This is too elementary for most of you guys but it was intended as a Kitplanes article. Building a low cost dyno. Revision 2 By Paul Lamar email: paul-lamar-la@ worldnet.att.net Wouldn't it be wonderful if everybody developing an engine for use in aircraft could afford a dyno. It would be a lot safer if people tested their intended engine for horsepower and static thrust on the ground instead of during the first flight. STATIC THRUST HP needs no introduction. Static thrust is that amount of force we get out of the engine, speed reduction unit (if you have one) and prop combinations that causes the airplane to initially accelerate. Acceleration is defined as a change in speed. What we want is maximum change in speed in the minimum amount of time so we can get up to takeoff speed in the shortest distance. IT IS NOT JUST A GOOD IDEA Isacc Newton laid down the law for this sort of thing over a hundred years ago. He said acceleration is proportional to force divided by weight. Simple enough. More force more acceleration. It is not just a good idea it is the LAW! If the force is divided by the weight in pounds we get a measure of the acceleration in units called "G's". One G is a change in speed of about 22 MPH in one second. If the static thrust (force) equals the weight, our airplane will be going 22 MPH the first second and 44 MPH after two seconds. The average speed for the first second is only sixteen feet per second so we will have traveled down the runway only sixteen feet. At the end of the next second our average speed is thirty two FPS plus sixty four FPS divided by two seconds or forty eight feet FPS. We are now ninety six feet down the runway and going 44 MPH. You see how important static thrust can be. The more the static thrust the better. One G is a wonderful acceleration. If only the average airplane had thrust equal to it's weight. Unfortunately the real world is not that wonderful or simple. PROP PITCH The thrust from the engine may increase or decrease as we travel down the runway depending on several factors the most important of which is prop pitch. If we have a fixed pitch prop and our pitch chosen is so high that the engine cannot spin the prop at max HP RPM while the airplane is stopped (typical for most fixed pitch prop airplanes) our thrust may increase as we accelerate down the runway and as the RPM comes up to max HP. If we have a constant speed propeller the thrust will will start out at the max possible and gradually decrease as we accelerate along the runway. Never the less the airplane will takeoff quicker with a constant speed propeller as we are getting more average HP out of the engine. MEASURING HP & STATIC THRUST HP is merely the product of torque and prop RPM. We can measure the torque by mounting the engine and prop combination on something that is free to rotate about the prop shaft axis. If we restrain this rotation and measure the force on a lever arm we will know the torque. If we allow the test stand to move forward and measure the force to restrain that motion we will know the static thrust. BUILDING A DYNO You can build a low cost dyno from a junked car rear axle, bathroom scale, ground adjustable prop and garage door spring to check HP and static thrust. Make a two by four and three quarter inch plywood frame about four feet high to hold the rear axle. Bolt the frame together with 3/8 carriage bolts and three inch drywall screws. Fabricate a two by four reinforced plywood firewall and bolt it to an old steel wheel. The wheel provides stiffness and a means to attach the firewall/wheel assembly to the axle. Take out the pinon gear and one axle shaft to reduce friction and leave in the differential spool for it's inner axle bearings. MOUNTING THE ENGINE Mount the engine with the motor mount you intend to use in the airplane to the firewall. Install the cowling to minimize the slip stream drag and improve the accuracy of the static thrust measurement. Mount the axle about four feet off the ground with a wide base two by four wood frame work. Streamline the frame work with styrofoam or thin sheet metal. Mount some cheap wheels or casters on the frame work to allow fore and aft motion. The axle allows the firewall to rotate about the prop axis. Stop the rotation with a two by four lever arm. Attach a streamlined two by two compression strut to the end of the lever arm and rest it on the bathroom scale. Make sure the strut is on the left side when the prop is rotating clockwise from