Spitfire Motor Choice?

[DH Flyer]

Please could I have some advice on choosing a motor?

[DH Flyer]

I am building a 31" wingspan Spitfire (balsa), aiming to finish it for full RC but no retracts.

I have tried to follow some basic rules of thumb

Do you feel that this would be a suitable choice for 3 cell Lipo?

http://www.brchobbies.co.uk/catalog/product_info.php?cPath=103_27&products_id=380

I'm sorry but I can't give you an idea of auw yet . . .

Many thanks

[Olly P]

Certainly, but we will need a little more info!

By 'motor' do you mean motor or engine?

what is the weight/span etc of the Spit?

Finally what flying experience do you have? a Spitfire is not a first model....

[Tim Mackey]

I guess as he mentions 3s lipo, and the link is to a motor, then he means motor

personally, I think you need something a little more robust and powerful - these little "bell" outrunners are OK for lightweight foamies, but for a conventional build, and 31" wingspan, I would suggest something  LIKE THIS

Its always best to have a little power in reserve, and not to be using the powertrain at its maximum constantly.

Think of a 1 ltr car doing motorway driving at 100mph all day, and then do it again in a nice 3 ltr limo....so much less strained

Edited By Tim Mackey on 20/05/2013 20:01:38

[Former Member]

[This posting has been removed]

[DH Flyer]

Your're right, I agree the first post would have left a lot to the imagination! it took me a while to tap out the rest of it on this tiny iPad keypad!

Thanks Tim Mackey, I'll go with your suggestion, that makes sense.

As for the flying ability, it would be a bit of a step up from anything I have done before, and I am not a member of any club, but I guess there is always an element of 'learning on the job'!

i'll approach it cautiously, and the 12" deep thick grass on the field is a very handy anti-crash device!

[DH Flyer]

Just wasted a few minutes trying to work out a thrust to weight ratio for a merlin engine on a 31" wingspan balsa spit!

[Tim Mackey]

Somewhat oversqaure I would imagine

[Martin McIntosh]

If you want a small Spit then go for the Nihuis 44" one which is a well proven design and save yourself a lot of wasted time and headaches. At 31" built from scratch without modern techniques it would probably have such a high wing loading that it would not fly or be so fast that it would not exactly fly anything like a Spit.

[DH Flyer]

Thanks for the advice.

In choosing a battery, do you think an 800mAH 20C 11.1V lipo would be correct? Would that lead to an acceptable flight time, bearing in mind that weight will be at a premium? Thanks

[Tim Mackey]

Do we have an estimated model weight yet?

800mahr is pretty small, and I would think you want more like 1200 minimum, especially for longer flight times

[DH Flyer]

To get an acceptable wing loading, I will try to aim for <15 ounces. I expect this will be possible . . . I agree 800 mAH is definitely on the small side, one calculator that I found since reckons that will give me only 3mins flying on full throttle!

Therefore I will choose something slightly larger. A 1000 mAH 3 cell Lipo weighs circa 120g or 4 ish ounces which is already a huge proportion of target weight! Perhaps I will just have to accept a very short flight time on each battery. What would you suggest? Thanks

[DH Flyer]

If 7.4v will be sufficient, then I needn't worry, as a 1300mAH 2 cell Lipo I have found is only 85grams or 3 ounces.

If the motor you suggested draws 15 amps max and the battery is 7.4v, then doesn't that result in 110 watts, which is over the 100 watts per pound suggested minimum? Am I on the right track?

[Steve Hargreaves - Moderator]

Kind of.....but the thing to remember is that the higher the current draw the shorter the flight time......if you went for a 3S battery you would only need around 10A to get the 110watts so you could get away with a smaller pack.....the flip side of course is that the extra cell will increase the weight so you are nearly back where you started but a great advantage is that you can use a smaller ESC (12A on 3S where as you would need an 18A unit on 2S)

Bottom line as they say is that there is no such thing as a free lunch & you will need to compromise....the skill with small models is lining up the compromises so they work in your favour....

[Tim Mackey]

Not quite

The motor MAXIMUM current is stated as 15A - a different thing altogether. It will only draw 15A when loaded to its maximum, with a 3 cell battery.

Data from website: Prop range: From APC-E 9 x 4.7SF (95W on 2S) to APC-E 7 x 5 (150W on 3S)

You can see that on the lower 2s voltage, the power is 95Watts.

Assuming your battery holds around 7V ( loaded ) then thats a current flow of approx 13.5A, so you could get away with a lightweight 15A ESC too. Be sure to check that your chosen battery will supply the current, without pushing the "C" rate too much.

13.5A / 1.3A ( capacity ) = approx 10C, so assuming your battery is at least 15C then you should be fine.

