Speed measurement of high & fast aircraft would likely be an area of expertise and interest for you guys?

 

Yesterday I flew a Mooney Rocket.  They do some nice STCs (e.g. JetProp), and claimed that 31x2400 with 20-21 gph (ROP TIT ~1600) would get you 215 KTAS at 18,000' and 203 KTAS at 12,000' (http://www.rocketengineering.com/content/rocket-specifications).

 

It was a windy day (~50 knots at 17.5), so I used a GPS groundspeed method described by Williams that looks pretty robust (http://williams.best.vwh.net/avform.html#3gs).  You just fly 3 headings separated by 120-deg, measure the GPS groundspeeds and use my spreadsheet to remove the wind effect and get true airspeed. 

 

I wouldn't expect to hit book numbers, particularly since this plane was heavier than standard (2228 vs 2068#), had two pilots (190 + 180 lb), and has TKS.  I was surprised though to measure true airspeeds as much as 20 knots below book and probably a 9-knot error in indicated airspeed.  What am I missing? 

 

My spreadsheet with data and formulas is on (http://AwlBiz.com/fly/True_Airspeed.xls).  Parameters for the 18,000' test given below:

 

 31  MP (in)
 24  RPM
 20.3  FF (gph)
 1,570  TIT (deg F)

 

 17,500 Indicated altitude
 30.30 Altimeter setting
 -8 OAT (deg C)  --> Density altitude 18,455

 

 162  GPS GS1 (on arbitrary HDG1)
 247  GPS GS2 (on HDG1 + 120)
 188  GPS GS3 (on HDG1 +240)
 155  KIAS
  
 195  KTAS based on GPS groundspeeds
 207  KTAS based on KIAS and DA
 217  KTAS expected from book

 

--

Craig "wanna go faster than my 160 KTAS Mooney M20J" Jimenez