Servers by jDrones

Benchmark Test of the Twin Engine Mapping Planes

(Nathan E) #1

This blog reflects my posts on RCGroups

I’m very interested in large area mapping/surveying at an affordable cost. To accomplish this goal, I’m beginning by researching the aircraft that can carry the most payload in the most efficient manner (subject to some constraints). There’s a ton of information online about new aircraft releases, and it’s great info. My goal is to test/verify it myself, and publish those results in a clean and accurate manner so that you can save yourself some time testing and tuning. I’d like to eliminate as many variables as possible and test the airframes on a level playing field. Ultimately I would like to sell the best setup as a ready-to-fly system to customers at a much lower price than others are charging (look at the eBee for example).

The planes I want to test are twin-engine foam planes under $500. In my opinion, they are the easiest and safest to hand launch for the amount of payload they carry. I’ve read, but not verified, that they are more yaw stable than flying wings. There have been quite a few released recently, and I’d like to benchmark their performance. The ones I want to test are as follows:

  • MyTwinDream (MTD) - Benchmark
  • Skywalker EVE-2000
  • Believer
  • X-UAV Clouds
  • Skywalker Titan
  • MFD Nimbus (maybe)

The independent variables I’d like to test on each aircraft are as follows:

  • Payload/AUW
  • Motor/prop combos
  • Airspeeds
  • CG locations

The dependent variables that I want to discover:

  • Takeoff ability
  • Cruise efficiency
  • Stall and maximum level speed
  • Glide speeds (l/d ratios)
  • Stall characteristics
  • Roll rates
  • Load factors before accelerated stalls

I’ll also publish/maybe model the internal fuselage dimensions so you can see if your particular camera fits. This will also be combined with other advice/observations from other threads into a detailed but concise, unbiased, and accurate report. I’d also post the pixhawk parameter files for you to use for your maiden flights. I’m not planning on a build log because that’s already covered in detail on other threads.

I have experience building and testing the Skywalker EVE-2000, and it will be a benchmark for me. I had consistent 2-hour autonomous flights at 16 m/s on a 16Ah 6s li-po battery. I’m sure the performance could be better with many upgrades, but I was happy to not spend more than $600 on the setup.

Please let me know if you have any useful insight into this process, or if you have data or suggestions that I should be aware of.

(Nathan E) #2

I’ve developed an equipment list that will be the same on each plane.

  • Pixhawk 2.1
  • Here M8N GPS module
  • mRo Next-Gen MS5525 Airspeed Sensor
  • Multistar 6s 12,000 (preferred) or 10,000 or 16,000 mAh battery pack
  • Skywalker 2816-12 500kv motors
  • Hobbywing Flyfun V5 40A ESC’s wiht active Freewheeling
  • Mauch 100A Current sensor and BEC
  • APC genuine 13x8E and EP props (to start - will change for tuning)
  • Hobbywing RPM sensor for Motor RPM
  • 16-gram HK digital coreless servo 1.4 kg*cm - .05 sec / 60deg
  • YEP 20A HV SBEC (using 6v output for servos)
  • 3DR 500mw SiK radio 900mhz
  • FrSky X8R 16ch receiver

(Nathan E) #3

Pre-Assembly Airframe Properties detailing weights and primary wing surface area (not tails).

(lucamax) #4

With what payload ?

For mapping , I would guess that minimum cruise speed would be an important parameter since the mapping precision depends on plane speed during shoots , the slower the better , if you want accuracy.

(Nathan E) #5

At the time, I used a Parrot Sequoia (1.2mp 4-band camera). It was very lightweight at maybe 100 grams.

I can see how minimum cruise speed would be a factor in the choice, however I do not think this will be an issue at the cruise speeds that we are expecting. These planes will go into the market that needs a very large area coverage area and not necessarily the highest accuracy.

For me, speed does not directly affect accuracy. Hotshoe feedback systems are able to attain a precise acquisition time, and by interpolating GPS positions, a very accurate position can be calculated. The speed does affect motion blur depending on altitude, however at high altitudes this is rarely a problem. A benchmark that I used for high detail image acquisition is included below.

Sensor: Sony APS-C 24mp (from Sony A6000)
Lens: 20mm prime E-mount
Flight Altitude: 400ft (122m)
GSD: 2.4cm
Cruise Speed: 17m/s
Shutter Speed: 1/800s (1.25ms)
Ground Blur/Smear: 2.1cm (<1 GSD)

Of course a lower blur would always be better, but the infomation that I found tells me that keeping the blur less than GSD is usually acceptable. If thousands of acres are sampled at that small GSD, it’s going to take a lot of time processing anyway. Wider lenses or sensors with less resolution would improve the smear. There are a lot of variables at play, but I think high accuracy is still achievable at the speeds we will be expecting.

