ORIGINAL: bjr_93tz
Hi David,
The trouble is that I'm giving away away so much efficiency running an 11'' prop but you get nice smooth turbine like power. The old .75 I'm used to is rated to put out about 2.4hp. 1300W just isn't going to cut the mustard at 100mph, BUT at 60-70MPH with a 12-13'' diameter prop 1300W is in the ballpark. The math says 8+lbs of thrust at 100mph = 2+horsepower.
I morally object to people flying a classic with a 14'' prop turning in the sub 10's
, it'd be like pulling the V-twin 2-stroke out of my TZ and replacing it with a 650cc single 4-stroke.
I think you hit it on the head. Your efficiency is falling off the steep end of the curve by using props that are way too small for the KV rating of the motors. I also agree that 1300W isn't going to give you 100 mph performance especially on 12" props. The figures I offered were really in a way of perspective in terms of achieving power with different types of motors under what I'd consider "ideal fuel" configurations. Just as turning 14" props on pre-turnaround classics is objectionable, loading up the same classic with 10 or 12 cells of lithium power is a bit much. You mentioned 10s 4Ah - that much battery has got to weigh a ton (~42 oz?). One winds up having to lug around close to 9 lbs of classic (or more on 12s) with a 10 or 11" prop expecting blistering performance. It might work in a dive but level flight and uplines will be ho-hum at best. Keep the amount of power roughly the same (although it doesn't need to be that high), reduce the weight by 20% (a 7.5 lb Curare is doable with the right construction and finish - I believe the Austrian ARF comes in at 7.5 all up) and then one might begin to approach ballistic performance.
The Scorpion on 10S (it's rated for 12S), pulling about 65A static with the 11x9 gave very similiar performance to the .75 two stroke and honestly up to about 9000rpm was very smooth but when pushed beyond that the whole thing got very bendy. I was running glow props on it as well, not the floppy E props. When you compare how solid the front end of a glow motor is to an electric you can see it's asking a lot from it. But it's all experimentation. The 8mm shaft wasn't flexing from what we could see but the ''spigot'' (for want of a better word) that the bearinges and stator are mounted into. On the rear mount arrangement my friend was using, you could grab the prop and bend it from side to side watching the can flex around in relation to the stator.
From your figures, you're pushing in excess of 2400W nominal on
very low kV motors! At that power rating and kV with rapid transitions to ~13K you may be getting close to prop tip cavitation. After a few flights do your prop tips look a little funky? If you want to turn a turbine, you've got to up that kV. The motor needs to be more "elastic". You will not find a single ducted fan motor in the 400 kV (let alone 330 kV!) configuration. Many an EDF is turned by inrunners capable of 2000+ kV.
Devin wanted to build a P8 and push it to 200 mph. It was realized that this was extremely difficult to achieve on a classic design with e-power but he says he's reached 180 mph. I can't recall what his model weighs and his airfoil might be thinner than design but he's simply turning a 4030-880 kV motor on 6s with a 10x10 prop. That is a motor that is designed to turn 19.5K nominal unloaded. I don't know what rpm's he's turning on the 10x10 but they are likely in the ~14-15K range. The motor is short, bearing supported in the rear of the can with no shaft extension, is front mounted (ideally with an X-mount but I don't know) and has a max current rating of 90A. So he is still producing the similar 2000W that your Scorpions are (22.2Vx90A) at a power loading of ~250 W/lb with an 8 lb model. The weight is of course achieved with the 6s pack which is ~60% the weight of a 10s pack or 50% of a 12s pack (assuming the same capacity).
Within the Scorpion brand, I think that if we want ballistic performance from 10-11" props on trike configurations, we might be better off targeting motors such as the HKIII-4025-890kV (6mm). There's a motor with a stator of only 25 mm capable of producing 2700W/95A continuous. Put an 11x7 on it with 6s and expect ~16K and ~2100W on a motor that weighs 10 oz and a battery that weighs 26 oz (2.5 lbs of power plant equivalent to 16 oz of fuel and a piped YS 60 FR). Because the motor is that much more elastic, it will transition smoothly to those rpm's without gross pressure differentials and resulting cavitation.
Don't let mine and my friends not-so-positive experience with the Scorpion outrunners put you off from experimenting. His was a S-4035-330 he intended to run ''rear mounted'' on 12S in a Falcon 120 but it was too bendy for that. He turned it around to a front mount, with a 10mm ply box around it with external rear bearing support and it's bench testing a lot better. Both motors up to about 9k were fine but started getting out of shape after that. I'm beginning to think a ''typical'' outrunner design doesn't scale well, just like we don't see too many .15+ sized engines based around the design of a Cox .049...
The above are just my thoughts and experimentation is definitely the key but I feel that one might do better on lighter configurations with smaller motors at higher KV's if one is aiming for equivalent '80's setups. If on the other hand, we're talking about modified late 80's, early 90's conventional gear models designed for 12-13" props on Hanno/YS LS like power, then using ~550KV motors is the way to go. If there is a black art to the electric equation, I think it is in finding the sweet spot between prop desired, motor KV and motor geometry. Shorter is better with outrunners to prevent flexing and ideally using 8 mm shafts on the 12-13" props while 6 mm shafts ought to work on the 10-11" props provided the can is short enough and the bearings are stout.
Man! Writing about e-power is akin to typing science class solutions - it's hard to keep track of all the darn units and abbreviations!
David