Servers by jDrones

L/d test flight


(Jace McCown) #5

hmm, cool! yeah thats why I’m asking right? thansk for the insight, looking through the prop data definitly makes me think its lower than the pitch speed (new term to me). Thanks!


(Samuel Tabor) #6

Hi Jace,

I do L/D tests in auto quite regularly. I use the soaring feature to allow climbs to use throttle, but descents to be zero throttle. Usually the soaring feature will trigger LOITER mode occasionally, but this can be suppressed by setting a large value for SOAR_VSPEED, eg 1000. Be sure to set SOAR_ALT_MIN and SOAR_ALT_CUTOFF as described here http://ardupilot.org/plane/docs/soaring.html

Using TECS_SPDWEIGHT=2 and a large value for PTCH2SRV_IMAX will be needed for good airspeed control with zero throttle.

If you want to map the glide polar, set up a rectangular mission. Copy the mission multiple times and use a DO_SET_AIRSPEED command to adjust the target airspeed.

Sam


(Matt Kear) #7

Hi @jace26
Specifically what parts of the flight envelope are you interested in finding the L/D ratio for? Are you going to use the data for a specific reason or is it more just to see if you can?
The reason I ask is because your Prop is going to have a big influence on that value. For example at 0 throttle it’s going to create significant drag, or assuming you have a tractor config the Prop wash will boost your lift.

It is tricky to characterise the aerodynamic performance of the aircraft through flight in this way.


(Jace McCown) #8

Cruise speed l/d for determining drag reduction device effectiveness and cruise speed for max range, max time aloft etc. We have a push config, not tractor. since we are a vtol plane, we are messing with cleaning up drag at cruise speed and then seeing how that affects our payload capacity, optimal cruise speeds ect. To some extent since these will be relative to later flights with drag reduction devices installed and for those flights the thrust isn’t as important as long as its somewhat consistent. Determining how l/d changes as a function of cruise speed is a little more difficult when trying to eliminate the prop. To be honest though, we know this isn’t going to be a lab quality result, at some level of measurement the prop will be in the noise.


(Nathan E) #9

Why not just put a folding prop on? That’ll effectively cut the drag and then the true unpowered glide will give you the L/d.


(Matt Kear) #10

Given your intended use of the data, may I suggest an alternative metric instead of L/D?
Is your aircraft a quad plane?
Also is the forward propulsion electric?

On the assumption that your aircraft is electric and you have current and voltage sensors attached and suitably calibrated, then a suggestion would be for you to use power and thrust as your metrics as these directly correlate to max endurance and max range flight conditions.

It is known that the maximum endurance of a fixed wing plane coinsides with the minimum power required for straight level unaccelerated flight. In my experience the velocity at which the aircraft will need to fly at to achieve this condition will always be quite close to the stall speed of the aircraft (within 20% of the stall speed). You need to make sure that your aircraft has a pitot tube for measuring airspeed and that you have adequately tuned ARSPD_FBW_MIN to be close/equal to your stall speed. Then you can run a series of experiments in which you leave your aircraft in loiter (with a wide turn radius) and fly it at series of discrete speeds. Then using the logged data you can correlate the velocity that flys at min power required.

Similarly to the above, the velocity that achieves maximum range in a fixed wing aircraft travelling in straight level and unaccelerated flight will coincide with the minimum thrust required. For a propeller aircraft it is a reasonable approximation to say:

thrust = power / velocity

Therfore using the same data from the flights suggested above you can now do a bit of work with the log flies to determine the velocity that corresponds to max endurance. And if you really want to find a lift over drag value it just so happens that to achieve max range you would need to fly at L/D_max and you could determine it from this equation:

L/D = weight / thrust

This is of course only applicable if flying at constant altitude and constant speed.

Using this method has the advantage that you are specifically flying in cruise conditions (your region of interest) and it reduces the effect of wind speed and direction, which would have a big impact on glide slope based approaches. Also you can fly your aircraft in loiter for longer periods of time, collecting more data, to improve your averages. Thus the experiment will likely have greater repeatability.

The disadvantage of this approach is that it relies on power and therefore current measurement, which is not the most reliable and will typically give noisy information. However, given that the experiment requires your aircraft to fly straight level and unaccelerated the current draw will be pretty constant and the measurement will be quite reasonable. Additionally, if you take the mean of your data for each flight velocity and you find that when you plot out velocity vs power (or velocity vs thrust) for all experimental conditions and you see a minimum power/thrust that occurs somewhere close to stall speed then it is a strong indication that the experiment worked and that the method’s experimental noise is reasonable.

If you do have a quad plane, use Q_assist and be sure to set Q_ASSIST_SPEED to be your stall speed. Then the plane will always be able to get out of trouble if it does stall.

I have messed around with getting my own quad plane to fly at slow speeds and was impressed at how well FBWB based modes fly at high alpha when an air speed indicator is fitted.

I hope all of that made sense. Not sure I explained it very well :sweat_smile:

Matt


(Nathan E) #11

You’re right that a power based approach will yield the best endurance. Unfortunately a level flight power-based approach compounds in complexity based on the propulsion efficiency curve. If you want to optimize both systems, it’s much easier to find the optimum of one and the. Optimize the other to it.

