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AndyD23/07/2016 20:40:05
718 forum posts
503 photos

014.jpg013.jpg012.jpg011.jpg010.jpg009.jpgI need Help,yes i know ive been told many times before,now thats out the way the problem is with my small B/H Chipmunk,i thought i try a new thread to reach more people.007.jpg

the poor ole thing wont fly with the cowl on without over heating,its running a asp 90 4 stroke with a petrol conversion,it great little flyer without the cowl but with it on i can start it an possably get one cicuit before over heating ,as for today went deadstick and ended up in the cornfield.

ive ducted the inlet with a good 4-1 exit and also lipped the front of the exit as well,im after ideas of what to try next,i dont want to give up on it as the sound is glorious on a slow flyby.

Peter Miller23/07/2016 21:12:47
10748 forum posts
1259 photos
10 articles

Try running as a glow. If it works OK then leave it that way.

Ask Just Engines for advice. They are pretty clued up on Petrol.

I do know from my one brief experience of a petrol 61 that they do run a lot hotter than glows.

Biggles' Elder Brother - Moderator23/07/2016 21:18:08
15748 forum posts
1460 photos

Andy - I have moved this to a more appropriate section of the forum - your Chippie, whilst pretty, will never pass as a Ballerina!


Former Member23/07/2016 21:34:25
3578 forum posts

[This posting has been removed]

AndyD23/07/2016 22:12:05
718 forum posts
503 photos

thx everyone

Chris Bott - Moderator23/07/2016 23:25:57
6724 forum posts
1392 photos
1 articles
Andy, even with the lip, could that lovely big exit hole actually be scooping air in to the cowl rather than letting it out?
I'm not sure what to suggest as a cure but wondered if that might be the problem?
Gary Manuel23/07/2016 23:44:04
2190 forum posts
1540 photos


Try adding ply baffles to the cowl around the inlet - channeling the air towards the engine, so that the only path for the air entering the inlet is across the cylinder head cooling fins. The hot air should then naturally find it's way out of the outlet.

I'm guessing that most of the air entering the inlet is finding it's way straight through the cowl without passing over the engine fins.

Baffles should be as close a fit to the engine as possible.

Something like this - without the baffles, the air would pass to the right of the cooling fins:


Edit - just looked closer at your photos, but can't quite make out what the black bits are - looks like you may have already tried this.


Edited By Gary Manuel on 23/07/2016 23:57:07

AndyD24/07/2016 00:01:34
718 forum posts
503 photos

hi Gary the engine is actually baffled but thinking more about it it seems to be more of a heat problem in the crankcase as even the spinner backplate gets hot,would redoing the baffle so it includes the whole engine be a good idea or is that defeating the baffle idea,im going to recheck the timing as well to see if its still spot on and not moved.

Thx Chris something else to try,i shall take some duct tape with me next time and try different size openings.

Ultymate24/07/2016 06:08:18
1701 forum posts
62 photos

I suspect your problem is that "glow" engines generally have less cooling fin area than pucker petrol engines as glow fuel has better cooling properties than petrol. You may therefore be between a rock and a hard place with this issue. sad

John F24/07/2016 07:59:43
1316 forum posts
51 photos

As Chris mentioned that huge hole underneath might be why there's not enough airflow over the engine; air coming in through that cowling bomb bay would negate any being pushed through at the front.

As you say try adding duct tape to cover 3/4 of that exit hole and see if cooling improves.

Mark a24/07/2016 09:49:05
321 forum posts
3 photos

Just bought one of these from Cosford last week. Going to use a Saito 82 fs so interested in fitting baffles in the cowl to cool that if needed.


AndyD24/07/2016 16:01:21
718 forum posts
503 photos

ive rechecked ther timing,spot on,and spent an hour tinkering in the garage with needle settings but they all seem about as good as i can get them,but cant do much more today without upsetting the neighbours,looks like it will be going electric,

electric..looks good cowl on no sticky out bits.


petrol..bad looks with no cowl but sounds fantastic.

Simon Chaddock24/07/2016 16:49:09
5616 forum posts
2982 photos

I suspect part of the problem is the front hole in your cowling is on the wrong side of the centre line.

Air leaves a prop with a 'corkscrew' motion so with your 'conventional' prop the intake is being partly baffled but then of course the full size Gipsy engine goes the other way round! wink 2

AndyD24/07/2016 17:47:25
718 forum posts
503 photos

i did think about just having sides and a top to the cowl but to totally destroy the cowl and its just the same put me off the idea.when fiddling this afternoon it still seems to be more the crankcase getting hotter and not the cylinder even with the cowl off,hot or not its so responsive so will keep trying for now,is there any way you could put some sort of heat sink on the engine.

Terry Walters24/07/2016 18:03:32
1829 forum posts
1068 photos

Andy - if the crankcase is getting so hot could it be that you have a problem with the bearings or crankshaft end float? i.e. mechanical heat being generated rather than the product of combustion?


McG 696924/07/2016 19:09:57
3021 forum posts
1143 photos


I really don't know anything about glow, petrol, 2-stroke or 4, nitro 5 or 10%... well, not at least in 'model' size.

... but if your crankcase is getting to 'hot' even without your cowl, from an 'logical' (autistic?) point of vue, I'm thinking of a kind of mechanical disturbance instead of a purely 'combustion heat' origin. Terry might be right regarding bearings, conrod play, axial tolerance of the crankshaft, ... ?

... Simon could have 'seized' part of your problem as well with the contra-rotating prop. But then that should be the case with other similar kits if anybody has made the same conversion to petrol... ?

... IMHO, your 'bomb bay' opening isn't that good as it is too much 'forward' and could (will?) accept some 'incoming' air flow. The best is to have it as far as back possible & possibly at the back end of the cowl, like creating some 'cheek' at the bottom end of the cowl. By the way, does your cowl has the same 'lateral cheeks' as the full size? Or is the cowl 'flat' against the fuselage on the sides?

Sorry, but I wished I could be of more assistance here. blush

Hakuna matata


BRU - BE / CTR Heat Control

kevin b24/07/2016 20:06:59
1776 forum posts
140 photos

It certainly looks like the cowl is tight up against the fuselage at the rear. Could that be the problem ?

I had an enquiry about an Airsail cowl from someone who said he had tried several manufacturers and theirs were all too big. After explaining how they fit I think he was ok. Didn't buy a cowl though. sad

AndyD24/07/2016 21:30:46
718 forum posts
503 photos

thx guys,i think i will redo the thinking make the inlet a fraction larger and moving the exit back and smaller,3-1 and maybe try and redsign the baffle/ducting as well,

im tempted to have alook in the crankcase but not the best mechanic or know what im looking for,ive also emailed just engines as well for any help,not sure how hot the crankcase should get running on petrol but you sure dont want to touch it,or even the prop nuts.trouble is i want to do a 120 but im not so sure now as that would be going in a simular cowl.

thx again everyone

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