Debunking Speed Bunk
It is convenient and practical to adopt the figure .88 for "effective" propeller pitch as a constant for use where it applies in this article. The figure, of course, is the reciprocal
By Joseph Hruby (EAA 6674) 5754 E. Grant Road Tucson, Arizona
T IS APPARENT THAT many of our brethren are not even remotely aware of what the maximum speed a Isingle-engine airplane with a fixed-pitch propeller can attain. An excellent article on the subject appeared in the July and August, 1971 issues of SPORT AVIATION, and another good one dealing with the widespread absurd stories on homebuilt speeds can be found in the August, 1970 issue. Here the author tries to show how easy it is to know what his "pride and joy" is capable of doing. But the clear and simple explanations must have been too much to comprehend for many. Or was it perhaps too painful to accept the truth and logic? At any rate the writer, dwelling on the matter, was provoked into making the accompanying table for direct reading for those not inclined toward arithmetic. The speeds shown in the table will read high by about three percent or more for all but the cleanest airplanes. This applies not only to homebuilts but also to factory creations, with the reminder that we are restricting this to single-engine planes with fixed-pitch propellers. The table was made up on the basis that a propeller always has a "slip" of 12 percent or that it has an "effective" pitch of 88 percent of the pitch as stamped on the hub. Now, on this matter of "slip" and "pitch", let's consider the following statements: "There is no known way of propelling an airplane other than by driving a 'jet' or stream of air or gases backwards. A propeller provides a simple means of creating such a 'jet'. It must be of a velocity greater than the speed of the airplane. It is the difference in the velocities that accounts for the
of .12 for "slip". "The highest probable speed that an airplane can attain in level and stabilized flight is a figure that is the product of the propeller pitch, as stamped on the hub, multiplied by the rpm." This ultra-simple formula is, in substance, the same as the one generally accepted which incorporates the following factors: 1. Propeller pitch 2. Revolutions per minute 3. Minutes in an hour 4. Feet in a mile 5. Allowance for propeller slip For an example, we have a hypothetical plane with a 48-in. (4 ft.) pitch propeller cruising at 2500 rpm. What is the speed? Speed = 4 x 2500 x 60 x .88 = 100 mph, or; 5280 Speed = 4 x 2500 x (60 x .88)= 100 mph, or; 5280 Speed = 4 x 2500 x = 100 mph, or simply; Speed = 4 x 2500 = 100 mph It must be obvious as to what the figure .88 does for us. When multiplied by 60 we get 5280, the same as feet in a mile. In one fell swoop we eliminate the factors for minutes in an hour, feet in a mile, and the allowance for propeller slip. The foregoing, of course, was not written for the edification of our professionals, but rather to justify the figures in the table, and to offer some food for thought for those who are about to write on the performance of their new homebuilt. Also, this may be of some interest to the old die-hard veterans who have learned to fly fast by "flying on the step".
term 'slip'."
PROPELLER REV'S PER MINUTE U
o U3
£
O* N
28' 30' 32' 34' 36' 38' 40' 42' 44' 46' 48' 49' 50' 52' 54' 56' 58' 60' 62' 64' 66' 67' 68' 70'
52 56
59 64 67 71 75 79 82 86 90 92 94 97 101 105 109 112 116 120 124 125 127 131
o o co es
53 57 61 65 69 73 77 81 84 88 92 94 96 99 104 107 111 115 119 122 127 128 130 134
o 10 co
es
55
59 63 66 71 74 78 82 86 90 94 96 98 102 106 110 113 117 121 125 129 131 133 137
o o •t (N 56 60 64 68 72 76 80 84 88 92 96 98 100 104 108 112 116 120 124 128 132 134 136 140
0
it> •t «>*
57 61 65 69 73 77 82 86 90 94 98 100 102 106 110 114 118 122 126 131 135 137 139 143
o o in s^ 58 62 66 71 75 79 83 88 91 96 101 102 104
108 112 117 121 125 129 133 138 140 142 146
o m
o o
59 64 68 72 76 81 85 90 93 98 102 104 106 110 115 119 123 127 132 136 140 142 145 149
60 65 69 73 78 82 87 91 95 100 104 106 108 112 117 121 126 130 134 139 143 145 147 152
in M
(O
SM
to ei
o o t(N
o IA
62 66 70 75 79 84 88 93 97 102 106 108 110 115 119 124 128 133 137 141 146 148 150 155
63 67 72 76 81 85 90 95 99 104 108 110 112 117 121 126 131 135 139 144 149 151 153 158
64 69 73 78 82 87 92 96 101 106 110 102 115 119 124 128 133 137 142 147 151 154 156 161
0
m
t~ (N
o o
0
e>i
00
65 70 74 79 84 88 93 98 103 108 112 104 117 121 126 131 135 140 145 150 154 157 159 164
OS
CM
o co
CN
co
o o •f
67 72 77 82 87 92 96 101 106 112 116 108 121 126 130 135 140 145 150 155 160 162 164 169
70 75 80 85 90 95 100 105 110 116 120 122 125 130 135 140 145 150 155 160 165 168 170 175
74 80 85 90 96 101 107 112 117 123 128 130 133 139 144 149 155 160 165 171 176 179 181 187
79 85 90 96 102 108 113 119 125 131 136 138 142 147 153 159 165 170 176 181 187 190 192 198
o
0 0
o 0
co
o 0 ce
o o 00
co
83 90 96 102 108 114 120 126 132 139 144 147 150 156 162 168 174 180 186 192 198 200 204 210
co
89 95 101 108 114 120 126 133 139 147 152 155 158 165 171 177 184 190 196 202 205 211 215 222
SPORT AVIATION
13
