RV-4 performance and VANS numbers.....

[Steve Sampson]

While I wait for a new starter motor to be shipped from the USA I wrote a little spreadsheet program to work from RPM and manifold pressure to HP developed through the 'rule of thumb', and then extrapolated from horsepower developed through VANS specification numbers, to TAS. I then compared that result, with the output from two triangular runs I did to find the TAS using the TASCALC program.

The results are incredibly similar. VANS numbers as usual are right on the money, and it seems I built it right!

Manifold RPM VANS Predict(mph) TASCALC(mph)
22 2290 184 184 Run 1 (3 legs at 8000')
22.5 2270 185 187 Run 2 (3 legs at 7800')

Most exciting of all, 2600rpm and 25" predicts exactly 200mph! Will I be able to get 25" at 8000'?

Now I know there are errors in the 'rule of thumb', but unless someone has a practical equation for me its good enough for now. 187mph at those settings doesnt seem too shabby.

 

[n5lp]

...Now I know there are errors in the 'rule of thumb', but unless someone has a practical equation for me its good enough for now. 187mph at those settings doesnt seem too shabby.

Steve, you don't say what engine you have but Kevin Horton has spreadsheets that calculate power for several versions of the O-360 http://go.phpwebhosting.com/~khorton/rv8/

Look down the left hand side of this page.

 

[smokyray]

By the Numbers..

Steve,
Most FP RV4's with proper fairings, semi-straight airframes and strong 0-320's and matched props (or in your case C/S) have simple RPM/Airspeed ratios. Van has used the number 48 in the builders manual for a total MP+RPM for 75% power. 45=65% 43=50%. At SL 24 squared most 0-320 powered RV4's indicate 160 knots in level flight, 23 squared 150 knots, 22, 140. Most FP props won't allow you to match the MP/RPM easily below 5K. The sweet spot of efficiency seems to be 8500' where I could get 22"MP with Ram-Air. Below 5K I normally flew at 22/23 with my FP MT prop.
When I finally got my RV4 "tweaked" (10 years of playing with it) it would do 170 Knots at 2700 RPM/28" at SL. TAS at 8500 was 165KTs at 65% power. Yours should be even better...

Smokey