsOnWheels 03-30-2020 11:01 AM

What are your TAS numbers? All the published numbers are true, not indicated, so that is the first place to start your comparison. Go up to 8,000ft density altitude. Run it wide open, note the RPM, TAS, FF and post those up.

EDIT: I see everyone beat me to it while I was yapping on the phone. I am a little slower than I would like, think my prop is a bit underpitched. I see 168kTAS at 8,000DA 2750 RPM. (O-360 FP Catto). I would expect about 172-3.

lr172 03-30-2020 12:21 PM

Quote:

Originally Posted by ebackscheider (Post 1418238)
Hey All, I'm new to an RV6 and I have a FP prop with a O360 power plant. I don't know the specs on the prop, but I will get that when I go to the airport next. I also have the old wheel pants and landing gear fairings.

At 2300 rpm I'm getting about 142kts indicated. I hear stories of 160+ kts in a 6, and talked to a guy yesterday that is getting 155 with an O320.

What do I start with to find where I'm bleeding speed from?

Thanks in advance for your thoughts!
Eric
eric at StopFlyingScared dot com

shouldn't compare speeds without considering fuel flow and altitude, if looking to compare. I get 159 knots (TAS) at 2730 RPM at 8.1 GPH at 8-9000' with my IO-320 (EI) and FP prop. Your numbers seem low. Make sure you have accurately calculated TAS. Also need to do a 4 point GPS test to verify your static setup. Mine was off by 10 knots until I made a static port fence.

with an FP prop, you need to be turning much closer to 2600 or 2700 for higher speeds. 2300 is way too low. If that RPM is achieverd with WOT at 8000', you are WAY over pitched. Just my opinion, but a properly pitched prop should turn 2700 somwhere between 8K' and 12K' (based upon your goals) when WOT and 100* ROP.

Larry

ebackscheider 03-30-2020 12:26 PM

Thanks!
 
Thanks all!

I will go do a test, maybe even later today. 8000 ft density, wide open and report back on the speeds I see.

I guess my old days of 23 sq with CS are hurting me :(

Eric

Sam Buchanan 03-30-2020 01:08 PM

Quote:

Originally Posted by ebackscheider (Post 1418279)
Thanks all!

I will go do a test, maybe even later today. 8000 ft density, wide open and report back on the speeds I see.

I guess my old days of 23 sq with CS are hurting me :(

Eric

I think you will like what you see once you put your -6 into high gear. :)

WingsOnWheels 03-30-2020 01:15 PM

Quote:

Originally Posted by ebackscheider (Post 1418279)
Thanks all!

I will go do a test, maybe even later today. 8000 ft density, wide open and report back on the speeds I see.

I guess my old days of 23 sq with CS are hurting me :(

Eric


Open her and see what she can do!

rvbuilder2002 03-30-2020 01:47 PM

Quote:

Originally Posted by ebackscheider (Post 1418279)
Thanks all!

I will go do a test, maybe even later today. 8000 ft density, wide open and report back on the speeds I see.

I guess my old days of 23 sq with CS are hurting me :(

Eric

Just be sure the speed you are looking to see is True Air Speed, not indicated (as already mentioned)

Indicated air speed is nearly useless for doing speed comparisons.

Using my fixed pitch O-360 powered RV-6A as an example - At 11,500 ft and a fuel flow of 7.8 - 8.0 GPH I can get true air speeds of about 195 MPH but my indicated airspeed is often 160 MPH or less.

ebackscheider 03-30-2020 07:11 PM

Some data
 
Did a short flight tonight

8,000 ft Density Alt
Turning about 2650 RPM

TAS right at 160 KTS - 184MPH

I don't have a fuel flow yet, so I have no idea where that was. The spike was some issue with the data.



Thanks for the help, I just need to push the throttle forward a bit more I guess.


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