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Feb 19, 2018, 03:10 PM
Aka... Chris Graham
NutBag's Avatar
Quote:
Originally Posted by whatturn
Slave to the scientific method that I am, I want to test my aerodynamics intuition and make a prediction on which airframe will end up being the most efficient plane for a given payload weight and see if I have a clue. Let’s stipulate 3000g for the payload weight. My guess is that the MTD will be the most efficient.

I’d be VERY curious what choice others who are following this thread would make, especially the pilots with lots of experience. What are your predictions?

And, let me join the chorus and say thank-you, thank-you, Nate for doing these tests and sharing the results.

NATE FOR PRESIDENT!

Geoff

Awesome thread!
Like many others Im also very curious to see the end results here, After just building a Believer that's set up purely for long range and return flying with vid feed rather than using automation and having a bit to do with some very long range MTD flights recently im keen to see your results.

After putting a few hours on my Believer, I think it may be a dark horse in this shootout
It's a lot more slippery than it looks and seems to carry weight well, Mine has a few mods and is also completely laminated in clear film/tape.

Keep up the great work Nate!

Chris.
Last edited by NutBag; Feb 19, 2018 at 08:08 PM.
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Feb 20, 2018, 02:30 PM
Air Crash Expert
sawman's Avatar
I've seen some pretty impressive numbers on the MTD before, so that's my pick. If only it had a bit more room inside.
Also very curious what would happen to the numbers if it had an extra 6 to 8 inches added to the wing tips.......
Feb 20, 2018, 02:39 PM
Registered User
Thread OP
Quote:
Originally Posted by sawman
Also very curious what would happen to the numbers if it had an extra 6 to 8 inches added to the wing tips.......
That would be a fun project to do once testing is complete - I have a 3D printer, and I could extend the airfoil just outboard of the engine mounts. I would need a longer carbon fiber tube spar though, and it would see higher bending stresses. Aileron authority would probably decrease too.
Feb 20, 2018, 03:20 PM
ARACE UAS
Pompecukor's Avatar
Interesting project naterater. As everyone here, I am also curious about your results.

Please answer me this. How did you mount 13 inch props on the MTD? I never actually tried mounting one, but the last time I measured it, I think I got something like 12.5inch as max that would fit (not be too close or touch the fuselage). This was about 2 years ago, so I could be remembering wrong.

PS: I am so happy that you are using "energy" in all your relevant data and not "current". This way it does not matter if you launched the X test with half empty battery and so on.
Well done.
Latest blog entry: longrangeFPV.com
Feb 20, 2018, 04:14 PM
Registered User
Thread OP
Quote:
Originally Posted by Pompecukor
How did you mount 13 inch props on the MTD?
Maybe the design has changed, but there is about 0.25" of clearance between the fuselage and the tips of the 13" props. The motors are about 1" long, and I made a special 3D printed adapter that puts the motors a little farther in front of the leading edge. Those mounts are about 7/8" thick. I still think the props would fit fine if that adapter wasn't there, so maybe the design of the plane has changed (but I doubt it). If you had very thin motors, the clearance between the fuselage and motor would decrease even more, probably to an uncomfortable level. FYI the extensions are to keep the props an equal distance in front of the leading edge for all models - the clouds will use the longest mounts while the MTD used relatively shorter ones.

Thank you for all of the positive comments everyone! As Pompecukor noted, I am attempting to give you an engineering/science based approach that allows you to make changes in your system and keep the results relevant. The physics that I learned while studying in college is being applied to the real-world. Reading other threads has helped me confirm some of the physics behind planes like the debate between 6s vs 4s batteries. I don't want to open up this debate here, but you should be able to predict that results of a 12,000mAh 4s with a 750kv motor are similar to a 8,000mAh 6s with a 500kv motor. They will both be delivering the same power (energy / time) and (current * voltage) to the motors and have similar performance characteristics.
Feb 20, 2018, 05:01 PM
ARACE UAS
Pompecukor's Avatar
Ahh, I think that you have a point right there.
I now remember when we first were considering motor kv and hence prop size. We just had and un-built unit on the table and just measured from the center of the firewall to the inner edge of the wing. Actually was one of my guys doing the measurement. I can see how he would not have thought to account for actual mounting (with motor) being much more forward and hence the clearance bigger.

Anyway naterater, as this is clearly a very nice endevuor which will probably help a lot of people with a lot of insight. Something that I like to appreciate from people (as you stated in your first post "...attempt to give back"). Please let me know if I can be of assistance in anyway.

