the maths be hind what size engine your plane needs

[Adrian Day]

Does anyone know if there is a maths equation for power to weight of rc builds ?
I know there is when it comes to motors but i need one for engines?
i need this to see if i have over powered my latest build - and an idea so i know how to work it out for my next one too..

[Martin Harris - Moderator]

If you have a Watts/Pound figure in mind then remember that there are approximately 750W to a horsepower. Manufacturers tend to be a little "optimistic" with their figures which are quoted at peak rpms so unless you can run a small prop without regard to noise, knock 20% off and you'll be in the ballpark.

 

For example, if an engine is advertised as giving 1.3 bhp at 15000 rpm, that's probably around 1 bhp in the "real world" and equivalent to 750W.

 

If in doubt, go up one size on your engine and remember that you can always throttle back!

[Tom Wright 2]

With eletric motors adrian its 100watts per pound, i am sure you can relate that to IC units,if not i am sure other members will come up with the answer. tim......beb.....martin

Edited By tom wright 2 on 07/02/2011 01:31:56

[Martin Harris - Moderator]

Better late than never, Tom!

[Tom Wright 2]

What are you doing up at this time in the morning?

[Tom Wright 2]

While your still awake how do you relate divergent pitch due to rearward CG in terms of elevator response / feel?

[Martin Harris - Moderator]

With some trepidation! Off to bed now though...

[Tom Wright 2]

What its only 2am ! good night.

[Adrian Day]

thank you Tom and Martin for your comments: very simple when you put the motor value ... i wish i was good with the figures for exchange to IC will have to look that onne up ? less someone can do the maths on the motor / ic exchange ... motors are a liitle more simple in figures i should go with them really but i crave the smell / sound and addiction of engines

[Mike Rolls]

It depends on the sort of performance you are after - but for adequate aerobatic performance in the 30-60 size range, which covers an enormous number of models, and assuming normal sports motors, one pound AUW (less fuel) per 0.1 cu.in of motor aize is a rough, but very effective, yardstick,

HTH

Mike

[Adrian Day]

Thank you Mike

that make sence and is a simple way but effective way of working out

im running my ESX-52 on a 52 Leo Engine, I Think its power is about right - with your equation I'm wanting it as a sports model.. but dont want it over powered..

Thank you again Adrian

[Biggles' Elder Brother - Moderator]

What's the model's all up weight and wingspan Aidrian?

 

BEB

[Adrian Day]

Hi BEB
Thanks for your msg

The all up weight is 2.5lb or 40 oz and a WS=50" Thats with 5 servo's receiver and 52 Leo engine at pres
Hope thats of some help

[Biggles' Elder Brother - Moderator]

 

 

Assuming a 2 stroke:

A 40 would fly it comfortably - it will take off, do basic aeros; loop etc - but not big. Vertical performance would be OK, but might be limited. Fine for a competant, but perhaps inexperienced, flier.

 

A 50-53 would be fine - sporty performance, big loops, bunts - close to unlimited vertical.

 

A 60 would whip it around - hooligan terriority! Prop hanging, 3ft take offs etc!

 

For 4 strokes:

 

Go 30% or so bigger. So a 40 becomes a 56. A 50 becomes 70 and 60 becomes a 90.

 

BEB

Edited By Biggles' Elder Brother - Moderator on 07/02/2011 14:22:15

Edited By Biggles' Elder Brother - Moderator on 07/02/2011 14:22:45

[flytilbroke]

I will be more conservative ( not politics), a reasonable .40 or .46 will pull that weight around with ease as it does not look to be a draggy type of model. A .53 will certainly give pretty good aero's, you can always throttle back.

[Adrian Day]

some great info the be getting on with .. and very welcomed info indeed. being new to this and having so much to learn .. great to gain knowladge from some pro's in the modeling world..
Thank you all so much for all your input i hope that the model is streamline in the none drag sence as i have tried to bring a new take on a sports model.. i do think that the front still need some redesigning but its a good start i think? first full design..

 

i can work well with the knowladge been given here to further my designing

best regards

Adrian

[Peter Beeney]

Having just caught up with this thread, my first thoughts were more optimistic, I think. I’ve got a couple of Irvine 53’s, one in an old Flair Patriot, and one in the club’s trainer, a standard Boomerang. Also a couple of Leo 46’s, I’ve never heard of a Leo 52, but that’s not in any way significant, or unusual, I’m usually way behind the times.

The Boomerang weighs 5.25lbs, has a wingspan of 61 inches and at this weight and power is very sprightly indeed. Not unlimited vertical, but if I tend to punt it around a bit it needs 10 standard rubber bands to hold the wing down. And even then it will flip up occasionally, if it’s a bit windy. The Patriot, at 6lbs and 60 inch span, again not vertical performance, but with an 11 by 8 at around 11,000 rpm, from a flat out shallow power dive it will pull straight up to the comfortable limit of viz.

If the Leo 52 is a 0.52 cubic inch two-stroke then then I’d consider Adrian’s model, at 2.5lb, less than half the weight, and 50 inch span, and with the motor on song turning say an 11 by 8, flying may require a level of concentration; if it were me, anyway; and with those ailerons, elevators and rudder the controls will be positive, to say the least. Interesting, is perhaps one way I might describe it.

Looking at the photo, impressive, surely the wing loading must be pretty low, around 11 to 13 ounces/sq ft? That’s a wild guess.

Very nice indeed!

