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Engine power

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AlbertBallVC03/02/2016 14:33:19
20 forum posts
2 photos

I have recently read that on old .60 engine from the early 1970s would be matched for power by a good modern .46 engine. If this is the case could anyone tell me what might be the best size modern engine to put in the following aerobatic planes. A 1969 42" span Moonprobe, the recommended engine back then was a .19 - .23. A 1979 52" span Diamond, the recommended engine is a piped .40 and lastly a Flair Pulsar biplane from the 1970's with a recommended .60. The reason for asking is that a smaller engine of equivalent power would be easier to fit in some of the airframes.

AVC03/02/2016 14:44:08
539 forum posts
147 photos

In my opinion (and this is just my opinion), it's not all about the power but also about the torque. A powerful engine with low torque will make the model fly faster, and considering that these models must already be fast "by nature", maybe the last thing that you want is this.

I'm an IC modeler, but I believe that for the cases you mention, maybe modern electric setups would fit better...

MattyB03/02/2016 15:53:37
2011 forum posts
31 photos

Yep, you only have to look at the way electric now dominates F3A to see it's advantages in this sphere. A big prop (often a contra nowadays) swinging slowly gives you less (or zero) torque to manage, and these big models fly nice and slow giving you more time to fly manoeuvres accurately. If these models are yet to be built I would definitely say go electric, as you can lighten them up some and fit a nice 4S-6S powertrain. If they have already been built and flown under IC power it might be best to stick with that, especially given access for changing the batteries might be tricky.

Bob Cotsford03/02/2016 16:05:39
8230 forum posts
454 photos

My old designs all have the size of engine that they were designed for, or even a little larger say a 46 instead of a 40. Bigger props, less noise. Going back to '69 designs I think that you could drop an engine size for a modern unit, say .15-.19 instead of .19-.23. The more recent designs I'd just go for the modern equivalent without the pipe.

Don Fry03/02/2016 16:12:51
4557 forum posts
54 photos

Bob is right, anyway, putting a smaller engine in will only result in more lead, the C of G hasn't altered with time.

AlbertBallVC03/02/2016 18:36:33
20 forum posts
2 photos

Many thanks for the responses. I didn't really consider the electric power mainly because of the outlay of buying the motors, battery packs and charger and I already have some engines spare, but never the less they are both interesting ideas. I have got a spare SC15A that I thought of putting in the Moonprobe but wasn't sure if it would be OK. I'll take your advice and put an Irvine .40 in the Diamond, however would and Irvine 53 or Jen .57 be unsuitable to put in the Pulsar bipe, the other engine I have is an OS61FX.

jeff2wings03/02/2016 19:03:51
804 forum posts
1967 photos

I agree with Bob and Donald , the SC15 will fly the moonprobe ok but keep it light, although it would make a good candidate for lecky like Martyn K's flea-fly,the 53 or 57 will be a good match for the Pulsar but the 61 more fun !devil

This may be useful to you

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Nigel Armstrong03/02/2016 20:17:36
24 forum posts
13 photos

wow! I built a Diamond just before I left school in1979. It flew very well with a HP 40 up front. One point you should also consider in "new V old" engine debate is weight. Yes a modern 53 or 55 size engine will out pull an old 61 from way back when. However, you should keep the tail light or you will end up sticking half a church roof under the engine!!

Owdlad04/02/2016 11:49:38
222 forum posts
140 photos

Power is king! stick with the recommended engine size. Why carry lead for balance up front when that weight can be used to provide can always throttle back

Bob Cotsford04/02/2016 12:37:20
8230 forum posts
454 photos

One point to bear in mind is that older engines - especially those pre schneurle porting were often lighter (and a shade narrower in the crankcase) than the later models and would only have rudimentary silencers so a 55ish engine with silencer won't be far off.

But a true 60 will still turn a bigger prop more efficiently and can be made to run quieter as a package.

Martyn K04/02/2016 13:31:54
5041 forum posts
3677 photos

I have an SC15 and its a lovely engine. Need to keep the airframe light though or to repeat what has already been mentioned, you will end up adding lead.

On my Ballerina, I didn't set the engine position until the model CG could be accurately measured. On the moon probe, I would be inclined to go for a long engine mount and perhaps move the lighter engine forward slightly to get the CG correct - then finish it off. Edge on the side of nose heavy - you will need to add less weight at the tail to get the CG to go back rather than the proverbial church roof to get it to go forward.


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