Electrifying VMAR Bulldog

[AlphaFSC]

Has anyone done an electric conversion on the Vmar S.A. Bulldog? Would appreciate any comments.

[Former Member]

[This posting has been removed]

[AlphaFSC]

Thanks for that. The Vmar kit is described as semi-scale, so hopefully it has a 'kinder' aerofoil to compensate. Wonder if anyone has done the intended i/c version and would like to comment?

[Peter Ford]

I just test flown my Vmar Bulldog and its absolutely spot on.

And I am electric!!

Radio:  Futaba 7C 2.4ghz

Battery 2300 5S 

Prop 14 X 10 (@ 7400rpm  800Watts (thats 100W / pound on 71/2 model)

ESC; Castle Creations 60A

Motor: Turnigy 50-55  (http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=2102) 

I am literally just back from the field. Had one 5 minute flight . Model took off in almost perfect trim. It is very stable, but still loops & rolls nicely.

The flaps are worth using as it tends to float on a bit without them. Very little trim change with flap down.

Checked Lipo battery with Cellmeter after the flight. I used about 65%

I was  charging up for a second flight but had to abandon due to rain.

[David Ashby - Moderator]

Nice one Peter, any piccies?

[Peter Ford]

I suppose you want to see this bit!

There is a Deans 'arming' socket glued into the bottom of the fuselage. Shown here 'armed'

(the battery plug was disconnected for this photo)

The Vmar kit has a very convenient 'fuel tank' hatch for battery access. A 2300ma lipo fits on a small ply tray fitted between front & rear formers.

I must say this model is 'Rolls Royce' quality. Some nice scale panel lines & rivet detail. I used all the supplied horns & clevises, hardware out the box.

The engine mount is alloy, cheap, and very adjustable. There was no 'hacking about' required to fit the motor.

The supplied spinner, whilst being a quality item, was better suited to a glow motor. So I used one of those ones with the alloy backplate + 2 screws.

I set the control throws as per the manual. CG is slightly forward of recommended position.

[Tim Mackey]

Nice one

[AlphaFSC]

Peter, thanks very much. Your last post answered all my questions in one go! Pleased the maiden flight went well - I'll be starting my build in a few weeks, thanks to your useful advice.

[LAN]

I just spotted this model, and since it's also available in Swedish colors it tickled my interest. Could you tell me if a 4-stroke would fit under the cowling? They only mention a 2-stroke in the specifications...

[AlphaFSC]

Peter, I'm puzzled as to how you got the balance point with your battery in the fuel compartment?

I've finished my build, yet to maiden. I'm using the same engine with a 5s 2500mAh Lipo (slightly heavier than yours, but not by much). Short of tying a sheep to the tail, there was no way to achieve the recommended 80mm balance point. I've had to put the battery laterally up against the back of the servo tray, and my receiver battery is up against the next bulkhead towards the tail. I just make 80mm then. AUW is 7.3 pounds and with a 13 x 8.5 prop the meter is reading 520 watts.

I don't think I've done anything daft, so looking at your photos is a bit perplexing.

I must echo your comments on the quality of this kit - quite outstanding.

[AlphaFSC]

For anyone still reading this thread - this was my first electric conversion and there was nothing daunting about it. Note the comments above about the balance point. Theaircraft flys very nicely. Just like the full-size version, it is very twitchy on the ailerons, so keep to the recommended throws and put in some exponential if in any doubt. Presence in the air and on the ground is very pleasing.

[robin hodge]

Hi Guys

 

Just bought the VMAR Bulldog in RAF colours. Also considering electrifying it. Good quality kit although I always find the hinges a bit tight on these ARTF models so I nearly always change them. Otherwise I'm very impressed with the kit - apart from the midget pilot. Thinking about changing him! I've already got a complete electric power train from the Tony Nijhuis Typhoon that I only flew once as it was a bit hairy at 62" span and 91/2 lbs so should power the Bulldog easily. Where did you get the mount from?

