Rob's Grumpy Tigercub build

[Rob Ashley]

Hmmm,

Not being a leccy expert I am a little confused about the results I am getting from my setup. The motors I bought state 430 watts each and gave an appropriate prop and battery which I have fitted (11x5). When I attached my Wattmeter to the setup and ran it in the garden, I was getting 288Watts at full power - quite disappointing really.

I put a larger diameter prop on (12x7) and retested it and found an initial peak of 395 - 405 Watts on as the motor reaches max rpm. The Watts then fall to around 340ish - so still not really much better.

Am I doing anything wrong or do I need to buy slightly bigger motors?

Any help would be really appreciated.

Rob

[Rob Ashley]

In the absence of any replies I went ahead and ordered another set of motors that have a quoted output of over 600W, 1450kv and 220g more thrust plus some manufacturers recommended props. Due to arrive in a couple of days so I guess I'll wait and see the results from those.

[Peter Miller]

Well Rob I couldn't reply because I have to ask George at 4Max what motor to use for anything!!!

My next model will have to be finished before I ask him as I expect it to weightunder 4 lbs by a long way and at first mention he said he thought 6 lbs.(Over my dead body!!!)

[Nigel R]

"Turnigy D3536/8 1000kv motors and 60Amp ESCs."

You could PM David Mellor, he has a ton of test data on these motors.

I also wouldn't take the battery / prop size recommendations on HK too seriously. Sadly.

I assume you have bought a 2 cell lipo (7.4V)? Bear in mind the real maximum on this motor is the current rating (30A) and on 7.4V lipo that's only about 225W. And given that these motors usually seem to draw a bit more power than the specs would suggest, your figures of 288W look about right.

If you went to 3S (11.1V nominal) then 30A would suggest a 330W max. You could probably expect to get away with an initial burst somewhere closer to 400W providing it dropped back down closer to 330W again. Batteries always sag a bit under load.

To get 430W you would need to be on 4 cells. Again I'd expect to find an initial burst much higher followed by a sag back down.

In short, you need a bigger lipo, I think. And probably different motors.

I believe you're replacing two .30 sized glows. With that in mind, personally I would (and have done on several projects) go with the 3536 motor, but in the 900kv variant. I have found that quite useful on 4 cell setup. With a 10x6 it will draw about 470W. This is pretty close to a 0.30 IC and on the limit for this motor. On the battery front a 4 cell 2200 - 2500mAh size would give 7 or 8 minutes in the air.

If you wanted/needed more power then you would want to go up to 3542 size or even 3548 (which will happily replace a 0.40 or 0.46).

Hope that helps

[Rob Ashley]

Peter - I'm sure your weight calcs will be spot on! I reckoned my GTC would be 7lb and current weight ready to fly is 6.5lb.

Nigel - thanks very much for that very useful info. I may well send a PM to David for his advice too.

I have been using a 3S 3000mAh battery throughout and had intended to try these motors on with a 4S battery - alas my 4S batteries arrived with different connectors so I need to do some soldering before a go any further along that avenue.

After I posted my results the other day I did look at the figures again and considered that I would only get near 430W with a 4S after re-reading the motor literature which states 2-4S. Typically, I have built the battery boxes and hatches to suit the 3S batteries I was intending to use.

The 1450kv motors are due to arrive in a day or 2 so I might fit those and look at the results from there - my initial thoughts were physical size of the motor is the same as what is currently fitted so I don't need to start carving away at the model to make them fit. I was also thinking that with a 9x6 prop and higher rpm I may get closer to the output of a 0.25-0.30 IC. Failing that I guess I'm looking at a little surgery to shoehorn a 4S into each nacelle.

I'm quite interested to see the results of the higher kv motors - I'll be back soon with info.

[Peter Miller]

This is why I love ic engines. I know exactly what they will do in any given model up to .52 size.

Having said that, these days lugging the gear across a field and up a hill, even if it is slight, is really a bit of a pain.

Now various members of the club also are complaining of the same thing and we need a five minute sit down to rcover....and they are about 15 years younger than I am.

Even worse, I have been given a couple of foamies and at the moment the only one that I don't lose sight of is a Sport Cub. Mind you when I have me cataracts done that will/should change.

In the mean time I am designing electric powered models.

Actually I have found that the Sport Cub is amazing. It does loops, outside loops, square loops, inverted and great four point rolls

[Tim Ballinger]

Rob,

Apologies,a bit late seeing this. I think both our models are around just under the 7 lb Mark and have a reasonable low wing loading. That together with the more effient blown effect over the wing means that the 680W ( I upped from my original motor size if you remember) I get from my set is quite lively . This comes from 2 3541 1270 motors on a 3S lipo .i use 3 blade 10x6 props and saw little difference with a 2 blade so stuck with the 3 blades.

The peak voltage always drops after the motor has started running. In my case, at full throttle, each motor holds at 30A with the 3S having dropped from the no load peak of 11.8 down to 10.8 V

Hope some of that helps. I only had a cheapo watt meter btw and it gave very erratic results so the above readings were from my Telemeterd 50A sensor( and voltage tap off the 3S) in the line to only one of the motors. Same values on both sides so more than happy withe measurements in the end.

Tim

[Nigel R]

Rob, 3S 3000mAh sounds reasonable enough. Probably well worth sticking the 1450kv motors on and seeing if you can get a sensible sort of prop to get a 350W - 400W reading.

