Something I have been wanting to know for a while. What is the Best Range Airspeed for the DJI FPV?

  

No, we’re not talking maximum range distance, or longest time aloft. What I’m curious about is what speed yields the most distance covered, with the least battery expenditure. This is something used in large aircraft, and it’s an important number to know in case you are in a low fuel situation, or you simply want to cover distance in the most efficient and conservative way.

  

Imagine you are doing a FPV flight and you are a few miles away. You lose track of time and battery levels and suddenly find yourself needing to return to home, but the battery is very low! What do you do? What speed gets you home while conserving the most battery? You’d hate to be just a short distance away, returning to land, only to have the FPV auto land (or crash) short of the home point possibly never to be found again. Had you returned at Best Range Airspeed, (covering the distance in the most conservative manner) you may have made it back.

  

Flight Tests: I conducted a series of five test flights, under identical conditions, and plotted the results below. For each test I started with a 100% full battery, launched to hover, put goggles on, entered Sport Mode and then flew the course climbing to 100’ AGL. Winds were light. Temperature about 70 deg F.

  

For each test I accelerated to 20, 30, 40, 50, and 60 MPH respectively and set the cruise control to hold that speed and altitude. My course was along a straight road and I went down the road 1.7 miles, turned around and flew straight back. I flew the exact route each time, only varying the speed on each test for a grand total of a 3.4-mile route.

  

  

I captured all the data from the post-flight logs, and measured data on the outbound leg, return leg, and then the totals. See the below chart.

  

  

Results: I found that at about 45 MPH, the DJI FPV covers the most amount of distance while using the least amount of battery. 45 MPH is the Best Range Airspeed and will get you any desired distance with the least battery used!

  

  

I had initially postulated that a slower airspeed would be better, but I found that the longer flight times to cover 3+ miles really ate into the battery. As you go faster and faster, at speeds above 50 mph, you certainly cover the distance quickly, but you end up really pushing the battery hard and eat up the juice rather quickly that way too. This is like a dragster. You get there quickly, but you rip through fuel at a crazy rate to do so.

  

Winds: I flew this test with light winds that were almost perfectly 90 degrees to the path of flight, so the winds shouldn’t make that much of an effect on the results. What is not known is how the FPV quad will perform in a headwind, or a tailwind, and what speeds might be best under those conditions. The biggest factor being that the quad will be exerting more, or less power to attain the requested ground speed with a headwind or tailwind.

That is really really interesting data.

My own experience agrees with what you show. Slow flight drains the battery almost the same as medium speed flight but the medium speed flight covers more ground.

If you're so inclined, I'd love to see the power used map Airdata provides. I'd also like to see the efficiency readings for each of the flights.

If you have a PayPal, I'll send $3 your way to add the wind feature to your Airdata account for a month. I'd be interested in seeing the wind information on these flights.

The drone does have to work harder in a side wind. It's flying a crab angle and the side component drains the battery a bit fast. You likely know this and correctly assumed the side component could be ignored if the wind was slow.

These are just requests from a stranger. You are of course under no obligation to provide this extra data.

I thank you for the great data you just provided.

Thanks Duane. Good to know on the crosswind, I did experience some of that, but not too much. With an out and back test route, I hoped to negate some (not all) of the wind effects.

I just looked at the Airdata battery efficiency numbers (will try to post pics later if needed) but they resemble my findings as well. Miles Per Battery were as follows. Flt 1 was 20 mph, Flt 5 was 60 mph.

1. 5.68
2. 6.98
3. 7.81
4. 7.37
5. 6.17
Edit: Here is that data on a graph. Might need more data points huh?

Hello there Buster1. Good day and thank you for sharing these informative information that you have tested and give. Great information and keep flying.

Thanks @Buster1 for this work, I was speculating myself what would be the right speed to get back home it is great to see this in numbers!

Great testing Buster.  Interesting results on what speed turned out best.

Hey guys,

I wanted to add in some data points. I did 2 more data flights, along the same route, same everything as before (except at 150' AGL this time vs the 100' before). Wind was slightly less too. I did these flights at 45 mph and 55 mph respectively and you can see where that data falls into the graphs. They are indicated by the red dots.

What was interesting (to me) was that the battery used amount (from my data off the drone) fell right into the curve nicely on the first graph. On the 2nd graph we are looking at the Airdata website numbers for Mi/Battery and what it calculates. Both data points plot along the curve, but are a touch higher, or more efficient. I'm thinking this is due to nearly calm winds that I had on the new flights Airdata estimated slightly more range for each battery.

Anyway, check it out!

One thing to keep in mind is that, In Normal and Sport mode, the aircraft will try to maintain ground speed (via GPS), not airspeed. So if there's a wind component, the drone will try to adjust its airspeed to compensate, and achieve the desired ground speed.

The other thing is that the losses due to a headwind aren't fully compensated for during the tailwind part of the trip. The reason being, the headwind has a longer time to act on the aircraft during the headwind leg than the tailwind has to act on the aircraft on the tailwind leg of the trip. So it doesn't even out, and you'll get different results in a wind with varying ground speeds.  This is one of those brain-twisters that pilots get during their ground school when working on their private. And it takes a while for it to kind of make sense .

In other words, I'd say be sure to conduct the test on a no wind day, to minimize that effect.

All great points Luciens, and correct too. Wind can drastically change speeds and energy used in every direction. I hope having ‘calm’ winds helps for accuracy in these tests.

One of my questions was like you mentioned about headwind or tailwind. In a headwind, the drone pushes harder to maintain the commanded ground speed... I think. The converse is true for tailwinds. It’s not like an airplane that maintains a constant airspeed in the air mass based on a thrust setting.

Thant you Buster 1 , very interesting information !
I was wondering myself what best range speed was for this drone !

Buster1 Posted at 5-3 13:29
All great points Luciens, and correct too. Wind can drastically change speeds and energy used in every direction. I hope having ‘calm’ winds helps for accuracy in these tests.

One of my questions was like you mentioned about headwind or tailwind. In a headwind, the drone pushes harder to maintain the commanded ground speed... I think. The converse is true for tailwinds. It’s not like an airplane that maintains a constant airspeed in the air mass based on a thrust setting.

Yes, in the GPS modes, it tries to maintain the speed reported by GPS. Which is technically ground speed. It's like the other GPS drones in that respect.

If you wanted to control for airspeed, you could do that in manual mode, but without an airspeed indicator it might be tricky. Maybe enter forward flight at a given attitude and power setting and just hold it there as best as possible. That's a method used in full scale when you don't have an airspeed indicator, so it might work reasonably well with the FPV drone in manual.

That would also compensate for headwinds and tail winds, as the idea there is to maintain the same speed through the airmass, regardless of ground speed. The distance covered would be different in each case, tho.

But still great experiment and great data for us to have!

eisenfliege Posted at 4-21 00:26
Thanks @Buster1 for this work, I was speculating myself what would be the right speed to get back home it is great to see this in numbers!

That's the benefit I'd see too!  
What is the best speed to fly back when I'm far away ...
     

It's very hard to find a reason why the rth speed is way slower than the most efficiency speed ... In any case very interesting tests. I was about to do the same for my long range flights. Should be interesting also I find the most efficiency descent rate (at 45mph).
The best way should be to have a osd efficiency indicator with the choice to engage autopilot...