You did ask for notes:
My first note would be that for whatever reason, you made a very protruding mount for the M5525. Was there any particular reason for that. Believe it or not, such a simple looking item will register in your result (As drag and turbulence). I would suggest either going with the same for all frames, to keep the field equal. Or using a more subtle mount, like the one you can see on nutbags Believer.

My next note is with regards to battery type/payload/takeoff. You did mention that you wanted to rate take-off too. Or rather "Discover....Takeoff ability..." among other things. You probably already know this, but I would like to point it out anyway. Even on 6s the voltage drop on the li-ion vs lipo WILL be a factor. This is with regards to payload capability.
That is to say if you load an MTD up to 5kg (AUW) and have a reasonable good takeoff with a motor/prop combo on lipo. You may find that with the exact same setup but with li-ion pack, you would have a worst take off. This will be completely irrelevant of flight efficiency. Simply that the high-load-voltage-drop that is one of the few disadvantage of li-ion packs will result in significantly lower static trust on the same prop/motor. Think motor kv * voltage.

Anyway I am almost sure that you are aware of this factor. So I am mostly just putting it out there for the general public that will most definitely be using your generous and detailed findings here as reference when the plan their next builds.
Latest blog entry: longrangeFPV.com
Feb 20, 2018, 05:20 PM
Registered User
Thread OP
Quote:
Originally Posted by Pompecukor
You did ask for notes:
My first note would be that for whatever reason, you made a very protruding mount for the M5525. Was there any particular reason for that. Believe it or not, such a simple looking item will register in your result (As drag and turbulence). I would suggest either going with the same for all frames, to keep the field equal. Or using a more subtle mount, like the one you can see on nutbags Believer.

...

You probably already know this, but I would like to point it out anyway. Even on 6s the voltage drop on the li-ion vs lipo WILL be a factor. This is with regards to payload capability.
Thank you very much for your notes. You're right that the pitot tube is far away from the fuselage. I probably overdid the isolation, but I wanted it to be completely away from any sources of parasite drag that could affect the results. It will be at the same height and relative position for all tests, and I didn't want it sticking out the nose for all of the bad landings that I plan to have. In addition, I'm not sure where the best place to mount the airspeed indicator is when using reverse thrust (which I plan to test in the future). I do like NutBag's mount, but I'm hesitant to switch now because MTD testing is complete and I don't want to change variables. I even thought about recalibrating the airspeed sensor, but I'd rather show indicated airspeed consistently between all of the tests to rule out as many atmospheric differences as possible.

On to your li-ion voltage-drop point: I did not see significant voltage-drop on takeoff from the li-ion packs when I did the endurance test. Maybe it becomes an issue in the heat or with smaller packs. I had 28,000mAh of battery, and I was pulling about 60amps during takeoff. It flew right out of my hand at over 5kg - but I did have a bit of a headwind. I didn't notice a difference between the li-ion and li-po except at landing - the ~40% lower voltage when the pack is discharged - it sure has an effect on the climbing ability. I'll keep an eye out for the voltage drop in the future though. Great notes!
Feb 20, 2018, 05:25 PM
Aka... Chris Graham
NutBag's Avatar
Hi Nate,

As Pompe mentioned, here’s a link to the pitot fairing I used... 2 sizes avail.
I understand your valid reasons for not wanting to change now and agree that you will get a more consistent result with it mounted up and away from air affected by the props.

I ended up cutting base off and lightly glueing in as to let it pop out if it gets hit on landing/transport.
Can thank rcgroups Mark Qvale (markq) for these great designs, he has lots of useful goodies on his thingi pages.
https://www.thingiverse.com/thing:2180278
Last edited by NutBag; Feb 20, 2018 at 05:37 PM.
Feb 20, 2018, 05:30 PM
Registered User
Thread OP
That's incredible... I didn't know that those existed, but our designs are so similar! It pretty much looks like I copied his design except mine isn't split in two; it inserts from the rear with access to the tubes. Thanks!
Feb 20, 2018, 05:39 PM
ARACE UAS
Pompecukor's Avatar
Quote:
Originally Posted by naterater

On to your li-ion voltage-drop point: I did not see significant voltage-drop on takeoff from the li-ion packs when I did the endurance test. Maybe it becomes an issue in the heat or with smaller packs. I had 28,000mAh of battery, and I was pulling about 60amps during takeoff. It flew right out of my hand at over 5kg - but I did have a bit of a headwind. I didn't notice a difference between the li-ion and li-po except at landing - the ~40% lower voltage when the pack is discharged - it sure has an effect on the climbing ability. I'll keep an eye out for the voltage drop in the future though. Great notes!
Not heat, but yeah, more relevant with smaller packs. You can even test it on the ground.
Just push WOT on a 10A lipo, then on a 14A li-ion (those would weight roughly the same). You will see the li-ion drop much lower than the lipo.
For sure if you are propped to (over) compensate for that there will be no issue. The plane will fly right out your hands. But if you are close to that edge, like you have 3.5kg max trust on WOT on lipo with with a 5kg MTD, which mean you need to trow it hard, but will take off. The exactly same MTD with li-ion will dip much more if not land to the ground.