[Mike Frith]

I bought the RAF version and converted it to electric. Used the following:

 

Motor: Emax BL4030

ESC: 80A

Prop: 14 x 12

Battery: 6S 2800 (2 x 3S)

Got around 25A = 500W

 

I customised mine a bit (to look like the full size one I flew in the University Air Squadron). I'll try and add some photos. I really enjoyed putting it together and adding those individual touches (call me strange!).

 

After reading around I made sure the CG was forward a bit. The instructions say 80mm back from leading edge without fuel so as I didn't know what fuel will add, made the CG around 75mm back. Balance was about right without batteries so put main and receiver batteries pretty much on the CG.

 

Maiden flight: 10 degrees of flap, 10 knots headwind. Got off the deck OK but took a while to climb out, difficult to keep the nose up and pretty hard to control. Turned onto crosswind and things got worse ... struggled to keep the nose up, just got faster. Hit the deck at speed, quite spectacular ... totalled. Very little of the airframe salvageable,  electrics all seem to work OK.

 

So, I'm thinking that the CG was probably too far forward ...as I struggled to keep the nose up. It was possibly a bit under powered too, though speed wasn't a problem.

 

I'm thinking of buying another one as I'm keen to see it in the air properly. Anyone want to help me get it set up and tested better the second time around? It would be great to get together with someone else who has one with electric conversion. Get in touch. I'm in Gloucester.

Edited By Mike Frith on 17/03/2010 22:44:57

[Mike Frith]

And now for the photos:

 

 

[Mike Frith]

... just had to change the pilot

Edited By Mike Frith on 17/03/2010 22:46:02

Edited By Mike Frith on 17/03/2010 22:46:58

[David Ashby - Moderator]

Sorry to hear about that Mike. How much did she weigh? 

[Mike Frith]

3.5kgs

[David Ashby - Moderator]

Hmm, that's 7.7lbs so 65 watts per lb - not enough Mike as you suspect. Many models fly better with a slightly forward C of G but it depends how nose heavy the model was I guess. You certainly needs more watts though. Will you be looking at a bigger motor? 

Edited By David Ashby - RCME Administrator on 18/03/2010 10:41:40

[Mike Frith]

Being fairly new to the electric lark, I sought the advice of someone who knows about motors ... they recommended this one. I expected it to be a bit low on power, but not as problematic as it was. However, next time I will be looking at a larger motor, probably a Turnigy 5055 or equivalent. Not sure what KV though.

[David Ashby - Moderator]

Well I would look to achieve 120 watts per lb or thereabouts 

Kv wise you're probably in 600-800 territory - give BRC or similar a call and they should advise.

[Danny Fenton]

Hi Mike, sorry to hear your test flight didn't go so well.

 

I am struggling with your set up a little bit, you are using 6S so around 24 volt, (20 under load perhaps) an 80A esc and a 480 kv motor, the prop is a 14 x 12 so would have expected more than 25A?? (but that is 500watts as you say)

The motor manaufacturers quote 51A using your set up with a 14 x 7.......... no wonder people new to electrics get confused

 

The motor is apparently good for 60A (1 minute) the esc is clearly good for up to 80A I think you could have tried upping the prop size a touch (If the ground clearance would allow it?) and getting the current/power up.

The other alternative is to go to a slightly bigger motor, but if the prop size cannot be increased you will have to go for more revs and that means a higher kV motor, not necessarily bigger. My 86" Chipmunk is on a 4240 which is only slightly bigger than you are using, but higher kV, still much smaller than Alpha and Peter are using.

 

I am still surprised the nose didn't come up, this is why I try and video all my maidens, so I can get everybody's oppinions if it goes wrong. As David says it does sound as though it was struggling for power and perhaps was close on 500 watts. Having said that my 14lb Hurricane cruises around  straight and level on 400 watts, (according to Eagle tree). But it does have over 2kW available for manouvres if need be. I doubt it would get off the deck at 400 watts

 

It does look a nice model, hope you get another

 

Cheers

Danny

[Tim Mackey]

Coincidentally, and hopefully of use to this debate - I am currently putting together an article for a future mag issue on converting IC to leccy and the model concerned is a low wing sport type model, and weighs in at 6lb+ .

