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Slowish RV6A

ReedC

Well Known Member
Hello,

I recently purchased an RV6a and it is not performing as I would expect. It has the following stats:

  • 1996 RV6A
  • 1100 Lbs empty
  • 160 hp O-320
  • Catto FP 70x70 Carbon with nickel edges
  • 91 Octane ethanol free auto gas
  • static RPM at DA of 5600, temp 15 C, is 2200 RPM as measured by a GRT EFIS

Speed Test conditions/results
  • DA of 8000 ft
  • PA 7000 ft
  • 4 passes 90 degrees apart, little wind
  • WOT , GRT EFIS showing 23.3 inches MP
  • fuel flow 9.2 Gph
  • RPM Avg 2660
  • IAS AVG 159 Mph
  • GS avg 173 Mph

The Catto people have been very responsive and indicate I should be seeing above 190 mph with these stats - putting me atleast 17 mph slow.

I did notice that several of the fairings have started to warp to varying degrees and separate slightly from the airframe. Most look ok, but one upper landing gear intersection fairing has maybe a 1/8 inch to 1/4 inch gap. I tried to tighten some of the fairings with Kapton tape for testing.

I also noticed there is no empenage fairing on the bottom of the horizontal stab/fuse intersection - see pic. Is this normal for an older RV6?


I would love input from the group on where to look:
  • engine performance? I am hoping you guys can help me rule this out with the RPM and fuel burn data.
  • excessive air frame drag?
  • prop tuning?

https://drive.google.com/file/d/0B24jmGdhTc40WDlObWo1TTBnMHZrSVg0TFFBTC1MREllSGpr/view?usp=sharing

https://drive.google.com/file/d/0B24jmGdhTc40OV9QS08wcFhmMFJGcDJNVlU1MDA2Q2tWdWh3/view?usp=sharing

https://drive.google.com/file/d/0B24jmGdhTc40SlA5MmVlZ3d3VzI2STdQMnJYZEdVWDhPWWp3/view?usp=sharing

https://drive.google.com/file/d/0B24jmGdhTc40NGRMZm82TFJOY3pfX2VpSW95WjhjOFFURUhn/view?usp=sharing

https://drive.google.com/file/d/0B24jmGdhTc40TXM2TThrR2VUMHJEUV9hY3ZRVldmU29oNVpn/view?usp=sharing


Thoughts?

Thank you,
Reed
 
I agree that 9 GPH should be producing at least upper 180's in a well performing 6. I get 185 at 8 GPH at 9K (catto 68X72). You need to determine if it is engine underperforming or drag/aerodynamics. I would ask craig what RPM that particular prop should be turning on a 160 HP RV-6 at 190 MPH. That should help you answer this question and strart your investigation. If Craig says that prop should be spinning 2750 to produce 190 (this is my quess, as your prop is very close to mine and I need to turn 2750 to get 189), then you should start your effots with the engine, as it is not able to produce enough power to spin it that fast.

Also, find the TAS calculator on this site. You need more math than a simple average of the 4 point speeds to get accurate results.

17 MPH below stock is a lot for drag issues. That type of speed loss would require issues that should be easily visible to the experienced.

Larry
 
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I also noticed there is no empenage fairing on the bottom of the horizontal stab/fuse intersection - see pic. Is this normal for an older RV6?

No. However, your second pic shows a std lower emp fairing.
 
Hi Larry,

Thanks for the leads

Sara @ Catto gave me the following:

?Top Speed at Target RPM- 191-195MPH@2750RPM?. Same as yours.

she did not indicate an altitude for this RPM.

I?m about 90 rpm low. I guess a compression check is in order.

I found some notes from the previous owner right after the Catto was installed. His 4 leg WOT test resulted in an avg gs of 183 mph and 2770 rpm. However, this was at 2000 feet PA vs my test at 7000 ft PA. Our airport elevation is 4600 and we are seeing DA closer to 5600 feet here so I cannot re run the exact test. What conclusions can be drawn from this? The airframe likely did not change much as it Looks the same.

Thanks,
Reed
 
What is your true airspeed in that condition? At 8000' DA it'll be a lot more than 159mph. The numbers on Van's site are true, not indicated, as that's the only way to normalize the values.

Also, check rigging. I've worked on several RVs recently that were wildly out of rig, probably to correct a heavy wing. An RV-8 I worked on last year picked up 10mph just by correcting the aileron/flap rigging.

Try a maximum speed run in level flight. Trim the elevator to neutral. Without touching the trim, return and land. Check to see if the trim tab is in trail with the elevator.
 
Hi Larry,

Inputing my IAS of 159 into an online TAS calculator gives me 179 mph. This is quite a bit higher than the 173 mph ground speed that the Garmin 430 showed when averaged over a square flight path. Not sure where the error lies.

Thanks for the tips on the rigging. I’ll go try the elevator trim flight tomorrow.

Reed
 
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Hi Larry,

Inputing my IAS of 159 into an online TAS calculator gives me 179 mph. This is quite a bit higher than the 173 mph ground speed that the Garmin 430 showed when averaged over a square flight path. Not sure where the error lies.

Thanks for the tips on the rigging. I?ll go try the elevator trim flight tomorrow.

Reed

The GS is of no consequence, it has no bearing on what you are trying to achieve.
There will always be variations from one airframe to another. When I bought my 8 I noted it's speed on my ferry trip home. A day latter after a lot of deep polishing & some repositioning of the u/c fairings I gained 4 kts under the same conditions.
Hope you can improve yr perf.
 
Hi Larry,

Thanks for the leads

Sara @ Catto gave me the following:

“Top Speed at Target RPM- 191-195MPH@2750RPM”. Same as yours.

she did not indicate an altitude for this RPM.

I’m about 90 rpm low. I guess a compression check is in order.

I found some notes from the previous owner right after the Catto was installed. His 4 leg WOT test resulted in an avg gs of 183 mph and 2770 rpm. However, this was at 2000 feet PA vs my test at 7000 ft PA. Our airport elevation is 4600 and we are seeing DA closer to 5600 feet here so I cannot re run the exact test. What conclusions can be drawn from this? The airframe likely did not change much as it Looks the same.

Thanks,
Reed

The fact that the plane used to turn 2770 with this prop and now won't is indicative of something sub-par with the engine, in my opinion. Engine will make more power down low. However, if Catto says that prop should be able to run at 2750 on a 6 with a 320, I would expect to be able to reach that RPM at 8K when ROP (9 GPH at 8K is ROP on a 320).

The fact that the previous owner was getting 183 at 2750 would point to some minor aerodynamic issues as well. Might be wise to start there, as it will be much easier. Do some research on proper aileron / flap rigging and clean up all of the obvious fairing issues. Once you get the plane as clean as you can (searching here will find lots of ideas and examples), then you can see how far off you are and address engine issues, if they exist.

I would definately confirm the mag timing and check the plugs now, though. Also, pay close attention to what happens during your mag checks at runup (RPM, EGT, any roughness, etc.) and report the results here. Ignition issues are the easiest to rectify.

The TAS calculator on this site has you put in 3 or 4 different ground speeds and tracks to compute actual TAS. Until you do this a few times, you can't trust the accuracy of your IAS and therefore any TAS computed from it.

Larry
 
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don't wanna sound harsh, but lookin' at those fairings, paint runs and missing screw(s?) tells me that she wasn't built to the highest standard.
I'd venture to say those fairings add up and loose 10mphish, the rest, as stated by others, may be due to mis-rigging (gear leg fairings are a good example).
The engine/prop combo and performance, though not stellar, look ok to me.
 
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