Shaunie Posted March 19, 2017 Share Posted March 19, 2017 So when it comes to flying surfaces we all know about boundary layer effects, to a greater or lesser degree of course. This layer can be a couple of mm thick I believe . So logic would say that with many of our model engines the fins are often only one or two mm apart. So in this case do we get much airflow between them at all? Is it simply that the airflow is already very turbulent due to the prop, cowling etc. Expecting BEB along at any moment to correct my thinking. Shaunie. Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted March 19, 2017 Share Posted March 19, 2017 Brilliant question Shaunie with correct logic to avoid still air around the motor, and that boundary layer is involved, as in any fluid. The boundary layer tends to stick at the surface, then in layers, pass over itself, but is neither a specific thickness in mm, and is influenced by movement, and available passage, temperature and air pressure. BEB will be along to straighten us out Quote Link to comment Share on other sites More sharing options...
PatMc Posted March 19, 2017 Share Posted March 19, 2017 Obviously the fins will be increasing the surface area. I'd have thought that even if there's only a minimal amount of air actualy moving through the fins any boundary layer would be cooled by the air passing over it. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted March 19, 2017 Share Posted March 19, 2017 Thinking out loud... Heat will be radiated to some degree and air close to the fins warmed by convection from the hot surface. Will this air expand into the flowing layer hence carrying heat away from the engine? On a wing, the surface is in fairly smooth airflow whereas the air around an engine will be very turbulent. Quote Link to comment Share on other sites More sharing options...
Shaunie Posted March 19, 2017 Author Share Posted March 19, 2017 What made me think about this is that many horticultural derived engines have a relatively small number of far apart fins whilst specifically designed model engines can sometimes have very fine fins. No doubt in my mind that fine fins look nicer and give more surface area but do they really cool any better? My suspicion that turbulence helps to break up the boundary layer seems to be right then. Shaunie. Quote Link to comment Share on other sites More sharing options...
PatMc Posted March 19, 2017 Share Posted March 19, 2017 I don't think breaking up the boundary layer really matters. The moving air that's in contact with it will be constantly cooling the BL by conduction. Quote Link to comment Share on other sites More sharing options...
OZ e flyer Posted March 20, 2017 Share Posted March 20, 2017 It also depends on the atmospheric conditions around the fins. Coming from an air conditioning trade background (which is different but the same general physics) heat exchange occurs dependant on surface area as well as the time spent in contact with the air exchanging the heat plus the difference in temp between air moving over the fin and the fin. Heat is exchanged from the fin to the air which causes the air to expand this helps minimise the boundary layer as it is "blown away" by the air moving past. Increase the airflow by placing the fins in an induced air stream and the heat exchange is more efficient. Increase the surface area (more fins) and you increase the release of energy from the block. If the fins are in a fairly static environment then the warm air is replaced only by the convection of the warm air rising and the boundary layer clings to the warm fins in its own little eddy currents of heating and cooling air. Lower the temperature of the air around the fins and the energy of the warm air is quickly absorbed by the low energy of the cool air and rejected making the boundary layer thinner. The hardest part of air conditioning for me to get my head around to begin with is that you can't add cold you can only reject heat(energy). Once you work that out it's easier to work out how fridges, air conditioners, radiators and almost any other heat exchange unit works. Sorry if that has bored you silly. It's a really simplified explanation but maybe a little long winded. TP Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted March 20, 2017 Share Posted March 20, 2017 Petrol engines have more heat to disperse, hence the deeper fins (I'd have said they have more surface area than the closely spaced but not very deep fins on a typical glow) and of course, chainsaws don't benefit from flying through the air (hopefully!) with the consequent stream of high speed airflow and a whirling propeller just in front. Talking of BEB - has anyone heard from the old boy (he must be old as his younger brother was a Camel squadron commander during WW1!) lately? I haven't seen any posts from him for a week or more...hope he's keeping well... Edited By Martin Harris on 20/03/2017 00:47:18 Quote Link to comment Share on other sites More sharing options...
onetenor Posted March 20, 2017 Share Posted March 20, 2017 Surely the fins acting on the airflow would create turbulence Quote Link to comment Share on other sites More sharing options...
cymaz Posted March 20, 2017 Share Posted March 20, 2017 Engine finning is a dark art. Too far apart and there is not enough surface area cooling. Too close and the air won't get into them. The fin must also taper to the edge to help draw the heat from the cylinder Quote Link to comment Share on other sites More sharing options...
Peter Miller Posted March 20, 2017 Share Posted March 20, 2017 This is why it is a good idea to baffle the cylinder to force the air flow through the fins. Look at speed model cowlings where the cowl is almost in contact with the fins. Quote Link to comment Share on other sites More sharing options...
John Tee Posted March 20, 2017 Share Posted March 20, 2017 Excuse my ignorance of the tecnicalities being discussed (boundary layers etc.), but the air is in contact with the cylinder head for fractions of a second so won't be picking up much heat. Having said that the heat must go somewhere as the air flows across the head - confused!! Was thinking a stationary model would pass more heat but if it's running the prop wash is flowing over the head so the stuation is the same. john Quote Link to comment Share on other sites More sharing options...
Peter Miller Posted March 20, 2017 Share Posted March 20, 2017 Boundary layer is the layer of air in contact with a surface moving through the air. You can see the same effect when the breeze is blowing but the air close to the ground is not moving as fast. THis will also explain why a model that is being landed too near the stal will suddeny drop the last few inches. I must confess that I personally doubt if there would be much boundary layer in an engine in the full blast is a popeller but forcing the airflow through the fins is important. Go and look at any aircooled aero engine, especially radials and see how closely some are baffled. Not so much the big American radials sitting out in the open but look at a Bristol Hercules in a cowl. Just a thought. I am sure that you clean your models down but if, like me you leave a film of dust on the wings and go and fly it that dust does NOT get blown off in flight because it is in the boundary layer Quote Link to comment Share on other sites More sharing options...
Martyn K Posted March 20, 2017 Share Posted March 20, 2017 Airflow through a car radiator seems to work OK and that has a similar gap size.. Martyn Quote Link to comment Share on other sites More sharing options...
Peter Christy Posted March 20, 2017 Share Posted March 20, 2017 On a typical model engine, the most important part to cool is actually the cylinder head! This is much more important than the cylinder jacket, though of course that does need some cooling too! Look at some boat engines, where the water cooling jacket only surrounds the head - not even a fan for the cylinder! Also most heli engines, where either an extra large head, or a clamp-on heat-sink for the head is required..... -- Pete Quote Link to comment Share on other sites More sharing options...
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