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Senior Design at 100mph - Part 3
1,672 viewsCalculations have to be backed up by real data at some point. That real data has to be gathered by someone, somehow.
That is why our next step was to perform thrust testing.
We first needed to test our engine, tune it and make sure everything was working. As well, we wanted the static thrust value to see how accurate the advertised specifications were.
Test Stand with Force Gauge
Our test stand, as shown above, is a simple wooden stand with the engine mounted on a floating platform connected to a force gauge. This allows the engine thrust to register as a force on our gauge.
Although it took a while to get the engine started, we eventually got it running smoothly and recorded a static thrust load of close to 12 lbs.
Trying to Start our Brand New Engine
All propellor driving engines lose thrust as the ambient velocity increases. Because of this, to accurately predict our top speed we need to know how our engine and propellor combination perform at speed.
Yours Truly, Intently Documentating
The thrust data at varying ambient velocities are called dynamic thrust data.
Dynamic Thrust Test Stand Mounted in Pickup Bed
The best way to test something like this would have been to put it in a wind tunnel where air velocity could be accurately controlled and laminar. Our main problem with this was the large quanties of motor fuel and exhaust fumes that our engine belched. Because we didn’t want to spend days scrubbing out wind tunnel vanes, we opted for a less controlled, but perfectly valid back-of-the-pickup test. Below you examine our methods (high in science).
John Fiddling With the Engine Prior to our First Test Run
This method is very simple.
1. Put the engine on a test stand in the back of a pickup.
2. Drive that pickup as fast as possible.
3. Try not to kill the person in back since he has the numbers you need.
John Trying to Keep all His Fingers
Our test stand was the same one we used for our static thrust test. Of course someone had to be back there reading the numbers and either writing them down, or memorizing them. That person absorbed most of the liquids and gases spewing out the exhaust.
This is a Good Way to Save Gas
Our testing method was to increment our speed by 10mph and check the force reading. We only dared go to 60mph with John in the back, since he is a very light guy and could be blown out by passing wind, or by a gust of wind. We would then just extrapolate the data we needed from our thrust curve. We assumed a linear curve, which is probably not true, but good enough for our case. Most curves stay fairly linear before dropping off suddenly.
Thrust (y-axis) is in pounds. Speed (x-axis) is in mph.
Our results were very encouraging. We had almost 12 lbs of static thrust and 8 lbs of thrust at 60mph. We extrapolate around 4lbs of thrust at 100mph, which is almost twice what we need to maintain that speed (2-2.5lbs of drag at 100mph). So we feel pretty confident that we will achieve our target airspeed. John also thinks that the engine is still running a bit rich, so with a little bit of tuning, we might eke a few more ounces out.
As a side note, John still smells like engine fuel.
Absolutely wonderful, sir. Brilliantly written and documented. I look forward to future posts about these talented engineering minds and their highly scientific logic and methods.
snork.
Next time, try using a WOOD prop instead. It won’t flex and you will get much more accurate results.
Just wasting some free time on Stumbleupon and I found your post . Not normally what I like to learn about, but it was definitely worth my time. Thanks.