(lucamax) #6

About precision with Pixhawk and hot shoe triggering , at best you should consider 200ms of precision in logs.

If you evaluate precisely stall speed I guess that it is possible to evaluate minimum cruise speed also , lets say , minimum speed 15-20% above stall speed and minimum cruise speed 10-20% above minimum speed depending on wind conditions.

About payload, mapping cameras usually goes from 400 to 1100 grams , so 900 grams , the weight of a Sony A7R with a small lens would be a fair choice.

I would suggest to make flights with payload and without to evaluated how the plane performances are affected by weight . I.e. choosing to fly with the same payload but a smaller battery.

(Tore Riise) #7

A tip; You´ll also have to consider the physical size of your payload.
We have previously used the MTD for this purpose, with a EOS-M and a TAU2 as payload. The EOS-M had to be mounted 90 deg off the flight direction due to the limited with of the plane. This of course reduces efficiency a lot, as you´ll have to fly more legs to get the same side lap.

Also, we got about 1:30 flight time with 11 Ah battery (4S).

We have also used sky walker X8´s in the past, and your thoughts about yaw-control are absolutely correct. That’s why we don’t use them anymore.

(Kikislater) #8

Nice discussion here but why considering only twin engine ? Like xuav talon is very efficient in 4s, 6s is not needed.
Also you mention that you compare them to ebee but there is no comparison first between planes and wings, and between heavy weight like there is in your chart and lightweight ebee. Lightweight should be considered due to country regulation.

What could be more interesting IMO in a mapping platform is :

  • landing like an ebee in a short space :
    - Reverse thrust (present in arduplane and working very well in my case),
    - deep stall at the end of reverse thrust (still need documentation),
  • in flight ground detection (120-200meters) like ebee with lidar or other sensor (Not available with arduplane platform).

(Nathan E) #9

I do not believe that this is true anymore. The code has improved, and PPK processing no longer ties the trigger with the actual shutter release. In addition, proper post-processing will interpolate camera position by precision timestamp between GPS log events (at 200ms or better). I believe the mission planner geotagging utility assigns a geotag for photos to the nearest GPS log event. That’s not what I will be doing.

That’s exactly what I’m doing! Great suggestion! I’ve noticed an increase in stall speed and a lot more induced drag at low airspeeds. The heavier payloads need to be flown faster to stay efficient.

The only comparison to the eBee that I will make is the price and area coverage. Those are the main two factors in my target market. There are hundreds of ebee-sized planes that I don’t care to compete against. The landing argument is entirely differnt when you only have to land once (whereas an eBee would be landing 5-10x for battery swaps). It’s a different philosophy, but an eBee cannot carry a DSLR or heavy sensor at all. The talon can, but that’s more than what it was designed for. Right now there is a gap in the market for heavy-lift, high endurance ready-to-fly UAS for mapping. That’s what this project is geared towards. If you don’t need the capacity, then enjoy the incredibly long flight times. At least you’ll have the flexibility to.

About 6s - sure it is not needed here for these planes, but I plan to integrate VTOL. VTOL will require significant thrust and power, and thaat’s more easily accomplished on 6s. The tradeoff in efficiency between 4s and 6s is small.

Your endurance seems about right in line with my initial test results on the MTD (will be published very soon). I was able to cruise for a little over ~2 hours on a 10 Ah battery (6s), and I’ve been testing in the cold where it’s not so good for battery capacity.

(Hugues) #10

Great project.
In my opinion( and experience), there is no such thing as one best frame. There are lots of best frames for specific uses, contexts, payloads and climates. You won’t be able to compare frames universally and objectively since they all require different motors, props and ESC configurations to be flown in their optimal config (for example you won’t be able to mount 13" props on the XUAV Clouds. Max prop size is 9", unless you don’t care about breaking them at each landing. Cf my videos on building and flying the XUAC Cloud frame).

In short, I would start first by defining a use case, a context and a payload objective. Then only would I research the best frame for it.
Just a thought.

(Nathan E) #11

@Hugues, thanks for the input. I think my use case is fairly well defined - long range/high payload mapping that can still be hand launched. You’re absolutely correct that certain airframes have limitations, especially in size. One size does not fit all. Maybe there’s a different way to think about this test - I’m finding the airframes with the worst efficiency. Do not use them unless you absolutely need to because of your payload characteristics.