the pilots perspective. Attach the base of the bathroom scale to the 3/4 inch plywood shelf. Attach a block of wood on the top of the scale to prevent the compression strut from blowing off the scale. Best have some positive means of limiting axle rotation to plus and minus twenty degrees in case something goes wrong. Attach the garage door spring to the opposite end of the axle and to a large tree or other suitable anchor. Use some sand bags to weigh down the whole rig to keep it from rolling over when blipping the throttle. CALIBRATING THE DYNO Calibrate the torque system with a large torque wrench. With a two foot lever arm double the scale reading to get foot pounds of torque. Calibrate the garage door spring using water as calibration weight. See my article on this subject in the Feb. 98 issue. TESTING Mount the engine and PSRU with an adjustable pitch prop. Balance the rig about the axle shaft so the engine does not rotate on it's own and zero is showing on the bathroom scale. Run the engine up to max RPM allowed by the pitch chosen. You now know the torque and prop RPM so the HP can be calculated. Stop the engine and lower the pitch until the engine will run at max HP RPM. Usually around 5000 RPM for a geared engine and around 2500 RPM for an aircraft engine. You now know the max HP. Repeat this process for any RPM of interest. Check the garage door spring for length and compare that to it's calibration numbers. You now know the approximate static thrust. The installed static thrust may be slightly more due to the better streamlining of the complete airplane. OPTIMIZING THE PROP PITCH If you are using a ground adjustable fixed pitch prop you may optimize it for the takeoff/cruise compromise by choosing a pitch that causes the engine to run several hundred RPM below peak power and twenty or thirty pounds below peak thrust. The engine will then rev up during takeoff. Hopefully to peak power for climb. MEASURING FUEL CONSUMPTION Make a fuel consumption gage out of a long piece of clear plastic hose mounted vertically. Measure how much fuel the hose will hold between marks using a chemistry lab calibrated beaker. Time the fuel consumption with a stop watch. Use an electric pump and an on/off switch to fill the hose again to repeat the fuel consumption test. Don't forget to correct for the temp. of the fuel. You now know the gallons per hour and the BSFC can be calculated from that information and the HP. Brake Specific Fuel Consumption or BFSC is a measure of the efficiency of the engine. It is in units of pounds of fuel burned for each HP for one hour. A good number for an aircraft engine is .42 pounds per HP hour. Some car engines are about .45 to . 55. Hook up a manifold pressure gage and record that as a function of HP. Knowing manifold pressure is very important to be sure your engine is developing the maximum possible HP at any RPM. I highly recommend bulding this dyno as it is better to find out on the ground what your engine prop combination is capable of rather than during the first flight. PROP |--------------- TORQUE in Lb/Ft ---------> RPM 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 1000 19 23 27 30 34 38 42 46 49 53 57 61 65 69 72 76 80 1100 21 25 29 33 38 42 46 50 54 59 63 67 71 75 80 84 88 1200 23 27 32 37 41 46 50 55 59 64 69 73 78 82 87 91 96 1300 25 30 35 40 45 49 54 59 64 69 74 79 84 89 94 99 104 1400 27 32 37 43 48 53 59 64 69 75 80 85 91 96 101 107 112 1500 29 34 40 46 51 57 63 69 74 80 86 91 97 103 108 114 120 1600 30 37 43 49 55 61 67 73 79 85 91 97 104 110 116 122 128 1700 32 39 45 52 58 65 71 78 84 91 97 104 110 116 123 129 136 1800 34 41 48 55 62 69 75 82 89 96 103 110 116 123 130 137 144 1900 36 43 51 58 65 72 80 87 94 101 108 116 123 130 137 145 152 2000 38 46 53 61 69 76 84 91 99 107 114 122 129 137 145 152 160 2100 40 48 56 64 72 80 88 96 104 112 120 128 136 144 152 160 168 2200 42 50 59 67 75 84 92 100 109 117 126 134 142 151 159 167 176 2300 44 53 61 70 79 88 96 105 114 123 131 140 149 158 166 175 184 2400 46 55 64 73 82 91 100 110 119 128 137 146 155 164 174 183 192 2500 48 57 67 76 86 95 105 114 124 133 143 152 162 171 181 190 200 2600 49 59 69 79 89 99 109 119 129 139 148 158 168 178 188 198 208 2700 51 62 72 82 92 103 113 123 134 144 154 164 175 185 195 206 216 2800 53 64 75 85 96 107 117 128 139 149 160 171 181 192 202 213 224 2900 55 66 77 88 99 110 121 132 143 155 166 177 188 199 210 221 232 3000 57 69 80 91 103 114 126 137 148 160 171 183 194 206 217 228 240 --