For example, if I choose a 16x5 prop, my peak efficiency will be significantly closer to the minimum controllable airspeed than with a 12x12 prop. I’ve done many tests this way, and some of the discussion is here: https://www.rcgroups.com/forums/showthread.php?3009817-Benchmark-Test-of-the-Twin-Engine-Electric-Mapping-Planes

Knowing the effective L/d is very important in the design stage of aircraft. I don’t think using RC forward propulsion systems are currently accurate enough to derive that from level flight (due to numerous complex energy losses in the system). I do agree with the glide-based approach.


(Matt Kear) #12

Yes, you are entirely right. I did not think about the effect on the propellers efficiency curve! Plus I made a mistake in my previous post as you can’t obtain the thrust from the electrical power draw without knowing the efficiency curves of the various propulsion systems like ESCs, motors, and props.

So with that, I guess I am also in the glide slope approach camp.

@Samuel_Tabor when you have done this previously do you use GPS positioning to get to distance travelled and baro to get height loss then calculate the slope?


(Nathan E) #13

One of the easiest ways is to use an RTK GPS and turn the raw logging on. Then RTKLIB can give you a precise trajectory. If you do a few loiter turns, the forward wind speed can be estimated and thus corrected for in the time domain. You could also use airspeed, but once again, that relies on another calibrated system. I like to keep my data with as few variables as possible, and the RTK GPS on a calm day provides me with excellent data.


(Matt Kear) #14

Great, thanks. That is good to know.

Out of curiosity have you tried using airspeed? If so what were the results like? I appreciate where you are coming from with minimising the number of calibrated sensors used. I was just wondering whether in this case, as the airspeed sensor is reducing the effect of wind on the data, would it work well enough (calm days are a rare occurrence near me).


(Jace McCown) #15

I can second getting good real air speed by loitering for a while first and getting the average gps GS to calibrate the AS. Then use the airspeed and the stable IMU vertical velocity to calculate real air slope regardless of the wind.

What I really want to do is incorporate a force sensor between the push motor and the frame to record thrust. We havent gotten around to that yet… but that would give you L/D just by knowing your aircraft weight, and the thrust in unaccelerated flight.


(Matt Kear) #16

Can you let us know how you get on if you do install a load sensor inline with you motor mount? I would be interested to hear about it.


(Jace McCown) #17

unlikely we’ll do it anytime soon :disappointed_relieved: but it would nearly ideal for this purpose unless I’m missing something.


(Samuel Tabor) #18

I used the target airspeed divided by the barometer derived rate of decent over a resonably long descent period e.g. 30-40s while flying in a straight line in low turbulence conditions.


(James Miller) #19

I’ve got to go with Sam’s method here. I’ve done many similar tests using his approach while attempting to measure the effect of vortex generator placement on l/D, and got repeatable results as long as the airspeed was reasonably well calibrated, and wind was known, and could be factored in. I was testing a glider, so the prop was a folder. Essential, I think.
My barometer/altimeter seems to have pretty stable calibration, provided nothing else was changed (like board placement, aerodynamic shape, etc,) and since I was shooting for optimum placement, deadly accuracy in absolute terms was not essential. That said, I think my results were pretty close even in absolute terms.
It’s possible to over think the problem, and until you know it is inadequate, I’d try the simpler approach.


(Jace McCown) #20

Cool. What did you find for your vortex generators?


(James Miller) #21

That’s not a short story.
If you calculate the proper dimensions of vortex generators for a wing of 22 cm chord at 14 m/s with the corresponding reynolds number, you find that most of the historical tests were performed wit generators that were far too large.
I found a product that bills itself as a turbulator that actually was a vortex generator tape in self-adhesive form and was pretty close to the proper size and spacing- 0.6 mm high instead of the 1mm that would have been ideal, so I used that for the generators. We all know the nasty tip stalls that some otherwise great aircraft have, and I was trying to find out if that problem could be reduced with devices to delay the stall at the tip.
In short, it worked partially on my much-abused but still true X-8, with the placement in the most effective position of 27% chord aft and a length of around 25 cm, with no detectable reduction of the L/D. The tip stall was reduced to the point where my fairly heavy X-8 would stall at around 7 M/sec. and a pitch angle held to around 5 degrees, with only a slight rolling tendency. I balanced the roll by varying the length of the generator strips until I got it as close to a straight-ahead stall as possible, but it never got to the point where you could say the problem was cured- only to the point where an alert pilot could dump the stick for less than a second to break the stall, still in stabilized mode, and it would fly right out of it—far better than the original condition. Here’s the link to the product.
https://www.topmodel.fr/en/product-detail-21380-turbulator-tape-white-5m-roll

Was this product truly making vortexes that would draw the free-stream air flow to the surface and keep it attached? Can’t really say, but it definitely worked to retard the stall.


(Jace McCown) #22

Thanks for the brief!


(Jace McCown) #23

So we did the glide L/D tests and it seemed to work okay. We didn’t ever pin down a number but got enough data to show that flying at x speed increases our L/D significanty over flying at x+ speed. It would be really hard to pin a number down to even a tenth because of the variance even in a single descent. A well tuned aircraft might do a better job at holding speed and maintainign a constant descent, but we saw quite a bit of variability. We also flew in some failry stiff winds, which didn’t help in acheiving prescise results, but didn’t really hurt the gross understanding we were tryign to achieve.


(James Miller) #24

The real world sure makes it tough to nail down a firm L/D number, I know. Here’s a well thought out tool I used to get a starting point for my estimates. My numbers matched this fairly well.
https://www.desmos.com/calculator/n3g05l9diz
I was not particularly concerned about absolute L/D, but stall. Also, I looked at my notes, and the pitch angle was 15 degrees, not 5 degrees.