Crossed with Steve LOL - the "not quite " bit was aimed at the OP

Edited By Tim Mackey on 22/05/2013 10:06:13

[Erfolg]

D is that Dave?

I have done a Guillows P47, which I think has a 34" wing, the slightly bigger alternative.

I used a CF2812 from memory, which was fast enough for level flight, but not enough guts to go up for some reason.

It has now been re-motored with a Turnigy 2217-16 to provide more urge. That is to get high enough to sort it out.

My model has a 20oz wing loading. I suspect that the flying speed will be higher than it should be to look anything like scale.

I anticipate trying it again very soon, when I get around to it.

[Erfolg]

Turnigy 2217

[DH Flyer]

Wow that P47 looks good! I look forward to finishing the Spit! This forum is a great encourager, seeing what others have done really helps me to stop thinking about it and just go and stick it together!

May I ask what the spec of the battery that you use is erfolg?

and no I'm not Dave, but Dan.

In a copy of thE RCMW plans catalogue there is an article about electric flight and I am finding that quite helpful. it develops what Steve and Tim have suggested. Thanks,

[Tim Mackey]

Wash your mouth out Dan

[Erfolg]

Dan

I think it is a 750, certainly a 3s. The battery size was chosen in part, as part of getting the CG in an acceptable place. To achieve this the battery box is in the cowl.

Which still required lead ballast. The re-motoring has reduced tha ballast a lot.

I do not know about Tim and others opinion, in my case I moved it to the 30% point, further forward than the plan, which was for FF and CL (which I think is more in line with the 30% but still quite far back).

The airframe does not weigh much, but by the time the servos, Lipo, ESC, Rx, go in, it became quite porky. Although my model is 1/16 sheet on the body sanded back to about 1/32, using cyno as PVA is to heavy, then the lightest glass cloth with WVP it still came out @ 20oz ft^2 according to my records.

I sprayed my model with Acrylic paint, as it is lighter than brush painting.

Edited By Erfolg on 22/05/2013 15:04:33

[DH Flyer]

Okay Erfolg, thanks for the details.

I like your installation of the battery. The spitfire has such a narrow nose in comparison that I may have issues doing something similar, but I'll have a go. I am looking forward to seeing what you do with the Natter!

After burning myself at the stake for the inadvertent heresy, I will go and start the fuselage! The wings are done but can't be sanded down until I can check the fit against the centre section.

Dan

[Erfolg]

Dan

It may be possible to put the Lipo on edge, if necessary.

It is worth bunging all the equipment in the general locations, taping to hold position to get a feel of where the CG is coming out, with a view of determining what has to happen to achieve what you want.

I used 5g servos on the aileron and a 9g on elevator. The Rx is a Frsky, 4ch as it is very light and compact.

click on the photo to see the detail, and by the local sagging, starved dog look, due to the very thin skin.

The wing is heavier though, essentially 1/16 skin.

I do not know about your Spitfire, the Guillows wing section, is rather peculiar, in the the LE is right on the bottom of the wing and sharp. Apparently it is said for RC it does not work well. So I raised it to an approx clark y position using Profi print out as a guide and provided a good radius to the LE.

There are probably threads for your Spitfire which provide good guidance and hints, tips and videos of the model flying.

Whose kit is it, I some how have not registered it?

Note the book in which I try and note all the parameters I think may be useful in the future, as to how it is set up.

Edited By Erfolg on 22/05/2013 16:42:54

Edited By Erfolg on 22/05/2013 16:45:22

[DH Flyer]

The plan I am working on is the 1963 Model Aircraftt plan by Stan Cole.

It was originally designed for free flight with a small i/c engine. Luckily it does not require much adapting apart from making control surfaces, reducing dihedral and adding an extra former to mount the motor on (I'm also going to leave the u/c off). I downloaded the plan from Outerzone.co.uk and I can find almost no trace of it on the Internet at all.

Just cutting out the fuselage formers . . . following what you said about the wing ribs I had another look at mine, I reckon they are okay

[Erfolg]

I see the tailplane is a lifting section.

You may want or benefit from somebody who knows about these things. Free flighters will know how they work. Lifting tailplanes were certainly prevalent with FF and some early RC models. You may find the odd model designed today with one, but I do not think they are common.

I totally agree with the UC thing, small models can do without the wight, drag and at such small sizes tend not to cope well with anything worse than a bowling green.

[DH Flyer]

Thanks for taking a look erfolg,

Next week someone is coming to stay with us who is a free flight aero modeller. He has competed in nationals in New Zealand and will definately be able to sort out any issues i have. In fact, i might just let him build it for me! So good timing