Screamin Demon - electric conversions
 

I was urged by another RCU regular to put up a thread about electrifying Screamin Demons. This is a topic of much interest, and I do want to arrive at a design config that covers most of the bases so this might be a good exercise to loin a few tings. For example:

- most universal mounting scheme, i.e. firewall? How far from nose, bla bla.
- largest comfortable/reasonable battery pack size
- good affordable and honking motor packages, i.e. HK 500 heli motors etc.

Currently, I am just fooling with an airframe I started a while back. It has relocated spars for CG mounting of the energy source (battery or a tank of chemicals), and it was made from selected lighter parts although I am sure I can make up the weight difference with the glass needed to keep the batteries from ripping themselves out of the airframe in hard turns. The airframe complete minus maybe 1 ounce of hatches sits at 8.5 oz right now. So I predict the entire airframe less hardware should come in a little south of 16oz.

I'm using the standard mounting plate (extended forward to 1-1/2" spinner) with two pairs of those Maxx heat sink clamp mounts.

This is my park flier version, I'm only going with a BL32 and 5S 2500 for now to keep weight down. I'd prefer a lower Kv and more blade area, however I'd also like to see how this fares. The same power package on a Shoestring EF1 ARF hauled pretty good.. although it was showing signs of preferring 7" props (or more hp into the 6"?). So it should be "sporty" on a Demon.

Hmm that was smart.. I'm at work and I don't have the pictures here. So I'll post this and add later.

I think it was time for this thread. I'm certainly not an "electric guy" at heart, but the possibilities are obvious even to me. These days, a viable E-setup that will hit 150+ mph can be had for $100.00 or less. For those of you contemplating electric, here's the key: When shopping for motors, look under EDF and Heli. These motors have the high KV ratings required for speed applications. Anything intended for a "Hotliner" is worth a look too. I avoid anything over 6S simply because the "High Voltage" ESC's and chargers (7S+) are vastly more expensive than the low-volt stuff.

The "Glow Equivalent" brushless motors like the E-Flite and Turnigy are not really suitable for speed due to their low KV ratings. The props that they swing are far larger and slower than the glow motor they supposedly match. This makes for great efficiency, but makes it difficult-to-impossible to find props with adequate pitch for truly high speeds.

One problem is weight. The main component of the weight of an E-system is the battery. This pretty much sets the upper limit of the power you can have. As the "big block" Demons have proven, deltas can get downright vicious if the wing loading gets too high. Launching becomes dicey as well. One advantage to electric is that you can bungee launch, only starting the motor after the launch. At some point in the quest for more power and speed, bungee launching will be the only option other than ROG.

Another problem is volume. In it's purest form, the Demon has damned little usable volume. Some vestige of a "fuselage" in the center section seems inevitable if the batteries are to be enclosed.

Mounting the motor is another issue. Heli and EDF motors all seem to be set up to mount at the front of the motor. This means mounting to the spinner ring, essentially. Firewall (rear) mounting seems a better choice, structurally. Most outrunners can have the shaft reversed, which allows mounting from the opposite end. Inrunners usually don't permit reversing the shaft.

I'd love to hear ideas from some of the hard-core electric guys out there. I only know enough to get in trouble.:p

Well I have one that is yet to fly, same deal as yours EDF motor at about 1.5 KW.

Demon park flier in the raw. Some hangar rash on the LE sheeting and capstrips to fix.. 7 places I think. Pays to finish airplanes and not leave them lying around I guess.

1lb 12.7oz as it sits here.

I could have taken it upstairs for a nice uncluttered backdrop, but why bother? :p It's like those hidden item puzzle games.. but no screen interaction here. Shame, should be able to click on the pulsejet and go deaf while watching my shop catch fire.

http://www.rcuniverse.com/forum/atta...mentid=2131281

Here's my Park Demon. 2.5 lbs. w/2650mah 4S. NTM 3548 900KV outrunner, 10-6 prop.http://www.rcuniverse.com/forum/atta...mentid=2131283 Only 80-90 mph, but GOBS of static thrust. Vertical launch right out of my hand.

