Which Size Engine Would You Recomend?

[Dai Fledermaus]

I was interested to read Steve Hargreaves article `Going Glow` in the May issue of RCM&E, particularly the last paragraph headed `Does Size Matter` in which he suggests fitting the largest size of engine recommended for any given model. I'm sure many of you will have read it so there's no need for me to quote his reasons for recommending this approach.

So, with his advice in mind, which size engine would you fit in an aileron trainer I'm building which has the option to fit a flat aerobatic wing at a later date. The recommended engine size is .25 to .40. However, here's rub, this design is about eighteen years old and the trainer version performed quite adequately with a K&B 28 which by todays standards is no fireball. I think I'm right in saying it develops less power than your average .25 does today.The designer also claimed that aerobatic version with even a decent .40, (for the time) was brisk and capable of a vertical climb.

So if you looking for an engine to suit both version which would you chose?

[stu knowles]

A good 46 like the OS 46FX will power oodles of models out there and is probably no heavier than a 35-40 from back in the day.

If the motor is too light you will end up carrying deadweight ballast, so for that reason alone using the biggest engine (within reason) has a lot going for it. It's easy to throttle down and fly right but if the motors flat out and struggling to cope, well that's never a fun thing.

So, if t'were mine a 46 and the best that I could afford, it should last you years.

stu k

[Jon H]

After over 20 years in this hobby and having flown most types of fixed wing model I am a firm believer that the approach recommended in that article is totally wrong for numerous reasons like additional expense wasted and a model that is (potentially) more difficult to fly. how many times have you heard a flyer complain his model 'wont slow down' for landing?? often this is caused by over powering as well as factors like the wrong propeller choice (too much pitch). It is often claimed a model will fly better with more power, this is not always the case. It should also be noted that most kit manufactures over spec their engine recommendations so that the model is guaranteed to fly. As I commented on another post, if (for example) a 60inch spitfire quotes 70-90 4 stroke then 70 will be plenty for scale flying, 80 gives a reserve if you get into bother, and 90+ is for toy aeroplane performance. Yes you can use the throttle, but how many people actually do?

So, in answer to your question I would buy a cheap Chinese 36 two stroke (ASP SC etc) and use it for the trainer, then buy another engine later. I suggest the 36 as its easily a match for an old 40 and yet is not much larger/heavier than an old 25 so you should not suffer other problems with c/g or fit in the bearers etc.

Finally the idea of 'dead weight' in lead never made sense to me. If you need a power of 10 (units not important) to fly the model, and your engine plus lead gives that and it flys well, adding an engine of power 15 with no lead makes no difference to the way the model flies unless you go faster that you did before. so the bigger and heavier engine is no improvement, you still have weight that is 'dead' and an engine is a lot dearer than lead

Edited By Jon Harper on 06/05/2014 14:45:22

[john stones 1 - Moderator]

I agree with Stu K

In my experience far more models suffer from being under powered than overpowered (the dreaded stall).

The idea of buying 2 cheaper engines rather than the better quality one, amounts to much the same costs. The larger sized one will be useful in a larger variety of future models, so to me is the best investment. Also the engine could be propped with less pitch to aid landings.

Chose your own brand but buy the best you can.

John

[cymaz]

Sc 52 or OS 46 ax would suit. The engine would do for the next plane after the trainer, depending on what you choose of course.

Edited By cymaz on 06/05/2014 16:11:27

[Jon H]

Stalling is related to angle of attack/boundary layer separation and has nothing to do with engine power or speed. this why CAP232's etc tend to be flicky recovering from loops etc as the tips will stall flicking the model over even if the airspeed is high.

As for the merits of cheap engines vs better quality ones then of course something higher quality will be better in the long term but ASP 36's are very cheap and you could buy at least 2 for the price of one OS.

I agree that you want to try and get an engine to fit a series of models, but in the event that it is not possible to do that as it would compromise one model or another then buying something cheap and cheerful is perfectly fine. I had an asp 36 for over 8 years before it finally died. Also given that this is a trainer it might have an argument with mother earth at some point so a cheaper more disposable engine might be a good idea?

If we were talking about a lovely 1/4 scale tiger moth or a really nice model then sure, go for the best you can get (I would use a laser 180 in my tiger moth example), but for a trainer its about doing the job.

Some more details about the model might help, if its the size/weight of a tutor 40 then a 46 is a great idea but it sounds smaller than that to me, hence my suggestion

[john stones 1 - Moderator]

Just my opinion mate, Underpowered models do lead pilots both inexperienced and experienced into a situation where a stall occurs. As it was a trainer being asked about I went with what I feel is the best option. Learning can be hard enough, I prefer a bit too much power and throttle back. I also believe 40 size models are the most common amongst club flyers

John

[Jon H]

Oh they are and I recommend a 46 to anyone with a 40 size trainer. But this model is saying 25-40 so I am guessing its about 50 inch not 60+ like the newer trainers and that makes a big difference.

