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RV-8 slowing down

donaziza

donaziza

Well Known Member
Hey Guys, Wonder if I could get some feedback on this. In the last bunch of months, My 8 has slowed from 152-156 kias, to 145-148 kias. I haven't gained weight:p, not aware of the engine losing power , ie the compressions are all good. I always wondered about the pitot tube. The wings, of course, are always loaded with bugs---I would "assume" the pitot gets loaded too??

It's a Dynon pitot tube, one of those heated kind that gives angle of attack. (The heat is broken)

Appreciate any help.
 
First run a NTPS 3-leg to establish IAS accuracy.
 
Last edited:
DanH said:
First run a NTPS 3-leg to establish IAS accuracy.
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This.

A static leak can cause a new error, a search of the archives will probably bring up a discussion of this from several years ago.

A "fast and dirty" check for a static leak is to make low passes over a long runway at different air speeds. When I developed a static leak, there were significant differences in indicated altitude that varied with ias.

Only do this if you are very comfortable making low passes, obviously do it at a safe altitude (I was level with the top of the control tower). Ideally use a very long runway at a quiet controlled field. This minimizes the chances of conflicting traffic.
 
Ed Williams Formula for TAS given 3 Groundspeeds

This is from Ed Williams Aviation Formulary. I have these formulas entered into a spreadsheet on my iPad, along with the formula for TAS given IAS and density altitude.
TAS and windspeed from three (GPS) groundspeeds.

Determine your groundspeed on three headings that differ by 120 degrees (eg 40, 160 and 280 degrees), call these v1, v2 and v3
Let:
vms = (v1^2 + v2^2 + v3^2)/3
a1= v1^2/vms -1
a2= v2^2/vms -1
a3= v3^2/vms -1
mu= (a1^2 + a2^2 + a3^2)/6​
Let bp and bm be the roots of the quadratic b^2 -b + mu =0 ie:
bp= 1/2 +sqrt(1/4-mu)
bm= mu/bp​
The TAS and windspeed are then given by sqrt(vms*bp) and sqrt(vms*bm) provided that the TAS exceeds the windspeed. If this is not the case, the roots are exchanged. This is a handy way to check your TAS (and the calibration of your airspeed indicator) using your GPS groundspeed, even though the wind is unknown.
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Here's the Ed Williams forumula I use to calculate TAS from IAS and density altitude:
For low-speed (M<0.3) airplanes the true airspeed can be obtained from CAS and the density altitude, DA.

TAS = CAS/(1-6.8755856*10^-6 * DA)^2.127940

Roughly, TAS increases by 1.5% per 1000ft.
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Ed Williams Spreadsheets

Come to find out, Ed Williams has a new website: Ed Williams Aviation Page. One of the links there is to an Excel spreadsheet containing the Wind Triangle formula, and many other handy formulas. Ed Williams recently retired as a scientist at the Lawrence Livermore Laboratory; He spent most of his career working on nuclear fusion. Smartest guy I ever knew.
 
Have you had the throttle linkage disconnected lately? If so, you sure you got the link back in the right hole?
 
You might try a compression check. I had a exhaust valve on number four develop a small leak which got my attention because a couple knots lost in cruise airspeed and 150 foot increase in take off roll. Both indications were insidious. Cleaned up the problem by lapping the valve. Once the problem was fixed, performance returned to normal.
 
Check your engine instruments, particularly tacho, and ASI to ensure evrything is telling the truth. I would 2nd a GPS calibration of the ASI, I use the NTPS method. Take care to fly headings and heights accurately.

Pete
 
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