How to quantify two different designs without wind tunnel
02-26-2010, 02:12 PM
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How to quantify two different designs without wind tunnel
How would I go about mathematically proving that one plane is more aerodynamically efficient than another without using a wind tunnel? The inputs I have are airspeed from a pitot tube, battery life and motor current draw, propeller rpm, and total flight time.. What other parameters would be useful? mathematical explanations would be helpful too. Thanks
02-26-2010, 02:31 PM
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RE: How to quantify two different designs without wind tunnel
Without a wind tunnel, a good flight sim would be the next way to go.
We did this with an airframe we are constructing to do a particular job and the results were quite interesting.
Especially the effect of changing airfoils.
We did this with an airframe we are constructing to do a particular job and the results were quite interesting.
Especially the effect of changing airfoils.
02-26-2010, 04:17 PM
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RE: How to quantify two different designs without wind tunnel
You're on the right track but unless there's a fairly gross amount of difference between the two designs any differences would be masked in the air differences between flights and even in the way you operate the controls from one flight to the next. Obviously I'm assuming that these are two relatively similar models designed to deliver higher performance. Otherwise there wouldn't need to be any testing or serious examination to decide which is better. I'm also assuming since you're interested in this sort of performance that these are electric powered sailplanes, serious all up last down type time duration models or similar style. For sailplanes it's not just one aspect but a blend of performance. You don't want to optimize duration at the expense of cruise speed. Doing so makes the model too limited to weather and wind conditions. Even for a pure duration model to be able to fly it in real world wind conditions requires that you have some ability to penetrate the wind.
If you're stuck with using the great outside world as a wind tunnel and want to see what these two models can do while looking for minor differences then you pretty much need to have a second pilot and fly both of them in the same air and in the same manner during a single flight in pretty much a side by side manner. To reduce the piloting influence it is best if they are set up to fly hands off in a stable manner and then use control inputs only as required to maintain location. Small inputs and patience should be used for all corrections to avoid trim drag from the inputs. Take them to the same height and then shut off the motors and allow them to glide down. Do this at various trim speeds with multiple flights. After some number of flights up around 6 to 26 you should have a pretty good idea of which model is better than the other.
Similar testing can be done by taking the two models downwind and then setting up to come back upwind by starting at the same point and altitude and set the same flying speed as best you can by eye. The one that crosses overhead with the most alititude is the cleanest and most efficient for that speed of fast cruising. Similarly the one that crosses first for the same altitude loss is the more efficient and low drag. Downwind spotters equipped with walkie talkies will help a lot with this sort of testing in making sure that both start from the same distance. And when at the same distance your angle from where you're flying will tell you that they are at the same altitude.
If you're stuck with doing one model at a time then all I can say is good luck. As I said unless you can perfectly match the air and piloting done during the flight or do some sort of hands off testing then the errors of difference from one flight test to the next makes any small differences invisible. At that point you may as well just use the flight sim option even though I really don't think that it is all that worthwhile other than for specific changes. Assuming it is the exact same design but with only one change at a time then there's likely some good information to be had with a GOOD flight sim that does a lot of the calculations needed to cover the entire model. But if it's two different planforms with lots of differences you're in the dark with the sim unless you can find out that it's covering off all the bases.
If you're stuck with using the great outside world as a wind tunnel and want to see what these two models can do while looking for minor differences then you pretty much need to have a second pilot and fly both of them in the same air and in the same manner during a single flight in pretty much a side by side manner. To reduce the piloting influence it is best if they are set up to fly hands off in a stable manner and then use control inputs only as required to maintain location. Small inputs and patience should be used for all corrections to avoid trim drag from the inputs. Take them to the same height and then shut off the motors and allow them to glide down. Do this at various trim speeds with multiple flights. After some number of flights up around 6 to 26 you should have a pretty good idea of which model is better than the other.
Similar testing can be done by taking the two models downwind and then setting up to come back upwind by starting at the same point and altitude and set the same flying speed as best you can by eye. The one that crosses overhead with the most alititude is the cleanest and most efficient for that speed of fast cruising. Similarly the one that crosses first for the same altitude loss is the more efficient and low drag. Downwind spotters equipped with walkie talkies will help a lot with this sort of testing in making sure that both start from the same distance. And when at the same distance your angle from where you're flying will tell you that they are at the same altitude.
If you're stuck with doing one model at a time then all I can say is good luck. As I said unless you can perfectly match the air and piloting done during the flight or do some sort of hands off testing then the errors of difference from one flight test to the next makes any small differences invisible. At that point you may as well just use the flight sim option even though I really don't think that it is all that worthwhile other than for specific changes. Assuming it is the exact same design but with only one change at a time then there's likely some good information to be had with a GOOD flight sim that does a lot of the calculations needed to cover the entire model. But if it's two different planforms with lots of differences you're in the dark with the sim unless you can find out that it's covering off all the bases.
