Firewall Forward Testing

I have come to the point in the project where I need to know if my engine and propeller will actually produce enough thrust to get the Piet into the air.  To evaluate this I am using the following criteria:

Ease of starting

Idle RPM

Transition to full throttle

Full static RPM

Thrust

My setup is a 164 CID Corvair, Wynne specs, OT-10 cam, 0.030 over, Zenith Bendix carburetor, long 'Y' intake manifold, hand made 66X35 propeller per Alvin Schuberts book, 93 octane, 3 into 1 into 'Y' exhaust with home made glass pack muffler

To measure thrust I am using a hanging type scale 0-200 lbs with a pulley to measure up to 400 lbs.

If I remember an article from Chris Heinz correctly, you should be producing about 1/3 the gross weight of the airplane in thrust.  This gives me a target of 360 lbs thrust.  I also found a rule of thumb that said you should get HP x 4 lbs of thrust with a cruise prop, or HP x 5 lbs thrust with a climb prop.  To work back to the 360 lbs of thrust desired, I would need to get 90 HP out of the engine with the cruise prop that I am using, or 72 HP if a switch to a climb prop (less pitch).  Here are the results of three test runs on my engine.

Timing Initial Low Jet High Jet RPM Thrust Transition
12 1.5 2.5 2300 200 Smooth
14 .5 1.5 2350 220 Rough
14 .5 1.0 2400 240 Rough

Well, 240 lbs of thrust doesn't seem to be enough.  Using the above formulas it appears that the engine is producing 60 HP (60HP x 4 = 240 lbs thrust).  I went to William Wynne's site and retrieved the following information about actual flying Corvair powered Pietenpols.  Since most of the engines are using different props, I adjusted the static RPM to reflect a 64x28 propeller using the rules of thumb in Schuberts book.  These rules say to add 100 RPM for each inch less in prop diameter, and for each 2 inches less pitch.

Owner Propeller Static RPM Cruise RPM Cruise MPH Adjustment Normalized RPM
Last Original 62'' climb 2600 2800 86    
Dave Karren 66x26 2600 3100 70 100 2700
David Cleveland 66x30 2850 2600 80 300 3150
Tom Brown 66x28 250 2450 70 200 2950
Jim Ballew 69 Warp 2800 2600 75    
Alan Swenson 62x26 3300 3000 80 -300 3000
Virl Deal 64x28 2500 2350 75 0 2500
Clifford Kumm 64x28 2500   80 0 2500
William Wynne 68 Warp 2700 2700 85    
Alvin Schubert 64x30 2950 2900 78 100 3050
Alvin Schubert 63x35 2600 2600 78 250 2850
Hans Van der Voort 64x34 2700     300 3000

When I apply the same adjustment to my propeller (66x35) I get a normalized RPM of 2950, which compares favorably with the other engines.  Then I went back to William's site to check the HP/Thrust section.  I retrieved the following.

Owner Propeller RPM Thrust
William's 601 66x41 2550 347
Larry and Cody Hudson 62x58 2445 225
William's shop engine 62x58 2520 231

William's engine with the Warp prop has the thrust that I was looking for.  The others have the same thrust that I currently have.

So, what next.  I made the prop 2 inches larger with the intent of trimming it if needed.  Once I get the current configuration running the best it can with smooth transition and max RPM, I will start trimming the prop and re-test.  Bringing it down to 64 inches should get me to 2600 RPM, and further into the Corvair HP range, but I will have to see what this does to thrust. I think I have too much pitch for a Piet, but I will continue to experiment. I wonder what type of thrust other Piets are flying on?

Update!!!

I now have the engine running at 2500 RPM Static with good starting, idle, and transition.  There were two changes I made.  First was to adjust the dwell from 28 to 34 degrees (and reset the timing to 14 degrees.  The next was to adjust the mixture with the aid of a "Colortune" see through sparkplug.  This device allows you to see the color of the flame inside the cylinder.  The best compromise I could was to have a slightly rich idle and top end with a good mid range.  This was at a main jet setting of 1 turn.  Setting the main to 0.5 turns gave a more blue flame, and more power, but the engine would not transition.  The resulting thrust reading is 280 lbs at 2500 RPM.  This moves my normalized RPMs up to 3050.  Pretty good.  I think I will leave everything where it is until flight testing (2 years away) and then decide if I need to trim the prop.

