Mike Connor

My Feedback: (5)

I designed, built and flown the glow version of my Delta and would like some thoughts on the electric motor mount I have come up with for my second prototype. My experience is limited with electric and would like the views of those that work with it daily. The pictures are a sequence of the first phase of assembly. There is no glue yet as the backplate will end up further back.

There will be a nose ring and cowling so cooling is an issue. I am looking at a strong Neu motor and plan on cooling slots near the nose ring. Maybe notches around the backplate mount that allows the air to flow around the motor and through the fuse area to cool the ESC and Battery. An opening near the rear of the hatch will let the air escape.

My question is does the mount shown with tri stock and maybe glass look strong enough for a strong motor and how much air flow is needed for cooling.

I also attached a pic of the finished glow version for reference. A 32" wing span and rtf weight of 2 lbs 11 oz. A $500 battery may make it a little heavier. [X(]

Thanks for any thoughts,
Mike

ozrcboy

Hi Mike,

Welcome to electric flight. Without looking at your photos in detail I am going to venture that your mount will be more than strong enough. Electric motors don't vibrate the way glow ones do, so the actual mount required to hold them doesn't need to be all that strong. For example, I have a 300watt motor on the back of my 1.2metre delta which is held in place by cable ties around the can just holding it firmly against the wing.

At 2lbs 11 oz you may have made your bird a bit heavier than needed for electric power - but I'm sure you will find a motor that makes her fly great.

$500 seems like a lot for a battery - sounds kind of special. How many cells/How much power/What KV motor and prop combo were you planning to fly. If you are yet to decide I would suggest if you want to make this thing absolutely hum along you probably want around 350-400watts of power - 3s or 4s will be the easiset way to achieve that with a fastish motor (2000-3000 revs per volt) and something like a 5x5 (lot depends on whether you go 3s or 4s).

Good luck with it.

Cheers,
Oz.

ozrcboy

Oh, sorry meant to say - cooling is quite important for electrics particularly when you amp it up . If you can find a way to get the air flowing over and through your motor that will generally allow you to, depending on the weather, abuse your motor a bit in terms of overdrawing current etc.

Mike Connor

My Feedback: (5)

Thanks for the reply. one of the electric power systems being considered would be similar to
Motor: Neu 1509/2.5D
Controller: CC80
Lipo: TP 4200 5s2p
Prop: 5x5 APC
If I get rid of the glow stuff the airframe and radio come out to 22 oz or 625 grams and then add the power system.
This Delta has been Doppler clocked at 120 mph (195 km/h) and I hope to well exceed that with electric so it needs to be strong.

Mike Connor

My Feedback: (5)

For cooling I was thinking something along these lines. I guess the air flows through the motor also so I could drill a couple of holes behind the motor. This is a front view.

ozrcboy

[X(]

Okay - you are looking for a little more power than I was thinking.

So, 5s and 2700rpm/v and a 5x5:

18.5v x 2700 x 5 = 249,750 inchs per minute pitch speed

Or as we humans like to use:

380km/hr or 236mph.

I kind of doubt it will go anywhere near that fast so you may be wasting lots of amps trying to get a pitch speed that may not work. My thinking is that the 5x5 will not produce enough thust to beat drag at anywhere near that speed. Just my opinion. Maybe dropping back to a 4s (which would bring your pitch speed down to around 300km/hr (190mph), or perhaps try and get a 5x4, or maybe even 5x3.5 prop (which would drop your pitch speeds to 300km/hr and 266km/hr respectively).

By the way - here's some of why I think that - my Delta flys flat and level at about 120km/hr on 340Watts (I know - it barely rates mentioning next to yours, but the maths should hold). Let's say your airframe has 75% the drag coeffiecient of mine, so it could do 160km/hr on the same power. Looking at that motor I note it can do 100A so at 18 volts that 1800W. Now the amount of power required is a cube of the velocity, so to fly twice as fast you need 8 times as much power. So, if you can fly at 160km/hr on 340W you will need 2.7kW to fly at 320km/hr.

