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  #1  
Old 01-25-2022, 02:18 PM
Taltruda Taltruda is online now
 
Join Date: Oct 2017
Location: Las Vegas, NV
Posts: 1,182
Default Slow -7a

I flew a 180 HP CS prop RV-7a the other day and it seemed 20 knots slower than I expected.. what should be causing it? IO-360, 180 HP, Whirlwind 2-Blade CS prop, electronic ignition, wheel pants and full fairings throughout..

7500 feet, 8500feet and 9500 feet. WOT played with RPM from 2300, 2400, 2500 and 2600. Leaned to 80 ROP, only saw 152 to 155 knots TAS. Even in the descent, it felt like it wasn’t as slippery as my -4.. it felt like it was dragging a parachute going downhill.. I don’t see any obvious signs of drag.. although the folded trailing edges are way under bent, could that be it?
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  #2  
Old 01-25-2022, 03:22 PM
Ralph Inkster Ralph Inkster is offline
 
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Location: Calgary, Alberta
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Default

Instrument accuracy?
Pitot leak?
Have you confirmed by flying side by side with another aircraft?
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  #3  
Old 01-25-2022, 03:42 PM
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Sam Buchanan Sam Buchanan is offline
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Location: North Alabama
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I doubt a -7A will ever feel as slippery as a good -4....
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  #4  
Old 01-25-2022, 04:32 PM
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Carl Froehlich Carl Froehlich is offline
 
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Location: Dogwood Airpark (VA42)
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Never underestimate the value of proper rigging. Rigging is not just “one and done”.

Carl
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  #5  
Old 01-25-2022, 04:36 PM
Paul 5r4 Paul 5r4 is offline
 
Join Date: Oct 2006
Location: Foley, Al
Posts: 685
Default agree

I have 7A with 160 and FP prop. Just looked at an inflight video and my numbers were...

6500 ft altitude leaned but still ROP
74% power
2460 RPM
8.7 GPH
IAS 140 kts
TAS 155 kts

Seems to me your numbers would be much better than mine.
I agree either slow for some reason or instrument error.
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Last edited by Paul 5r4 : 01-25-2022 at 04:39 PM.
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  #6  
Old 01-25-2022, 09:07 PM
Taltruda Taltruda is online now
 
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Location: Las Vegas, NV
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I’ll have to fly next to another RV with it to check indicated airspeeds, but the winds were very light the other day.. we flew out to an airport and back, it took 1:00 there and 1:02 back.. my -4 does it around 45 to 48 minutes. I also can climb to altitude faster than the -7a, the -7a seemed to struggle above 8500 feet..

When checking “Rigging” are you checking the flaps are up, the ailerons are even with the flaps and wingtips, and the elevator is in trail during cruise? You can’t really change incidence once the plane is built, right? What else in “rigging” could be holding the -7a 20 knots slow? Weak engine? Surely a nosewheel can’t show it down THAT much, can it?
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  #7  
Old 01-25-2022, 09:35 PM
00Dan 00Dan is offline
 
Join Date: Nov 2016
Location: Atlanta, GA
Posts: 273
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Engine data is always worth a look at. The CS prop can hide symptoms that the engine isn't making power (whereas with a fixed pitch prop you'd clearly see the reduction in RPM if something was off).

Did one of the cylinder pairs (1/3 or 2/4) exhibit markedly lower EGT/CHT than the other?
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  #8  
Old 01-26-2022, 03:39 AM
Taltruda Taltruda is online now
 
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Location: Las Vegas, NV
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What would cylinder pairs indicate? All CHTs were about 380 ish in cruise, while the one labeled #4 (I suspect is really #3, as #3 is typically the hottest one) lives around 400.

My -4 this time of year has CHTs in the 330 range, and it takes aluminum foil on them to get them that warm..
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  #9  
Old 01-26-2022, 07:52 AM
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jcarne jcarne is offline
 
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Location: Worland, Wyoming
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Of course fix the trailing edges at some point but I can't see rigging slowing it down that much but I've been wrong before. I'm guessing instrument error.

Take up a GPS and do some box runs to see what the real speed is.

My -7A gets pretty much Van's published numbers.
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  #10  
Old 01-26-2022, 09:30 AM
Taltruda Taltruda is online now
 
Join Date: Oct 2017
Location: Las Vegas, NV
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Quote:
Originally Posted by jcarne View Post
Of course fix the trailing edges at some point but I can't see rigging slowing it down that much but I've been wrong before. I'm guessing instrument error.

Take up a GPS and do some box runs to see what the real speed is.

My -7A gets pretty much Van's published numbers.
We didn’t do gps box or triangle, but we flew a known route, and the gps was within a few knots of the TAS both ways.. plus the known route which typically takes 45 to 48 minutes, took one hour both directions.. winds were light at altitude, I think this plane is just 20 knots slow. Even in the descent, RVs are really slippery and pick up speed.. this one felt like it was being “held back”.
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