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Old Nov 10, 2014, 07:19 PM
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For what it's worth, the concept of using a heli motor on a classic pattern model has been done before. A couple of years ago a fellow over on the other forum framed up a P8 and used a 600 size heli motor with very high KV (880 comes to mind) to turn a 10x10 APC-E prop. His goal was to break the 200 mph barrier with it (not impossible but pretty close) so his model was very sleek (gear doors). The cores were not sheeted but rather covered in a G10 like material which leaves them ready to shoot with paint. I think the model turned out reasonably light and the speed was indeed impressive. I seem to recall he hit around 170 mph or thereabouts. The prop sounded like it was cavitating in a video he posted. Here's the link to his build thread:

http://www.rcuniverse.com/forum/clas...oenix-8-a.html

He was pushing close to 2000W on 6s.

Doug, congrats on your success this year and I look forward to hearing what you and Lucien come up with. it would be great if he and Scorpion could come up with an electric 15K+ and 16 oz of fuel equivalent solution. I'm thinking this will require 8s and 800+ KV motors. The main issue as already discussed will be current draw. The only real solution to the problem is to build the classics to 1-1.5 lbs lighter for e-power so one can load up with 6000-8000 mAh of bats.

David
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Old Nov 10, 2014, 08:48 PM
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Oakland township, Michigan
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Nice job Doug. Pardon my ignorance, but David, when you make reference to a 15K+ equivalent do you have a prop in mind? I am just curious. For the most part, it is difficult to make a direct comparison between glow and electric because the props can be quite a bit different.


Teo
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Old Nov 10, 2014, 10:36 PM
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Montreal, Canada
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Quote:
Originally Posted by viva_peru View Post
Nice job Doug. Pardon my ignorance, but David, when you make reference to a 15K+ equivalent do you have a prop in mind? I am just curious. For the most part, it is difficult to make a direct comparison between glow and electric because the props can be quite a bit different.


Teo
Teo,

exactly, the issue is not so much that props between glow and electric are different (they don't have to be) but rather that the power plants generally have different characteristics.

Electric outrunner motors have a lot of torque and they will deliver it right off the bat at low RPM so they are able to turn a larger prop resulting in a "more efficient" power setup. For a sport model or a scale aerobat, this is fine but what we're after is an electric version of the glow engine these classics were powered with back in the day. It is not strictly necessary as Doug's setup has shown but the 1980's classic pattern model was designed to be flown fast and has little need for 3D type torque (which e-motors can typically do very well).

So when I say 15K+, the prop I have in mind is the "classic 1980's" prop which is somewhere between an 11x7 and an 11x8. The issue is that if one takes the typical "60 size" outrunner and puts an 11x8 prop on it, the RPM on a 6s pack will usually be somewhere between 11 and 12K rpm which not only doesn't deliver the power needed but it also results in much lower speed compared to that of the model when powered with a short stroke classic piped engine.

The goal with e-power, IMO, is to approach the same type of flight pattern and behavior that was achieved with the 1980's glow engine using similar props. It is not strictly necessary to "pitch down" as much as 11x7 (which was needed due to the "lowish" torque at low rpm of short stroke glow engines) but obtaining a similar type of flight envelope and flight time on a higher revving smaller prop is of interest with the classics. For the designs which came later and were intended for the long stroke engines, going up to 12x10 or 12x12 props is also desirable but they still need to turn these at appropriate RPM and produce a similar amount of power and thrust as the LS engines did.

While the current production long stroke NR Speed/13 produces more thrust and power than most of the engines did in the 80's, replicating that performance with e-power is also desirable. Approaching 2.5 bhp (1850+ W) at 15.5K rpm, the NR is delivering the goods that were sought for the larger rear exhaust classics (e.g., EU-1A, Brushfire, Aurora). Being able to turn an 11x9 or 12x8 with an input power of over 1800 W on an electric outrunner is not something that many 60 size motors do well.

Of course, these are just my personal views.

David
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Old Nov 11, 2014, 03:01 PM
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Oakland township, Michigan
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Hi David,

Thank you for the answer. I was not sure if you were trying to match performance between glow and electric by matching RPM and props as well. I agree with you that with the current crop of outrunners it is difficult to match the RPM's while limited to a 6S pack and while using a glow size prop as well (at least in terms of diameter).

I am just wondering about if in the case of electric power, the balance between speed and thrust needs to be a little different, meaning, a little less speed and a little bit more thrust?

