What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Slow -7a

Taltruda

Well Known Member
Friend
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?
 
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.
 
Last edited:
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?
 
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?
 
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..
 
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.
 
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”.
 
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
 
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.
I’m retired. Not really concerned about all those #s. Just like to keep it below 10 gph.
 
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
 
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
 
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.
 
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.
 
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.
 
Back
Top