[Tim Ballinger]

Rob,

Should have said I use a 3700 capacity lipo. Gives me 8- 10 mins.

tim

[Rob Ashley]

Chaps, thanks for all of your advice - I am most grateful.

Nigel - I will stick the 1450kv jobs in and see what readings I get. Looking at the motor specs they are rated to 655W on 4S, so with the right prop I think I can get a useful 400ish W per motor which should give enough performance without stressing the setup too much.

Tim - Great to hear from you again. I do remember you changing your motors come to think of it and with your flight test data with similar weights gives me more confidence in mine. Incidentally I have 1 battery per motor in each nacelle so I'm hoping power to endurance will be a good balance. I can see me ending up with a whole raft of elec props to get the best balance. I like the idea of 3 bladers though.

Peter - I'm with you at the moment. I know what I am getting with IC and don't with elec despite many calculations and research. I feel your pain with the lugging of kit to the flying site, we are lucky at our club being able to park directly at the site. I see your Ohmen is due to be published next month too - honestly, you put the rest of us to shame...

[Peter Miller]

Well the OHmen is a pretty simple model but it does fly like a dream and totally aerobatic.

Now for my next one, well that is different!!

Se my thread MAK 15 MP

**LINK**

I have started cutting out parts. sent Grahama Ashby the PDF files of my plan. His comment was OMG!!

[Rob Ashley]

Yes Peter she certainly is different, but what a model, very unique shape. Looking forward to seeing the plans.

[Rob Ashley]

Ok, new motors in and tested. Didn't go that well as I hoped, as no matter how much tightened the prop nut the prop shot off the motor and smashed into the cupboard door, splintered and scuffed the transmitter I was holding at the time. Hey ho.

Motors are holding about 31A at full chat and producing 330ish W, but more interestingly the model weighs 2.9kg with a static thrust of 3.54kg - so should be ok. What I found interesting here is the thrust is greater than the manufactures' quoted but the watts are less. Still thrust / weight ratio is a much better performance indicator than watts / weight.

She has moved under her own power for the first time - I managed to taxi around the garden for a while, setup the failsafe check for loose items and conduct a range check.

Edited By Rob Ashley on 24/08/2018 18:44:12

[Rob Ashley]

Have to admit the dog doesn't look that impressed, but I am quite happy with it.

There are still some lettering I need to source and add on the top wing - I mean it must have a name right? I need to change the deans connectors to XT60s and I need to mould another canopy and paint it blue to match the underneath but otherwise the jobs list is complete and ready for her maiden flight.

[Tim Ballinger]

Looking forward to the flight pics.

Tim

[Peter Miller]

I find that there are various places on EBay that do very good vinyl lttering at reasonable prices

**LINK**

THis is the one that I use at the moment since the last one seems to have given up

[john stones 1 - Moderator]

He's a lovely looking dog Rob, models not bad either.

[Nigel R]

Looks great - and very yellow! Hope the maiden goes smoothly.


If those are the standard aluminium adaptors they are sometimes a little lacking in quality.

I find a Spot of cyano can help when tightening then up.

[Piers Bowlan]

Or perhaps spring washers may help stop the prop nuts coming lose? Did you balance the props, it may help if not? Lovely looking model by the way Rob, great workmanship, as usual.

[Rob Ashley]

Hi All,

Peter - thank you for the link for the lettering I will give them a look later on ( have some jobs to finish for my wife first).

Nigel - Yes she is a little yellow at the moment isn't she. I wanted to fly her first and make sure all was well before completing the covering job. I intend to compliment the blue underwing with something on the top surfaces similar to Peter's original. Likely including painting the canopy the same shade blue and add some lettering etc. plus that means I don't have to do cockpit detailing...

I hadn't thought of cyano on the prop shaft - thanks for the tip I will give that a go.

Piers - I have swapped the original prop nuts for loc nuts, oddly it was the adaptor that came loose and the nut was still tight when I picked up the bits. I replaced that adaptor and so far the this one seems fine, but as Nigel says a drop of cyano should help matters. I do balance my props and think it is important to do so, there are quite substantial differences in manufactures tolerances sometimes.

Anyhow - it looks like time for a maiden flight - hopefully report back later today.

[Nigel R]

Might be worth trying a different adaptor if you have a few from a spare motor. Also a wipe with ipa might remove any thing slippery from motor shaft and collet.

[Rob Ashley]

Nigel - IPA? That's a beer where I'm from

[Peter Miller]

Here is another company that I use. "Stickers Aviation". They have ahuge number of all sorts of stickers incluing American aircraft markings (Pre War) They are sensibly priced an you always get some extras with the order. They are based in Greece but deliver very fast, as fast as most UK shops.

**LINK**

They are on Ebay

They even do 41 SQuadron badges and that makes the truly ACE as far as I am concerned.

[Rob Ashley]

OK so with the rain set in for today I thought I'd take the time to add some of the trim.

Just needs a canopy and I need to order the letters. Saw tooth cut individually using a ply template. Wasn't hard to do apart from taking time to reduce the number of air bubbles between the two coverings. Yellow is Oracover and blue is Easycoat.

[Rob Ashley]

In slightly more interesting news I did manage 2 flights yesterday. She flies excellently! Zero rudder and aileron trim required but I did need quite a bit of +ve pitch trim. I have since moved the CG back 5mm and see on the next flight.

Rolls are very axial and she will loop from level flight at half throttle - so my concerns about the power I was getting from the 3S setup were quite unfounded. This model is a real credit to the designer - well done Peter Miller!