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  #11  
Old 01-26-2022, 09:39 AM
wcalvert's Avatar
wcalvert wcalvert is offline
 
Join Date: Jan 2019
Location: Anacortes Wa
Posts: 443
Wink

Forgot to put the gear up ... ?

Seriously, if you're convinced that the airspeed indications are valid, then you are 1. not making rated power or 2. not getting that power converted to thrust or 3. have more drag than expected.

Confirm your indications, then start working through the list. You're the one who knows the most about the build/engine/prop etc.

Cheers
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Bill (Wild) VA-165 '90-'93
Anacortes, Wa

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  #12  
Old 01-26-2022, 01:16 PM
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pilotkms pilotkms is offline
 
Join Date: Mar 2007
Location: WARNER ROBINS, GA
Posts: 576
Default Nother set of data

Short trip this morning.
180 hp, Hartzell BAF.
6500
2400 rpm, 22 mp
160 TAS, 145 IAS
66% HP, 9.4 gph
Recent flite along side an RV8, appeared my TAS May be 4 kts slow.
Im retired. Not really concerned about all those #s. Just like to keep it below 10 gph.
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RV 7A RV #9700 May 2017
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Built in SoCal KCCB, now in GA @ KPXE
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  #13  
Old 01-26-2022, 03:07 PM
Hwood Hwood is offline
 
Join Date: Feb 2005
Location: Central Alabama
Posts: 346
Talking

Perhaps it's that third gear leg?

Sorry, I couldn't resist! Please don't hit me!
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  #14  
Old 01-26-2022, 03:56 PM
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RV7A Flyer RV7A Flyer is offline
 
Join Date: Jul 2013
Location: US
Posts: 2,653
Default

Quote:
Originally Posted by Taltruda View Post
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 wasnt as slippery as my -4.. it felt like it was dragging a parachute going downhill.. I dont see any obvious signs of drag.. although the folded trailing edges are way under bent, could that be it?
Let's assume a book value of 170 KTAS, more or less. You say you're seeing 15-18 knots slower. Since a change in speed goes as the cube root of the change in horsepower, all other things being equal, I calculate that if it's related to the engine, you are getting 75% of the power that you should be getting. Perhaps you have a cylinder that's not making power?

Vn/Vo = (Pn/Po)^1/3
Rewriting
Po*(Vn/Vo)^3 = Pn
Let's just 180 HP...it's entirely irrelevant...
180 * (155/170)^3 = 180 * .757 = 136
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  #15  
Old 01-26-2022, 07:08 PM
calpilot calpilot is offline
 
Join Date: Aug 2018
Location: Independence
Posts: 230
Default

Sounds about correct to me!!!

DAR Gary

I have a "Super" RV-7, lots of mods, and see an honest 165 knots at 9500 feet, about 6-9 more than a more stock unit.
Remember..... We all want to go with our hair on fire.... but there are a lot of inaccuracies in prop, MP gauge, tach, leaning, cams, ignition, pistons, and drag reduction. Make sure you are comparing "apples to apples".

DAR Gary
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  #16  
Old 01-28-2022, 06:05 PM
jconard27 jconard27 is offline
 
Join Date: Dec 2021
Location: West Lakeland
Posts: 7
Default

CalPilot-

My O-320 powered RV-7 did an honest 165 kts true all day long....

Of course it was light.

And the only mods were a Sam James cowl, and lots of extra love.
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  #17  
Old 01-28-2022, 06:53 PM
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RV7A Flyer RV7A Flyer is offline
 
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Location: US
Posts: 2,653
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RV-7A w/ Lyc IO-360-M1B, Hartzell BA CS prop, a bit heavy due to interior and accessories, but I see 170 TAS at modest altitudes, running best power and 2500 RPM.

I suspect the OP has a cylinder that is not making power (or at least, not anywhere near rated power) to be that slow.
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  #18  
Old 01-29-2022, 05:37 AM
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DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 10,701
Default

Confirmation is a good thing. Download the true airspeed flight test spreadsheet here:

https://www.danhorton.net/Misc/NTPS_gps-pec.XLS

Inputs are GPS groundspeed and (at any heading) the resulting ground track. Fly early in the calm AM, before thermals, altitude hold on, throttle/mixture locked. Spinning the heading bug 120 degrees is convenient. Allow time to stabilize on each new heading.
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