I have one framed up with an E-Flite 32 outrunner (50kV? 720kV? or something low) for a buddy - he's not expecting or wanting 150, but he'll have a great sport flier with tractor pull at launch.

WOW..that battery looks like a brick...!
The third spar looks like a nice solution.
That motor sure is nice and compact, you could cowl that thing pretty easily.

It may look big, but it will just barely squeeze into the root airfoil, whereas the 6s will not. Still, I'll likely bump the hatch up a bit to hold a 6S just in case.

Here is a killer 28mm motor for $50: https://www.youtube.com/watch?v=M1wSkhei_vg

Better get one of those and check it out!

That's NOT a 28mm motor, Mike. It a 36mm motor, hence the first 2 digits of the model number. If you use a 2200mah 6S pack, You'll only get about 1 min. or so at full scream and the pack will need a C-rating of 60-70 to stay healthy. That pack weighs about 13 oz. and the motor weighs 8+ oz. with a prop adapter.

Oh duh yes, it is a 36mm,... I guess I was thinking "hot inrunner" and typed 28mm. I doubt I would load it to 140A like that guy though.

NOTE: the accuracy of all posts is inversely proportional to the number of hours after mid evening the post is made.. :cool:

While more expensive they require much higher current for the same power. High current has it's own expenses in wiring, connectors and batteries. Plus higher current has installation issues that aren't there with higher voltage systems. It's a tradeoff for each plane, especially for the bigger ones that need more power.

For larger planes, high voltage makes a lot of sense. Double the voltage and you only need half the current for the same wattage. I'm only concerned with smaller airplanes where the weight and bulk of an 8-12S battery becomes impractical.

You pretty much covered it all, the only other major point you missed was cooling.
With any electrical component heat is a major concern solid state electronics are the most sensitive followed by your motor coils and magnets. When you design an electric a fundamental part of you design has got to be an integrated cooling system. If you take this to the point they did in the SR71 you can use this to produce thrust as well (not a lot granted) but it will be at it's maximum when both drag and waste heat are at their peak as well so it does help.

Here is one with a 28mm motor.;)
http://www.rcuniverse.com/forum/atta...mentid=2131485

I have got to get my catapult built before it get beat up just sitting on the shelf.

Proper cooling is mandatory for any electric setup. Every electric airplane can have a smoke system. Once.

I looked at the 1600kV version of that 36mm motor, and it is a much better candidate! At 75-85A on 6S, it looks like it will handle 6.5x6.5 or 7x7 props easily with about 1500W input. Of course it can be pushed harder.

But a pair of Nanotech 3S 3300's side by side plus that guy is starting to look like an appealing power package, as far as inrunners go anyhow. 80-85A and pitch speeds in the 180's on 7" prop.

Especially if you look at manufactures specifications as being merely a "baseline" guide.;)

A 6X8 would be far more "entertaining" on that motor.
With a big motor like that can easily handle 150+% spec wattage levels as long as you get good airflow around and through it. But when you push them that hard you really need to have a good system of cooling a diffuser area behind the intake along with a bypass intake for the motor case cooling. Using an expansion chamber and a exit nozzle with the a intake as above I have put 1500W input through a 670W rated motor many times and it still hasn't cooked it yet, although flight times have been 4 minutes max.

Good to know, I have no sense for the margins there.

Rule of thumb for electrical designs is to spec them at about 75% of actual smoke level. Millage may vary...

Making progress. Need to build a vertical fin now.http://www.rcuniverse.com/forum/atta...mentid=2132336

I dare ya to go with a clear plastic fin...!

Nice try, Chuck.:D

Chuck I am toying with of the idea of doing one sans fin or winglets using that prandtl stuff Dryden FRC has been playing with.

degrees of twist = 12X the AOI at cl (at cruise speed) 1/4 span to the tip twist center at 1/4 cord. If I understand it correctly.

This idea is supposed to reduce drag by 17%

You could cut out a sheet metal pattern, clamp off the portion that you don't want to twist, then clamp wood handles across the ends of the sheet metal pattern to use as levers and crank in the proper amount of twist.
Once you are happy, tape off the metal or cover it with iron on film and build a foam core wing right on top of the sheet metal form.
The form should be trussed in a jig to lock in it's shape.