Dyno

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

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