Slow -7a

[wcalvert]

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

[pilotkms]

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.
I’m retired. Not really concerned about all those #s. Just like to keep it below 10 gph.

[Hwood]

Perhaps it's that third gear leg?

Sorry, I couldn't resist! Please don't hit me!

[RV7A Flyer]

Originally Posted by Taltruda

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?

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

[calpilot]

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

[jconard27]

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.

[RV7A Flyer]

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.

[DanH]

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.