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Oct 26, 2018, 12:03 PM
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V900 speed trials


How fast does a stock V900 really go?

When Stuart Warne was still leading the Durafly Ship at Hobby King, I did some work for him on the upgrade to the EFX to the EFXTRA. The basic difference was a larger motor and prop and clipped tips.

A "stock" EFX with the supplied black prop barely got over 90mph. Putting a "better" APC prop on it made it go 99MPH! Stuart wanted it to go faster.... how much I asked. Lots... he said... I told him, sorry, but to go LOTS faster requires more energy than you can fit in the fuselage. Since 4S was really the size limit (with any reasonable amount of capacity), with some motor changes and better props, I was able to get the EFXtra to 110mph level flight and close to 135mph in a terminal (straight down) dive.

A couple years later, in steps the V900 from Eflite with claims of 120+MPH. With all the work and inflight data I collected on the EFX and EXFtra, I had to see if there was some magic in the V900 that made it go 10MPH faster on basically the same 4s power system. It takes GOBS more power to go 10MPH faster (level flight) at 100+MPH!

Well ladies and gentlemen, it does go 120mph as advertised, but only in a steep dive! It does about 110MPH straight and level, and surprise surprise, it's just about the same power level as the EFXtra! it's just that and EFXtra does it with a 7 x 6.5 prop and a V900 with a 8x8 . Pretty much proving that if you make good decisions (motor/prop), POWER is king, not what is in front of it.

Here is some of the on-board data I collected with a Castle logging 75Amp ESC substituted for the stock 60amp esc provided with the model and an airspeed sensor mounted on the model. The airspeed sensor is logging MPH (not knots as my Taranis is calling out in the video). It was verified against my GPS on a clam day up to 70MPH and with my car speedometer. If there are any Taranis geeks out there watching, can you tell me how to set up the telemetry so she only calls out the value and not the "knots" tag too? can't seem to turn that off. I can't even find the "Knots" sound file on the TX!

anyway... I got with a 4S-1800 65c Graphine

110MPH at 50.57amps, 686 watts and 15348 RPM in straight and level flight.

in a steep dive it climbed to:
124MPH and because the prop was starting to go into windmill the current/power and rpm dropped to:
42.99Amps, 577 watts, 15180RPM

I also ran the model on a second flight with a 4s-2200 65C graphine. Results were very similar, it may have gone an MPH or two faster, but it took more power to do so.

So claims of other guys getting there's to go 130-140mph by using GPS speed or radar need to put a pitot tube on theirs and try again. it would take a steep dive and 5-6S to go that fast with this model!

It is a great flying model and more than fast enough for me. After some more testing, it will probably go the way I did with one of my EFXtras...... FPV!

Video of the 4S-1800 is below.

V900 SPEED RUN NO1 (4 min 50 sec)
Last edited by Tom Hunt; Nov 25, 2018 at 10:38 AM. Reason: spelling
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Oct 26, 2018, 12:57 PM
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Great information and video.
Oct 26, 2018, 01:24 PM
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Interesting. 15,348 rpm with 8" pitch equates to 116 mph, which is right in the ballpark allowing for a bit of slip.

The OpenTX Knots sound files are in sounds\en\system btw - knot0.wav and knot1.wav (I have no idea why there are two). To get rid of the units you could perhaps set a Gvar equal to the speed value, then play the value of the Gvr.
Latest blog entry: Eachine QX65 FPV quad review
Oct 26, 2018, 01:27 PM
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Quote:
Originally Posted by Bill Glover
Interesting. 15,348 rpm with 8" pitch equates to 116 mph, which is right in the ballpark allowing for a bit of slip.

The OpenTX Knots sound files are in sounds\en\system btw - knot0.wav and knot1.wav (I have no idea why there are two). To get rid of the units you could perhaps set a Gvar equal to the speed value, then play the value of the Gvr.
Thanks Bill

I'll get the TX on the "puta" and fix it up.
Oct 26, 2018, 02:15 PM
Registered User
Cool info. So with a stiff wind behind it maybe 120 mph… And not meaningfully faster than the EFXtra.

It’s interesting how subjective the perception of speed is. Especially how big a roll sounde plays…
Oct 26, 2018, 02:19 PM
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Originally Posted by Herrsavage
Cool info. So with a stiff wind behind it maybe 120 mph… And not meaningfully faster than the EFXtra.

It’s interesting how subjective the perception of speed is. Especially how big a roll sound plays…
with a stiff tail wind... 30+ she could do 140! I'm not flying it in 30+ winds in any direction!
Oct 26, 2018, 05:32 PM
Registered User
interesting. 1260 kv * 8 pitch = 10080, 1780 kv * 6.5 = 11570. so just from pitch speed calculation efxtra should go a bit faster? what power does efxtra draw on full throttle at the highest speed?
Oct 27, 2018, 12:02 AM
Registered User
Tom, what would be your guesstimate for a realistic top speed with the EFXtra, completely stock including the 4S prop, on 5S?