You can even see this v-drop on ecalc scenarios. I know it is too synthetic, but does help to find some starting points for theory-crafting.
Note, that I can blindly tell you that most people that will read this topic are not going to use the platform for mapping or similar commercial enterprise. Simple FPV.
Latest blog entry: longrangeFPV.com
Feb 20, 2018, 06:23 PM
ARACE UAS
Pompecukor's Avatar
And before I forget to mention it. Please be very careful with using reverse trusting on twins. Yoi mentioned wanting to use it later on.

Most people (I am almost sure even you just now) forget that the reverse trust on tractors not only slows the plane (acts as air brake) but also has the undesired effect of eliminating lift on a large area of the wing. 2x12 inch length in your case. It is almost like at that moment (when revers throttle is active) your wings are suddenly shorter buy 24 inches.
Yeap. Bad idea!

Reverse trust is idea for pushers, but not for twins.
You would be braking and at the same time dropping out of the sky (loosing attitude without gliding). Especially as it is for landing that you would use it for, where you are trying to get close to stall speed anyway. So makes it even more dangerous.

You can get some speed control done. But do not expect such awesomeness as you see from Skywalker X8 and co. videos.
Also the Arduplane implementation still leaves a lot to be desired. There are 20 different parameters that need to be adjusted (with sub-par documentation) and even with that you will not get consistent results. Also some bugs present in the executions too. It is free/open source, so we take what we get. hence not complaining.

So think twice about it. I am speaking from personal experience, but I guess that ia obvious at this point.
Last edited by Pompecukor; Feb 21, 2018 at 05:14 AM.
Feb 21, 2018, 09:27 AM
Registered User
parajared's Avatar
Quote:
Very interesting, subbed. I'd expect you to get better results with 10x7 props. Unfortunately very few high pitched CW/CCW pairs are available. Banggood had some APC clones in 8x8 and 9x9 pairs, but I havent seen above 9".
Someone (ArxAngelRC I think) said he tested with both CCW, CW props and two CW props and experience little to no performance difference.
Feb 21, 2018, 11:02 AM
Registered User
mike_o's Avatar
Quote:
Originally Posted by parajared
Someone (ArxAngelRC I think) said he tested with both CCW, CW props and two CW props and experience little to no performance difference.
Under normal condition probably not. But if the wrong motor quits, then you'd have wanted counter rotation. Try a search for "critical engine" for more info on the phenomenon



By using counter rotating props, you'll get two non-critical motors

On this flight, my 2500mm twin lost one motor right after launch, but still survived
Diamond 2500 Twin FPV maiden (3 min 47 sec)
Last edited by mike_o; Feb 21, 2018 at 11:13 AM.
Feb 21, 2018, 11:19 AM
ARACE UAS
Pompecukor's Avatar
Rc motors hardly "quit". Usually the last part ever fail.
Further more on this (RC) scale there is hardly any relevance of the dirrection.
Absolutly not worth the trouble limiting yourself in prop choice.
Nicks friend teste (simulated) single motor fail and his MTD flew quite fine even after. If it was not the title of the video we not even know that it is flying on single motor. As far as I am aware it was not counter rotating.

Not saying there will absolutly not be a difference during a failure. What I am saying is the it is quite flyable. Both manned or auto. If it was flying wing, with no way to compensate on vertical stabilizer, then it would obviously me predominant. But no one here is flying a twin flying wing

So IMO it only matters on full scale planes. Not RC ones.
Latest blog entry: longrangeFPV.com
Feb 21, 2018, 11:36 AM
Registered User
mike_o's Avatar
Quote:
Originally Posted by Pompecukor
Rc motors hardly "quit". Usually the last part ever fail....not worth the trouble limiting yourself in prop choice....
I tend to agree. But if your ideal prop turns out to be a size/pitch that is available in CW/CCW pairs, then use them.

Quote:
Originally Posted by Pompecukor
...Nicks friend tested (simulated) single motor fail and his MTD flew quite fine even after. . As far as I am aware it was not counter rotating...
But it may have been the non-critical motor


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