Thanks,

Teo
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Old Nov 11, 2014, 05:56 PM
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United States, IL, Swansea
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A lot classic ballistic designs are limited on prop diameter. On my UFO a 12" prop was a stretch. The nose wheel was into the leading edge of the wing when retracted so I was at the maximum on ground clearance. that is one of the reasons I chose a Tipo 750 for my new project. There is more room for a longer nose strut, but I don't want to make it too long and make it look out of place.
I would like to run a 12" prop, but have room for a 13" if need be. If we were willing to jack the airplane way in the air and run a 16" prop, thrust would not be an issue, but a classic model flying a 70 miles an hour would do nothing for me personally.

Doug's setup would suit me fine. It sounds like it's powerful enough to get the job done for anything I'd want to do. I will probably never compete. Not from lack of want, but there isn't that much activity around me, and I'm not much into traveling.

Doug.
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Old Nov 11, 2014, 06:50 PM
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Oakland township, Michigan
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Ok. That makes sense. I was not thinking a 16 in prop, more like around 14" or so. I did some bench testing on the APC 13x10 some time ago and the data that I collected (current, voltage and RPM) were kinda odd. After playing with the online calculator I was able to get the prediction to match the test results. In the end, I came to the conclusion that the prop was probably partially stalled when tested on the bench. That got me thinking that as the plane slows down, you start to loose prop efficiency when you probably need the thrust the the most.

In terms of speed, I was thinking more like topping out at 90 mph or so. Being able to achieve that speed is much easier in terms of input power needed so I was wondering why not bring down the speed a little. That is all.

Thanks,

Teo
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Old Nov 11, 2014, 09:00 PM
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Montreal, Canada
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Teo,

what we choose motor and prop wise to power our classics with e-power is entirely up to us, but like Doug (Jet Painter) mentions, most trike gear classics are limited to a 13" prop max - and there are few among them. Most of the classics were designed to turn 11" props or in the case of the long stroke designs, 12" (occasionally 13") props.

Earlier in the thread (or perhaps it was another) I was voicing how I was of the opinion that the current energy density of LiPo's doesn't really allow us to "purely replicate" the short stroke piped glow experience. To do so and have the flight time that a 16 oz fuel tank allows, the capacity of the batteries would have to double (or close) or the voltage would have to increase substantially. It is possible to obtain the 1850+ W on a 10s setup without much difficulty but the amount of "juice" that we need to carry is still the same and a 10s 5000 mAh battery is almost twice as heavy as a 6s/5000. And that is not to mention the additional weight carried by the larger motor and the huge prop that is being turned. So, with current outrunner technology, there is really only one solution as far as I can see: increase the KV of the motors (thereby increasing rpm on smaller props) while increasing the energy density of the batteries so a 6s/8K or 10K weighs the same as what a 6s/5K weighs today (otherwise put, do the same with a 10s/5K for what is a 6s/5K today). As battery technology is advancing rapidly I expect that the energy density desired will be within reach in 5-10 years time. That might be too long for some of us so the only other "reverse engineering" solution is to cut down the weight of the airframe/components so that we can carry 2 x 6s/4K or 5K packs. It's no easy feat as a battery that size weighs no less than 800g or close to 29 oz. So in order to achieve the "range" today of a SS piped classic 60 on 16 oz of fuel we'd need to reduce the weight of electric models by about 1.8 lbs - before batteries...

In summary, we can bring down the speed of the models to 90-100 mph without too much of an issue but having the necessary torque and thrust on "classic props" for uplines does require more rpm which when straight and level translates into higher speeds. If you want your classic to fly at 90 mph when it can/should fly at 150, simply pull back on the throttle.

Anyhow, I think I might be digressing Doug's thread here. The conversation was spurred by the desire to look into the possibility of using high KV heli outrunners for the purpose of our classics. The main issue seems to be shaft diameter and resulting strength. We'll see what Lucien and Doug come up with.

David
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Old Nov 11, 2014, 10:03 PM
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Thank you for the information David and I will leave it at that. I now understand that the intent is to truly replicate not only the look but also the feel and performance of planes of that era. Totally agree that speed is much more power hungry than thrust. Now back to Doug's thread...

Thanks,

Teo
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Old Nov 12, 2014, 12:42 AM
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Fatherof4's Avatar
Rancho Santa Margarita, CA
Joined May 2006
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No, do not stop.

This is exactly the dialogue I want to have on this thread.

I will post some additional thoughts later.
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