The use case, to be more specific, is to map as much area with the Sony A6000 as possible. Broken down, that means I’m primarily looking to find the airframe with the longest range when carrying about 500 grams of payload. Since mapping can happen in the wind, airspeeeds and turning capability will play a factor in the decision as well.

I see what you mean, but because the goal is primarily to map the most area, small prop size is a major limitation. With ESC’s that have braking capability, I’ve seen about a 95% of my landings don’t damage props that extend below the bottom of the aircraft. In addition, props could be treated like a consumable. They aren’t expensive compared to the additional value they add to the system by extending the range. Even if they broke on every flight, I would gladly pay $15 per flight to double my scan area. On initial results, that’s 1000 acres. Props are easy to change and having to change them doesn’t affect my objective enough for me to cap the max size. I also will be testing up to 14" props on the Clouds (the max size that prevents fuselage contact).

On a final note, prop size is a major variable in VTOL. These twins may use vectored thrust in the future if everything goes according to plan.

(Hugues) #12

About the motor/prop choice for the XUAV Clouds platform, I did a while back some bench tests that lead me to go for 9" props on 6S battery as best efficient combo :

(Nathan E) #13

@Hugues, Thank you for the insights and video. I like how you documented your results. I strongly feel that bench tests of props used for aircraft do not provide accurate data for fixed-wing aircraft. I have spent quite a bit of time analyzing the computer modeled performance data of APC props on

My findings were similar to yours - lower pitched props give higher thrust per watt on the ground; HOWEVER, when the advance ratio (oncoming airspeed) is not still, higher pitched props have a significant efficiency boost. I’ll illustrate an example: 12x6 vs 12x12

0 airspeed (on the ground)
12x6 @ 7000 RPM = 171 watts and 3.2 lbf of thrust
12x12 @ 6000 RPM = 179 watts and 2.5 lbf of thrust - the higher pitched prop loses

36mph airspeed (in the air)
12x6 @ 6000 RPM = 75 watts and .785 lbf of thrust
12x12 @ 4000 RPM = 73 watts and .817 lbf of thrust - the higher pitched prop wins
9x4.5 @ 9000RPM = 83 watts and .816 lbf of thrust

To further back the data, you will be able to see the results posted here soon. They’re already up on the RC Groups page in my first post. 9" props may work well for takeoff and be a better all-around prop for your purpose. Because I am mainly focusing on range, the 9" and smaller props do not fit my purpose as well.

Unfortunately most motors are tested for multirotor applications, and manufacturers are publishing efficiency in grams per watt. Efficiency in airplane props with forward movement can truly be expressed in a unitless number related to the amount of work or power transferred to the air as thrust. A big reason that I use APC props is because of the published performance data - even if it’s computer-generated and theoretical.

(Hugues) #14

very interesting and quite logical (a higher pitch with faster moving air is physically analogue to a smaller pitch with slower moving air). You’re right my test objectives was to define the best combo (with constraints such as a max prop size) to hand launch without the need to use a catapult or any other launching device.

So your objective is endurance comparison between different airframes, motors, props, etc. Endurance optimum requires thus solving a multi variable non linear equation where main variables are:
-prop size
-prop pitch (and prop airfoil)
-RPM level that is minimally required to maintain your plane 's wished speed
-motor model and characteristics (Kv, weight, winding resistance, …) that runs at its optimum efficiency at that RPM (and with all other variables taken into account)
-Airframe aerodynamic characteristics (wing size, wing airfoil profile, CS factor, etc)
-All up weight
-Battery chemistry and behaviour when temperature increases during use
-I probably forget many more variables…like external real world variables : wind, temperature, air density, turbulences, …

(Nathan E) #15


You’re right there are a LOT of variables. I’m trying to keep as many of them consistent (although that is nearly impossible) while changing the most important independent variables - All up weight, cruise airspeed, and prop choice. I’m also transferring the entire electrical and power control system from airframe to airframe to eliminate the equipment variables. After I find the best airframe for the job, then I’ll tackle the rest of those other ones. I also agree with you about hand launching being one of the constraints.

I don’t think a great, long, multivariable nonlinear equation is the best practical method to communicate the results to the audience here, so I’ll do my best to simplify the results while keeping them relevant. Another variable that may be significant is the ESC to add to your list. haha.