Anyway - looks like an interesting project - will be interested to hear how it turns out.

ozrcboy

Oh, your diagram of cooling - not sure exactly which bit sits where but the critical thing is that there is an exit for the air.

Mike Connor

My Feedback: (5)

The new world speed record for electrics set at the German speed cup a couple of months ago is 225 mph or 363 km/hr and my Delta is not going to break that record I am sure. I understand your math and agree. My main concern with the high rpm motors is the lack of thrust with the small diameter props. A lot of money for a combination that may not work well. The attached frontal view shows the slim 6% airfoil. A Neu motor will cut the frontal area of the center section way down.

ozrcboy

Well there are plenty of prop options around that 5-6" mark, and you can always cut the prop down. I've never played with 5s so can't really say what your amp draw will be like. i know my 2800kv motor on 4s draws around 30A on 6x4 and 5x5 props, but you are obviously going to be going a bit faster.

I'm thinking you are going to end up needing a 6" or greater prop to generate the thrust you need to. Maybe even a 7x5 (although that might send the amps through the roof). Good news is its easy enough to change props and have a play. Do you have an amp metre to check your current draw? Also don't forget that with the high pitch props (like the 5x5) amp draw can actually increase once the plane is flying because the prop is stalled in static testing.

Oh, other thing was I woudln't worry too much about the small props. Your point is fair enough - they don't produce as much thrust watt for watt, but by the same token pylon racers all go for these small fast props because the first problem is getting enough pitch speed - the second problem is getting enough thrust to overcome drag as the speed builds. Clearly your delta is also a low drag ship so the same principles should hold.

The reduction in drag by getting rid of your glow motor will no doubt give you a nice easy free improvement although my understanding is that it is mainly the drag related to the bernoulli/lift affect over the wings that slows you the most.

You may find the program motocalc useful - you feed in your motor/battery/prop stats, airframe etc, and it will spit out estimates of amp draws, top speed, pitch speed etc - I've never bought it cause I'm a cheapskate and I can do a good guestimate back of the napkin style. Given the $ you are spending on your setup Motocalc is probably pretty cheap in comparision, and a useful planning tool.

Cheers,
oz.

vicman

My Feedback: (10)

Mike, I noticed Tower had a 5s 2200 battery on clearance for under a bill. Little small on capacity but good for a practice run at a lot less $.

Mike Connor

My Feedback: (5)

My ThrustHP calculator shows the 6x4 with 1.6 times the load of the 5x5. You are finding these two props equal from a current stand point? I may need a 6" or larger diameter and only testing will confirm that. However, I was surprised to see a Weston Magnum 40 glow converted to electric with a 5x5 prop and do very well. No amp meter yet. I may have to rob a bank to get a good start in high speed electrics. There is a guy building one of these for electric and he is considering a 1000w Outrunner. Not a fair comparison but should be interesting. As far as drag from the Bernoulli/lift affect, I hope the symmetrical wing will be at a very low angle of attack when over 100 mph.

vicman,
Thanks for the heads up. That may be something to consider. 3 to 4 minutes flight times are normal for me with a 6 oz tank.

vicman

My Feedback: (10)

Yea, I thought about snagging one for a long desired EDF project but then realized how much I would need to spend for a charger. Mine is only good for 3s.

ozrcboy

Hi Mike,

No - I don't find them the same - the 6x4 draws a lot more current on my setup, and doesn't go as fast - on the other hand I'm only tooling around at sublight speeds like 120km/hr. However, for your setup you are going to be aiming to get well out past 200km/hr and so you will need a lot more thrust than I do. Further your motor/batteries can handle a lot more amps than mine can.

Cheers,
Oz.

Mike Connor

My Feedback: (5)

Thanks for the replies. I think I will get with the local experts before dropping a chunk on electric equipment. Too bad components couldn't be more interchangeable.