More Performance Modifications
Mustang II Experimental Airplane N727RH

Updated December 27, 2009
(first published: July 16, 2004)
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These photos incorporate deviations from the plans.
Use of this information for Your aircraft is obviously AT YOUR OWN RISK.

Modifications in progress

This page will be a 'work in progress' to share what I'm doing with modifications. On July 3, 2004 I started an attempt to reduce cooling drag by reducing the original Twin Comanche cowl inlets by 33%. Original openings totaled 55 inē, now at 36 inē. They are filled with balsa, foam and tape for trial purposes.

My goal with these modifications is to maximize the 75% power cruise at 2400 RPM (near 8,000' DA) - currently at 166 KTAS, while still being able to climb and cruise at 17,000' when tailwinds favor that.

Reduced cowl openings
Reduced cowl
openings
Previously 55inē,
now 36 inē
(107 KB)		 Top of climb at 17,000'
Top of climb
at 17,000'
GW=1509#
Climb power =
full throttle (15.0")
2500 RPM
(138 KB)		 Cruise at 17,000'
Cruise at 17,000'
IAS 107 K, TAS 144 K
FF= 6.2 GPH, OAT= -2.7°C
DA=18,900'
(136 KB)		 Cruise at 7,000'
Cruise at 7,000'
IAS 145 K, TAS 166 K
FF= 8.8 GPH, OAT= +17.6°C
2400 RPM, DA=8,900'
(Above optimum Density Altitude)
(133 KB)
The primary purpose was to collect CHT and oil temperature data to confirm that the reduced inlets would not over-temp the engine. They were normal even though the atmosphere was ISA +13° at sea level (28°C).
Cruise data was collected at 17,000'; 15,000'; 13,000'; 11,000', 7,000' and 6,000'.
I erred by reducing the throttle at 6,000' to set MP for 75% cruise for a standard day: 23.1" MP, when I should have corrected for the non-standard temperature by running at 23.6" MP - just about full throttle - which would have been very close to the optimum density altitude for a 75% power setting at 2400 RPM.
I estimate that approximately one more knot would have been achieved if I'd collected 75% data at the optimum altitude.
New data will be collected when the fiberglass work is complete, which should smooth the flow into the engine somewhat.
Plans are to also reduce the cowl flap outlet area and later, add exhaust nozzles.


December 5, 2009

Cruise flight test and airspeed calibration

The intent was to determine what my "normal best cruise speed" is at optimum altitude at 75% power, full throttle and see if GPS verified speeds agreed with the IAS and calculated TAS.
Test was flown at 8,500', OAT -3° C, QNH 30.21", 1785 lbs., CG 72.5", full throttle, 21.8" MP, 2400 RPM, mixture at 75° ROP / 8.6 GPH.
I calculated the DA to be 8050', which is very close to what the PA-30 manual shows to be the DA with best TAS at 75% power. Considering the power setting chart for the engine, I believe the engine was producing 72.5% power under these conditions. If I had to do over, I would re-run the same test 1,000' lower and I believe that would have generated the desired 75%.
IAS was 139 K, yielding a [calculated] TAS of 159 K
Groundspeeds on the four cardinal headings: 360°=151 K, 090°=184 K, 180°=174 K, 270°=137 K.
Using the calculator found on this page : yielded an average TAS of 160.4 K.
Therefore my airspeed indicator appears to be indicating 1 K slow at cruise. (My data will be rounded to 1 knot)
I'm slightly disappointed that my previous "best cruise" was 166 K TAS, but will now "claim" and file 160 K TAS.
Although at a reduced gross weight and slightly lower altitude, (e.g. 7,000') the TAS may be as much as 5 K higher. In the past on long trips as fuel burns off, I have seen 3 to 5 K increases in TAS.
If I try this again at a lower, more "normal" gross weight at a slightly lower altitude, I would expect a couple more knots. I'll report it here if I do this.


December 21, 2009

Slow flight airspeed calibration

The purpose of this test was to see what my "actual" stall speed is compared to other Mustang IIs.
The lowest altitude with smooth air this day was 3,500'. QNH 30.16", OAT +3.8° C, 1,550 lbs., full flap, 50 KIAS held during test.
For reference, power-off stalls were performed, full flap with the first buffet felt at 47 KIAS, and the break occurring at 45 KIAS.
Clean stall was 4 knots higher at 49 KIAS
Calculated TAS=52 K, PA=3280', DA=2700'
While flying a steady Indicated airspeed of 50 K with full flap - (a nice, round, easy-to-see, easy-to-hold number that was close to stall):
Groundspeeds on the four cardinal headings: 360°=61 K, 090°=74 K, 180°=69 K, 270°=55 K.
Using the calculator found on this page : yielded an average "actual" TAS of 64.3 K.
While I held an IAS of 50 K, the DA corrections would yield a TAS computed as being 52 K.
Comparing this to the GPS verified "actual" TAS of 64 K tells me my airspeed indicator is reading 12 K slow in this regime. (64 minus 52.)
Thus I'll revise my "published" stall (break) speed to be 12 K higher than indicated: Vso 57 K (CAS) with full flap, Vs 61 K (CAS) clean for purposes of comparing to other Mustang IIs.
"To be continued..."

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Previous modifications page (canopy / windshield in 2001)

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