 

HP RPM Thrust AC Gross   HP RPM Thrust AC Gross     Diameter Area 68 67 66 65 64 63 62
50 2300 200 600   50 2300 250 750     69 340 0.99 0.97 0.96 0.94 0.93 0.91 0.899
55 2350 220 660   55 2350 275 825     68 335 1 0.99 0.97 0.96 0.94 0.93 0.912
60 2400 240 720   60 2400 300 900     67 331 1.01 1 0.99 0.97 0.96 0.94 0.925
65 2450 260 780   65 2450 325 975     66 326 1.03 1.02 1 0.98 0.97 0.95 0.939
70 2500 280 840   70 2500 350 1050     65 321 1.05 1.03 1.02 1 0.98 0.97 0.954
75 2550 300 900   75 2550 375 1125     64 316 1.06 1.05 1.03 1.02 1 0.98 0.969
80 2600 320 960   80 2600 400 1200     63 311 1.08 1.06 1.05 1.03 1.02 1 0.984
85 2650 340 1020   85 2650 425 1275     62 306 1.1 1.08 1.06 1.05 1.03 1.02 1
90 2700 360 1080   90 2700 450 1350                      
95 2750 380 1140   95 2750 475 1425                      
100 2800 400 1200   100 2800 500 1500     Speed       Pitch        
105 2850 420 1260   105 2850 525 1575     RPM 26 28 30 32 35 37 39 41
110 2900 440 1320   110 2900 550 1650     2400 52 56 60 64 70 74 78 82
115 2950 460 1380   115 2950 575 1725     2450 53 57 61 65 71 76 80 84
120 3000 480 1440   120 3000 600 1800     2500 54 58 63 67 73 77 81 85
cruise   climb     2550 55 60 64 68 74 79 83 87
                    2600 56 61 65 69 76 80 85 89
                    2650 57 62 66 71 77 82 86 91
RPM 2750         RPM Actual Fit Delta   2700 59 63 68 72 79 83 88 92
Pitch 35         1000 50 50     2750 60 64 69 73 80 85 89 94
Speed 80         1100 59 53 3   2800 61 65 70 75 82 86 91 96
Effeciency 0.88         1200 68 56 4   2850 62 67 71 76 83 88 93 97
          1300 77 61 5   2900 63 68 73 77 85 89 94 99
          1400 86 67 6   2950 64 69 74 79 86 91 96 101
          1500 96 74 7   3000 65 70 75 80 88 93 98 103
          1600 106 83 9   3050 66 71 76 81 89 94 99 104
          1700 116 93 11   3100 67 72 78 83 90 96 101 106
          1800 121 106 13   3150 68 74 79 84 92 97 102 108
          1900 126 121 15   2000 43 47 50 53 58 62 65 68
          2000 132 139 18                    
          2100 154 159 20   Required Thrust
          2200 177 183 24     L/D
          2300 200 210 27   Weight 6 7 8 9 10 11 12 13
          2400 240 240 31   700 117 100 88 78 70 64 58 54
          2500 280 275 35   750 125 107 94 83 75 68 63 58
          2600   314 39   800 133 114 100 89 80 73 67 62
          2700   358 44   850 142 121 106 94 85 77 71 65
          2800   408 49   900 150 129 113 100 90 82 75 69
          2900   463 55   950 158 136 119 106 95 86 79 73
          3000   523 61   1000 167 143 125 111 100 91 83 77
                    1050 175 150 131 117 105 95 88 81
Projected Numbers                 1100 183 157 138 122 110 100 92 85
Max Gross 1100                 1150 192 164 144 128 115 105 96 88
Max RPM 2900                 1200 200 171 150 133 120 109 100 92
Max Thrust 440                 1250 208 179 156 139 125 114 104 96
Max MPH 85                 1300 217 186 163 144 130 118 108 100
Cruise PRM 2550                 1350 225 193 169 150 135 123 113 104
Cruise MPH 74                 1400 233 200 175 156 140 127 117 108
Min Level RPM 2000                 1450 242 207 181 161 145 132 121 112
Min Level MPH 58                 1500 250 214 188 167 150 136 125 115