Now from the onset, I was aiming for around 90 Watts per pound, as the model concerned is not a rocket ship - and IMO neither is the bulldog

I wanted to use what certain equipment that I had to hand - especially a spare motor which was from another earlier project ( no not the camel ) and this has a Kv of 480 which is the same speed as yours according to Danny - although I coulodnt actually find the motor specs when I did a quick google around. However, I will be using a 4s battery, and larger prop - 'cos I dont want high revs, or noise.....one of the adavntages of leccy stuff is the mix and matchability of components to produce different results . The motor was originally run on 8s LiFe packs, and later on 6s Lipo...again same as you. It used a smaller prop ( 12 x 6 )  as the model involved required a higher pitch speed. This setup produced 11000RPM, 43A static, and 900 watts, so I too am struggling with your data.

On the new project, 4s lipo and a 15 x 8 wooden prop produces 43 A , and a very usable 630 watts at a nice low 6250 RPM, which should do me just fine.

My point is, before you go buying more motors, experiment some more, and double check your equipment and data -  cos you ought to be getting a heck lot more than 25A on a large 14 x 12 prop!!

Edited By Timbo - Administrator on 18/03/2010 14:25:06

[Tim Mackey]

Just noticed aftter looking back at your figures....your packs are only 2800 mahr capacity, I strongly suspect that this could be part of your issue - that big prop on a 480kv motor with 6s battery should be pulling pretty darn big currents TBH, and I think your packs may be depressing voltage badly under that load.

You havent mentioned whether you are using a wattmeter?

[Mike Frith]

Thanks, guys, for joining in.

 

OK ... so much to respond to. To be more specific on the numbers (which weren't quite right in my original post), I bench tested the motor with 3 different prop sizes 14 x 7, 15 x 10 and 14 x 12. The 14 x 7 gave around 25A, but the 15 x 10 and 14 x 12 gave around 26.5A peak which settled back to 25A after the initial 10 seconds. These figures correspond with others who have tested the motor (see http://www.giantcod.co.uk/epower-bl4030-385kv-brushless-outrunner-motor-p-257.html). Emax seem to over inflate their own performance figures. It's only a 380kV which I'm suspecting is a bit low anyway (reputedly gives around 6500rpm).

 

So, 26.5A x 22V = 583W peak or 25A x 22V = 550W nominal

 

I was advised that this was adequate for the model (around 75 watts per pound) as I didn't want something too hard to handle.

 

I don't quite see why 2800mah batteries should be a problem at 26A. They can discharge at 30C which would be over 80A, so it shouldn't be so demanding, right?

 

To be honest, seeing how it flew, I don't think power was the problem. I think the CG was just too far forward. It was something I found hard to get right in the workshop, and wished now that I had spent more time playing with this. I guess I didn't quite think that such a slight difference would result in such an uncontrollable aircraft. My years of experience with rc gliders told me that, but I think the Bulldog is a sensitive, even twitchy, model which isn't very forgiving at the edge of the envelope ... from what I've read of others who have flown it.

 

As for videoing the maiden ... got the wife to do that but she lost the model just after it left the ground

[Tim Mackey]

Ah well the data has changed - there will be big difference between a 480Kv and a 380Kv motor - hence the lower current.

I suspected the batteries only based on 

1) not knowing the C rating at that time

2) 480 kv @ 22V on big prop could be pulling maybe 60A or so in which case 2.8Ahr aint much - in fact they would be working at over 20C

yp, 380Kv is toolow, and your way forward with it, is either even more volts which puts the pitch speed very high, or even bigger prop which may start getting a bit silly scale wise.

 

I would cut your losses and go for a higher Kv motor - maybe 500 or so.