RDJeff

My Feedback: (11)

In the vast experience of this group, I'm hoping somebody can answer this question. How closely can a person cowl a diesel without it burning up the balsa? I'm finishing up a Brodak Thunderbird, and powering it with a dieselized O.S. FP-40, and am wondering how much room I need to allow around the cylinder and head. The cowl looks to have adequate cooling intake/exhaust area. Thanks!

djlyon

On FAI TRs where we used a solid cowl with an aerodynamic duct we came to a 1/16 of an inch or closer. With an aerodynamic duct we got a lot of air thru the fins on the cylinder and head.

Denis

Recycled Flyer

Ithink that a stunt tuned diesel won't reach wood burn temperatures unless perhaps it is in intimate contact with the exhaust outlet - but even then Idoubt it considering the 'wet' environmet' around most diesels.

If you can see past the outside of the fins it should be alright in the air but idling for too long may cause issues.

pe reivers

I have had an engine cowl closed up a sliding fit to the fins. No wood burning, just excellent cooling.

greggles47

Steve Rothwell, the manufacturer of the mighty R250 (among others) has a very specific set of instructions given with engines. He states that cowl should be around a 0.5mm (.020") from the cooling fins and about 1mm from the head of his engines in a Team Race model.

The other spec is to have the cowl exhaust about 1.5 to 3 times the inlet area.

See here http://www.smrpl.com.au/page4/page12/page12.html

I can see no reason for changing this for a stunt model. The basic aim of Steve's approach is to force cool air through the engines fins, and a slower model will also benefit from having the air forced to work in the same way.

I wouldn't think that having any part of the cowl touching the cooling area of the engine would be a good idea, and unlike Chris, I believe that scorching the wood would indeed happen from any contact.

Good luck.

Greg

Recycled Flyer

Hi Greg,
the only reason that I can think of to use more seperation gap between the cowl and the engine in a stunt model is that the cowl is seperate from the nacelle, whereas on a Team racer it is one piece with it.

0.5mm clearance is fine if you have no movement in the airframe but if you do (as in with a removable cowling) I would want a bit more to stop inadvertant rubbing.

Forced cooling is not such an issue with rich mixtures and higher oil contents typically found in stunt engines.

Cheers.

P.S. We are only refering to a dieselised OS 40 FP running at the most 9000 rpm in a stunt model here - not a hot under the collar combat or team race engine doing at least 16000rpm.

greggles47

OK Mate, build one - fly it and let me know!




Greg

RDJeff

My Feedback: (11)

I was hoping for a little more than 9000 rpm, maybe 10,000. Anyway, this model was designed for a Fox .35, a fairly small engine, so I had to completely rebuild the cowl, and was wondering how much clearance I needed around the engine so I wouldn't bake the wood or blister the paint. The top of the cowl was resting on the cylinder head, so I wound up having to relieve a lot of the wood in that spot.

Recycled Flyer

Hi Greg,
if its any help here I have run my dieselised MVVS 40 with the boost port plugged by a soft piece of pine and then without.
The inspected piece of wood was of course oil soaked but not what I would call scorched.

Bear in mind though the temperature difference between the outside of a head and the outside of the lower crankcase can vary as much as 200ºC you can well see that where you consider contact to be is very determining.

In my Mercury Monarch with a PAW 19 from my youth I did spy a certain 'blackness' on the parts of the cowl that did come into contact with the head of the engine but concluded that it was a combination of carbon from the exhaust mixed with fine alloy particles worn from the head and quite stained your fingers upon touch.
Once the blackness was cleaned away the paint surfcae underneath was lightly and permanently stained but the actual underlying wood was fine.

The paint or surface finish is the real issue here, as Jeff states, and the 9000 rpm with your stunt diesel was a bit of a stab in the dark I must admit.
But do you really want 10 000 on the clock with a diesel? (I thought that aiming for 9500 with my PAW 40 was pushing it and a few have commented that it is far better aiming for less.)

Anyway Greg, you are right, build it and all will be reveled!

greggles47

Chris,

Your piece of pine was on the inlet side, inside the case? Correct? If so you would expect the temperature to be a lot lower than the exhaust side. The reverse would be disastrous for the running of the engine.

The black stuff you mentioned was probably burnt, as the balsa/ply contacting the engine would have been insulating it from any cooling, & Id guess it'd get hot enough to scorch.

I wouldn't expect a well built modern model to have a cowl that would move by more than .5mm, In fact it should be almost as immovable as one built into the fuse as per TR models. Apart from any thing else the sound of a vibrating cowl would drive me to distraction.

Generally I think a motor stuck out in the breeze would receive the most cooling air, and yet I never hear of diesel being over cooled, so how could a close fitting cowl which would achieve maximum cooling be a bad thing?

TO RD,

Please excuse our bickering your original question seems to have been answered, If I were in your situation I would aim for clearance somewhere between .5 to 1mm all around, and maybe have the head exposed to the air outside the cowl. Certainly have no points touching the engine.

Good Luck

Greg

RDJeff

My Feedback: (11)

Thanks everyone for the input. I only have tight clearance around the head, and will be sure to allow some space there. Seems that would be the hottest part of the engine anyway.