I ask not only because I am personally curious, but because I almost always get asked when I am out flying or when I show friends a video or whatever.. I usually say 120ish.. (I’ve become hooked on 5S.. )
Oct 27, 2018, 05:34 AM
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Quote:
Originally Posted by Herrsavage
Tom, what would be your guesstimate for a realistic top speed with the EFXtra, completely stock including the 4S prop, on 5S?

I ask not only because I am personally curious, but because I almost always get asked when I am out flying or when I show friends a video or whatever.. I usually say 120ish.. (I’ve become hooked on 5S.. )
Herrsavage

here is the data on 4s on the stock prop (7x6.5) on an 1800kv motor (before the production went to 1750)
50.03amps 680 watts 24400 rpm 109 MPH

you can see the wattage and speed are nearly the same as the stock V900.

it took almost 200 watts to go 10 MPH faster between the best I could do with a stock EFX to the proposed EFXtra.

to push the EFXtra to 5s (assuming that the ESC and motor can handle the extra wattage) and assuming the power goes up far less than the increase in voltage would dictate (poorer motor and battery efficiency), and let's not forget that drag goes up with the square of the change in speed, I think you would be lucky to break 120 (level flight).

I think you could improve your chances by dropping the prop to a 6 x 6 on 5S, but I just think it would just improve efficiency some of the system.

good luck!

Tom
Oct 27, 2018, 06:19 AM
Registered User
Thanks. I have been flying my EFXtra completely stock only on 5S of late, and really like it. I also recently made a Multiplex Funjet Ultra with a complete power system, including 4S prop, from an EFXtra. Also like it a lot and want to leave it that way for a while.

But you really think a 6x6 prop would be an improvement? If so I'll try it - I think I ordered some recently to test... (ps Just checked. Got some Gemfan packaged 6x6 "Speed" props.. Came in pairs for cheap from HK...)
Last edited by Herrsavage; Oct 27, 2018 at 07:10 AM.
Oct 27, 2018, 07:12 AM
Registered User
I have a question on the V900. When I measure the output on a watt meter using a 4S 1800 mah, I am getting over 70 amps and 1000 watts. Why such a big difference? Could there be that much difference between static and flight power draw?
Oct 27, 2018, 08:17 AM
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Quote:
Originally Posted by tclaridge
I have a question on the V900. When I measure the output on a watt meter using a 4S 1800 mah, I am getting over 70 amps and 1000 watts. Why such a big difference? Could there be that much difference between static and flight power draw?
yes, very much so.

Props that have a pitch/diameter ration of >.8 are partially or fully stalled without moving them through the air. An 8x8 is what we call a "square" prop (P/D =1) and depending on the shape and size (chord) of the blades, these props give much higher amp draw statically.

some prop designs (usually high aspect ratio ones like the "square" MAS and the pylon APC props) static and WOT (flying) current numbers are very close. The APC E series 8x8 prop supplied with the V900 has a LOT of blade area and it takes a lot more energy to fight all the drag of the stalled prop.

all props unload in the air as you reach max speed and you would expect the current to come down. The lower the P/D ratio, generally the greater the unload (in current) in flight.

Tom
Last edited by Tom Hunt; Oct 27, 2018 at 08:22 AM.
Oct 27, 2018, 08:21 AM
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Quote:
Originally Posted by Herrsavage
Thanks. I have been flying my EFXtra completely stock only on 5S of late, and really like it. I also recently made a Multiplex Funjet Ultra with a complete power system, including 4S prop, from an EFXtra. Also like it a lot and want to leave it that way for a while.

But you really think a 6x6 prop would be an improvement? If so I'll try it - I think I ordered some recently to test... (ps Just checked. Got some Gemfan packaged 6x6 "Speed" props.. Came in pairs for cheap from HK...)
a 6x6 if nothing else should improve the electrical efficiency of the system at WOT, reducing the waste heat. it wil make everything last longer as we know that heat is the enemy of electronic equipment.

I would like to at least see some (static) numbers of your stock EFXtra on 5S.

ps, not a "Fan" of Gemfan props..

a wise aeronautical engineer once told me so many years ago that the difference between a good prop and a bad prop (from those who know how to design props) is about 5%. Gemfan props are 20% worse!
Oct 27, 2018, 08:29 AM
Registered User
I believe they were labeled as Turnigy on the HK website, but the packaging is Gemfan. When I first got into five inch quads I quite liked the 5045 Gemfan props I had. They were not the toughest, but they flew fine - and were way better than what most quads were coming with. So so far I can’t complain about them, but I don’t have all that much experience with them, and well basically none on airplanes.

I think I have a Graupner 6x6 too, but those props are a PIA to mount with the weird hole sizes and everything..
Oct 28, 2018, 04:40 AM
Registered User
Hakaiii's Avatar
interesting data, thank you for sharing.

prop slip is 0.946 (5.4%) which means the prop/hull combination is very well tuned !

by the way, the gemfan props (sq 4.7", sq 5", sq 5.5" and sq 6) are EXACT copies of the graupner cam props (wich are very efficient and silent props)

I use a 6x6 (indistinctly Graupner or GEMFAN) on a stock old gen EFX on 3S with nice overall perfomance


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