Home > VansAirForceForums

-Advertise in here!
- Today's Posts | Insert Pics

Keep VAF Going
Donate methods

Point your
camera app here
to donate fast.

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

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.

RV-7 Tipper
Bill (Wild) VA-165 '90-'93
Anacortes, Wa

First Flight 7/17/2021!!
- Been there, Donated 2022
Reply With Quote
Old 01-26-2022, 01:16 PM
pilotkms's Avatar
pilotkms pilotkms is offline
Join Date: Mar 2007
Posts: 576
Default Nother set of data

Short trip this morning.
180 hp, Hartzell BAF.
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.
RV 7A RV #9700 May 2017
N325KS the Flying K
Built in SoCal KCCB, now in GA @ KPXE
900+ Hours & 7X cross the USA
OSH flyin 2018 & 2019 & Petit Jean 2019 & 2021
Reply With Quote
Old 01-26-2022, 03:07 PM
Hwood Hwood is offline
Join Date: Feb 2005
Location: Central Alabama
Posts: 346

Perhaps it's that third gear leg?

Sorry, I couldn't resist! Please don't hit me!
Finishing up an -8
Exempt but paid
Reply With Quote
Old 01-26-2022, 03:56 PM
RV7A Flyer's Avatar
RV7A Flyer RV7A Flyer is offline
Join Date: Jul 2013
Location: US
Posts: 2,653

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
Po*(Vn/Vo)^3 = Pn
Let's just 180's entirely irrelevant...
180 * (155/170)^3 = 180 * .757 = 136
2022 Dues paid!
Reply With Quote
Old 01-26-2022, 07:08 PM
calpilot calpilot is offline
Join Date: Aug 2018
Location: Independence
Posts: 230

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
Reply With Quote
Old 01-28-2022, 06:05 PM
jconard27 jconard27 is offline
Join Date: Dec 2021
Location: West Lakeland
Posts: 7


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.
Reply With Quote
Old 01-28-2022, 06:53 PM
RV7A Flyer's Avatar
RV7A Flyer RV7A Flyer is offline
Join Date: Jul 2013
Location: US
Posts: 2,653

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.
2022 Dues paid!
Reply With Quote
Old 01-29-2022, 05:37 AM
DanH's Avatar
DanH DanH is offline
Join Date: Oct 2005
Location: 08A
Posts: 10,701

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

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.
Dan Horton
Barrett IO-390
Reply With Quote

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

All times are GMT -6. The time now is 06:33 PM.

The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.