PB

[Adrian Day]

sorry all i was wrong on the engine in this model - the leo i have is a 46 and thats in my other model

Link

the 52 that i have for this model is a ASP S52A two stroke glow engine -

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=8989
Hi Peter

thanks for you kind words you know your models yes indeed your are about correct when you say the wing loading is low 12.5 there about ... Im in two minds what size engine i should settle with ? being new to this all the post have been so helpful .. im getting some great feed back and i can see its a big hill to climb to getting to know all there is in this field...

Peter you are not behind the times with the engine i said.. sorry i was mistaken listed in the links above are the engines i have ...

i do think that this will be a little sporty with a 52 but i was thinking maybe running it a little slow and maybe just using the mid to full speeds for aerobatics either that or i may change the engine for a leo 46 and see how that reacts... maybe someone could give a little advice on what they would think best???

 

Here is the new picture with wing cover on .... thank you for you comment on the picture let me know you thoughts? i would be grateful for your comments be it good bad or the ugly ???

 

REAR VIEW

 FRONT VIEW

Edited By Adrian Day on 08/02/2011 01:05:47

Edited By David Ashby - RCME Administrator on 08/02/2011 05:03:29

Edited By David Ashby - RCME Administrator on 08/02/2011 05:03:51

Edited By David Ashby - RCME Administrator on 08/02/2011 05:04:41

[David Ashby - Moderator]

Adrian, can you check this out please, thanks.

Edited By David Ashby - RCME Administrator on 08/02/2011 05:05:58

[John Muir]

Hi Adrian,

 

How about double checking the weight of your model. Your engine weighs over a pound, your servos, rx, battery and fuel tank must be 10-12oz minimum, which leaves the weight of your covered airframe alone at less than 14oz! Looking at the pictures, it's a conventionally constructed balsa airframe with fully sheeted wings and no obvious signs of lightening holes, so I'd expect a model like this to come in at about 4-5lb AUW (or if I built it, considerably more). Frankly, if it does weigh 2.5lb including all the gear and engine, then your building skills are amazing, or else you've discovered a new type of ultra light balsa wood and I want some.

 

A 2.5lb model with a 46 or a 52 in it will be absolutely ballistic, and will stay in the air even with the engine idling, making it a pain to land. I have a 36 inch span sports model weighing a bit over 2.5 pounds which flies very nicely on an old OS25FP and a 9x5, thank you very much.

 

Just going by the look and size of the model, I reckon your Leo 46 will do very nicely. The 52 will be a little 'hotter', but would work well too. If it really is lighter than it looks, then a 32-36 will do the job very well, in line with the rough formula Mike Rolls gave you earlier.

 

Cheers,

 

John.

[Biggles' Elder Brother - Moderator]

John has a point there Aidrian. I too was surprised at the 21/2 ib figure for a 50" span model with what looks like a sheet sided fuselage. It does seem remarkably light!

 

Assuming the AUW (including engine and radio gear etc) is a little more than 21/2 I'd still be tempted by the 52. The throttle lever works both ways! It would give you easy criusing at 1/3 throttle and loads of power for "blatting" around when you're confidence is higher. Its not impossible, but it is very difficult, to have too much power!

 

BEB

[Martin Harris - Moderator]

Funnily enough I thought the same about the AUW but kept quiet as I felt ashamed that I couldn't think of getting anywhere near that light myself but as others admit the same I'd better come out of the woodwork!

 

One consideration with the engine, unless the pictures give the wrong impression, is its weight. Where does the C of G come out (with the silencer fitted of course)? It does seem to have quite a long nose. 

 

However on looking at the pictures again, if the tail area and moment are as small as they look you may need a fairly forward C of G anyway - have you calculated it theoretically yet?

Edited By Martin Harris on 08/02/2011 11:38:35

[Adrian Day]

Hi John, BEB & Martin

 

i have added a pictrue of the model on scales and yes she is a very light model i get my supplies from a great company near me the balsa cabin they hand pick the weight your looking for ....

I have gone for the lightest i can get hold of ... and have to confess that i have just realise that i havent weighed the model with the battery or the fuel tank.. so its still got a little to add but everything else is there but the model at present is just under 2.3/4lb if my scales are correct sad to say they the only thing i have .

 

as yet i havent worked out the CoG Yet as not 100% on how to work it out ??? if anyone can help again would be grateful???
think its 1/3 of the wing from front but a little unsure yes im going for more front than back maybe im wrong to do this ??

and as i keep saying im new to this so this is really my first big step to a full design /build ..

sorry if i havent answered all your questions and the things i dont understand im working through i dont claim to be great at this thats why im asking so many questions Im so gratefull for all your input and your patience
best regards to you all
Adrian

[Biggles' Elder Brother - Moderator]

Well Adrian you are to be congratulated - it is a very light airframe.

 

OK, to business. We do need to sort the CoG out because if any lead is needed to get it right then we need to build that into the calculations. With the fuel tank and batterry added, then balance at 1/3 back from the leading edge is a good place to start. That will be a "safe" CoG. You might find that when you maiden the model the CoG could come a little further forward than that - but that is a good place to begin.

 

Check that out and let's see if she does need any lead - I hope not as it would be a pity to add weight!

 

Just to satisfy the doubting academic engineer in me can you also check your scales? Just stick a 1kg bag of sugar of something like that on it to check it reads OK. Not that I doubt you - I doubt all measuring instruments - its my job!

 

BEB

[Martin Harris - Moderator]

There are lots of "rule of thumb" methods to calculate a safe starting point - mostlly assuming a conventional planform. If (and I still can't decide from the photos) your tail area and its moment arm are a little smaller than "average" then it's important to use a method which takes these into account.

 

I've had good results from this online calculator...