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  #1  
Old 04-29-2016, 12:13 PM
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acam37 acam37 is offline
 
Join Date: Aug 2012
Location: Lufkin Tx
Posts: 636
Default Another Performance ?

We've been flying our rebuilt -4 since October and slowly working out all the usual bugs after a build. We were having an engine stumble issue which was finally narrowed down to the carb. After the swap and enlarging the jet because it was running too lean, now we seem to have lost about 100 rpm and 10-15mph at WOT. Even after leaning to ROP and LOP. Our Catto 3 blade is apparently under pitched and was already giving us less than advertised cruise and top speed numbers, but now its downright embarrissing. 150-160 indicated, 170-175 TAS and GPS ground speed 180-200 depending on the atmospheric conditions. With a completely fresh 160hp 0320 I expect to see better numbers than that.
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  #2  
Old 04-29-2016, 12:37 PM
CATPart CATPart is offline
 
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Location: up up and away
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Default

Please say if mph or kts, but I assume you mean mph.

I cruise at an average of 190 mph ground speed at 2500 rpm with my Bernie Warnke wood prop, 160 hp o320. I thought that was pretty good.
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  #3  
Old 04-29-2016, 12:57 PM
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acam37 acam37 is offline
 
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Sorry, all my speeds are MPH. Maybe My expectations are too unrealistic? But something changed when I went from a MA-4SPA 10-5009 to a 10-2517 and added a Vans gascolator. My performance went down instead of up.
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Last edited by acam37 : 04-29-2016 at 01:30 PM.
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  #4  
Old 04-30-2016, 11:30 AM
Vac Vac is online now
 
Join Date: Jul 2009
Location: Niceville, Florida
Posts: 358
Default Round Numbers

Arlie,

A good snapshot of overall performance can be had at 8000', wide-open throttle (which is kind of a nominal 75% cruise configuration). The advertised performance numbers for the -4 on the Van's website show 192 MPH TAS under those conditions with a 160 HP engine.

Things do depend on how well your pitot/static system is calibrated, but some GPS data runs under those conditions (or adjusting altitude to approximate the 8K', standard condition) will give you a reasonable ball park comparison so that you can do a real apples/apples assessment without doing too much extra testing.

There's quite a bit of good info over on Kevin Horton's site on how to dial in the pitot/static system, and lots of different spreadsheets that can provide good data based on four- or three- leg GPS runs.

By way of comparison, here are the numbers for our very-stock -4 turning a 2-blade 68 x 72 Gen 2 Catto at different altitudes and conditions. All of the data is the result of test and have been corrected for conditions, so it might provide some insight into conditions other that 75% cruise for a stock airplane with a similar engine and fairly close propeller combination.

https://drive.google.com/file/d/0B8E...ew?usp=sharing

The bottom line is that you should be pretty close to the numbers published under the "Performance" tab on the Van's webpage.

Please give due consideration to how fast you fly, as structural margin is reduced at speeds above Vno (180 MPH IAS for the RV-4) and and flutter margin is reduced above Vne (210 MPH for the RV-4) which is based on true, not indicated, airspeed.

Good luck and feel to free to drop a line if you have any questions or would like a spreadsheet that might help crunch some of your airspeed flight test data.

Fly safe,

Vac
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Last edited by Vac : 04-30-2016 at 11:33 AM.
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  #5  
Old 05-03-2016, 03:26 PM
Rick RV-4 Rick RV-4 is offline
 
Join Date: Aug 2006
Posts: 226
Default Confused?

Arlie,

You mention a TAS that does not match your groundspeed. No wind, they should be the same. If your average GS is higher than your TAS, you might have a pitot/static issue. If you are getting an average (not TOP) groundspeed of 180-200, that seems pretty good to me.

Just for comparison's sake, I cruise at around 184 TAS (MPH) at 8,000 with about 58% power. With an O-360 and a Sensenich metal prop I can just get over 200 MPH TAS, and still have some power left (but would exceed the recommended 2,700 max RPM).

Cheers,

Rick

Last edited by Rick RV-4 : 05-05-2016 at 07:47 AM.
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  #6  
Old 05-09-2016, 07:33 AM
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acam37 acam37 is offline
 
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Default Actual test numbers

After speaking with Nicole Catto about prop performance, I took the plane up to 8000' to record some actual numbers. I flew four directions at WOT and averaged the speeds. GPS ground speed averaged 186.25mph with the fastest at 189mph. The average TAS was 181.75mph with the fastest at 184mph. It's not the fastest -4 out there but I can live with these numbers.
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  #7  
Old 05-09-2016, 08:32 AM
aerhed aerhed is offline
 
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Location: Big Sandy, WY
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Your average GS is your TAS. Where are you getting TAS's to average?
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  #8  
Old 05-09-2016, 09:30 AM
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Off the EFIS. I averaged the speeds like I did the GPS ground speed. Is that the correct way to do it?
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  #9  
Old 05-09-2016, 10:42 AM
Christopher Murphy Christopher Murphy is offline
 
Join Date: Feb 2007
Location: colorado
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Default Forget about your efis

Do the gps test average your ground speed. Thats it!
Anything attached to your pitot static most likely will not be exactly correct

Cm
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  #10  
Old 05-09-2016, 11:07 AM
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acam37 acam37 is offline
 
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I sent my engine type, HP, GPS speed, static and max RPM at 8k to Catto. They should be able to tell me if my prop is the correct pitch and size. I think we will also do a pitot/static check to make sure the airspeed is reading correctly.
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