One model I used when learning to fly was a telemaster 66 with a span of...yup, 66 inches and it flew very well on an ancient enya 29. I have since flown one on an Irvine 40 and it was awful as it was just too much power, escpecially for my poor student as it kept running away. Granted the prop selection was not ideal but still, it was an issue and it eventually lead to the models end

Another plus for a model with a little less power (not underpowered) is that it will teach better technique for things like go arounds. If they can just punch it out of trouble that's not always a great thing. Given that an instructor should be on a buddy box at this point an accident is unlikely.

In any event, some more details of this specific model would be helpful

[Dai Fledermaus]

Thanks for you interest fellers. This is the design in question LINK

[crispin church]

i was just going to say what plane well done for link

what is the gap where the engine fits then go from there even better if built with everything in it put it on a cog stand and add weight where the engine would be to see how much the max weight the engine could be before you have to start thinking about moving things back

dont get a little engine then have to added weight to balance it out you mite even be able to fit a .60 which will be a good one for next plane and on wards

[iqon]

40 - 46

[Peter Miller]

Having looked at the model I would suggest an SC .32. I fly aerobatic models that size with the SC .32. More than enough power.

On a .40 to .46 it will climb straight out of sight vertically.

Yes, I know, you can throttle back. Only thing is, most people seem to fly with the throttle fully open and forget about the intermediate positions until they become experienced.

[Jon H]

Thanks Peter! At least someone agrees with me. The 32 would be more than enough power for sure but the 36 is in the same crankcase so presents no problem in that regard. Either engine would be perfectly fine. You could use a 10x6 for sport flying or an 11x5 for training. My engine (36) used to turn these propellers without any problem at all.

crispin a 60 is utter madness and would tear the model apart. There are many cases where a model will have more than enough power and still require lead in the nose. Most scale models are this way and something like a sopwith pup with a short nose will require lead. If you take the flair pup as an example it needs about 1lb of lead when powered by a 52 4 stroke. if you use the 'bigger engine' method you will need something like a 120 which will neither fit the model or power it correctly. Most likely it would pull the wings off.

The best policy is to power to the model sufficiently (with a small reserve) and then balance it as required. Just shoving a huge engine in the model is not a clever idea.

It should also be noted that full size aircraft such as the hurricane and spitfire (which we model often) have areas on the engine mounts and in the tails for lead to be added. Adding weight for balance is a standard practice

Edited By Jon Harper on 06/05/2014 19:40:34

[Peter Miller]

I always use a 10X 6 on my .32s,

The .36 would be just as good although I have never used one.

From some of the comments I am reminded of a club that I was in. They used to fly the old KK Super 60s on a .60 two stroke. Insanity but fun to watch when the wings snapped.

[Jon H]

Super 60?? they used to be overpowered with OS 48 4 strokes!

[Peter Miller]

I upset the secretary by doing aerobatics with my Super 60 on an early Saito 30!!!!

Edited By Peter Miller on 06/05/2014 19:45:10

[john stones 1 - Moderator]

Relax fella's

They where just different opinions and the comments came in one instance from a gentleman who flys mostly scale and is far from a boy racer. Just different views. myself I am 59 never had a wing fold Peter, I build and fly them right. I learned how to reading Boddo, Peter Miller etc. I learned to fly on a Ripmax trainer 60ish inch Enya 19

How about a nice fourstroke Colin

John

[John Muir]

Another vote for a .32 or .36, or possibly an OS 40LA which is roughly the same weight and power. A modern 40 with ball bearings will be considerably heavier than a K&B 28. You might have balance problems with a modern 40 and need lead in the tail. Also, modern engines are nearly all 'ABC' type engines and if your plane is overpowered to the extent you have to pootle around with the engine just a little above idle you'll find the engine will run too cool. When you actually need to throttle up it'll just cough and splutter. An engine that needs to run nearer the top end of its throttle range will stay hotter and will respond cleanly when you need it to. I'd say a 32/36 will provide ample aerobatic performance in that model later on as well and you won't need to upgrade. It's not as if the trainer you're looking at will go any faster with a big engine as it's got a thick wing section and loads of drag and that will limit the top speed anyway.

John.

[Jon H]

I don't only fly scale, My old ripmax Harmony was massively overpowered with a pair of ASP 52's. it as a hoot! but the airframe began showing signs of distress after a little while so I was forced to retire it

I have a similarly 'large' performance from a number of my current models, others (like my little nieuport) are powered quite sedately.

Also John has a valid point about the 40LA, I forgot about him! But they are dearer than the ASP's and (bizarrely) probably will not last as long due to the tendency for the liners to shed their plating after a while. OS have suffered badly with this for some years.

[AlexJ]

My first model was 50inch span, a Pilot QB15H (must be early 80's !), unsurprisingly it was powered by an OS 15 FP plain bearing cooker of an engine! The thing flew very well, it was quite lively, actually.

ATB

Alex

[Jon H]

The FP series were great, better than the LA in my experience

[john stones 1 - Moderator]

I would agree there jon, bit of a let down for OS the L.A engines

John

[Dai Fledermaus]

Thanks for your advice gentlemen as always it's very much appreciated.

Cols