PRME Engines User Community

[preivers]

Part one: preamble.
Many model airplane pilots move to gas engines after they gained first experience with glow engines. So the engine is fitted up against the engine mounting pod or firewall, and the cowl then is fitted around it. For looks, air intakes are provided just like the original 100% plane, and if possible nothing should stick out of the cowl and no large openings are allowed. Due to limited space, or the desire to not alter a fuselage to accept the large two stroke muffler systems, it is often decided to trade engine performance for best looks, and install a compact in-cowl muffler.

In general, and for ease of reasoning, it can be assumed that in a two stroke internal combustion engine about twice the amount of power that is delivered to the propeller is generated as waste heat that must be removed by the cooling system. (cooling is ~22% of total energy in fuel, power is only ~12%)
Glow engines have much internal cooling due to the high latent heat of evaporation of the methanol fuel. Gasoline is about 1/10th of that amount, so there is little internal cooling. Consequently gas engines run hotter, and more heat must be removed by the cooling air. Compact muffler systems worsen the situation, in that they increase the cylinder temperature.
The power an engine can generate is limited by amount of heat that can be removed with an air cooled system, which in turn is limited by the physical size of the cooling fins and the amount of air flowing through those fins. Air that flows around the engine does not contribute to cooling. That is why most high performance two stroke engines in motorcycles now are water cooled.
Even the best of engines will fail or break down prematurely if allowed to run too hot, and model airplane air cooled engines are very easy to abuse in this respect since they lack a blower and shrouds to force the required amount of fresh air through the engine cooling fins.

As a general accepted rule, modelers *should* use an air outlet area at least twice as large as the air inlet. This air outlet should be located in a zone of low air pressure and assist in extracting cooling air. The air flow inside the cowl however tends to take the path of least resistance, which is around, and not through the cooling fins. Unless the cylinder is directly hit by the free air stream, and ample air exits are present in a low pressure zone of the cowl, the only way to get anywhere near sufficient cooling is by using a system that forces the air to flow through the engine cooling fins.
Such an air guiding system is known as baffling and shrouding. With baffling, however simple, air can only flow through the engine cooling fins, and is not free to take the path of least resistance around the engine. There still is a need to have sufficient moving air through the engine, but the baffling sees to it that all available air is used to best advantage. This will be explained later.

Very few general rules apply for good cowl design:
1) There is an air entry into the cowl to allow cooling air in. The entry is located in a high pressure zone
2) There is an air exit slot to allow the hot air out into the main air stream. This exit, which must be at least twice as large as the inlet, is located in a low pressure zone, and it's shape should allow the extracted air to merge with the main air flow.
3) The cowl shape should allow for a smooth guiding of the main air stream.
4) The complement of a good cowl design is a good baffle and cylinder shrouding (jacket) design, such that ALL air entering the cowl will pass through the engine cooling fins.

Many model airplane designers are not aware of the need for a good cowl design. All too often cowls are tight fitting around the fuselage without any air exit, and it is up to the modeler to do what he thinks is right. The modeller in turn tends to rely on the plane designer’s expertise.
To my current knowledge, no systems are offered that provide any help in solving the air cooled engine’s cooling problem. The general thought about this is that in a moving plane there is so much cooling air flow available, that the engine will not get in distress. However, there are flight situations that have a very low forward speed, or even no air speed at all, like in verticals, and 3D flight with prop hanging and torque rolls.

Interesting links:

Troy built models about cooling: http://www.troybuiltmodels.com/ns/learn/engine-air-cooling-easy-how-to/

pressure in front of the cowl:
http://naca.central.cranfield.ac.uk/reports/1938/naca-tn-673.pdf

Total pressure difference over the baffles:
http://naca.central.cranfield.ac.uk/reports/1941/naca-report-720.pdf

other interesting stuff
http://www.rcshowcase.com./html/faq.html#How%20Important%20is%20cooling?
http://www.liquidcooledairpower.com/lc-longertbo.shtml
http://www.liquidcooledairpower.com/lc-topendheat.shtml
http://naca.central.cranfield.ac.uk/reports/1929/naca-tn-328.pdf
http://naca.central.cranfield.ac.uk/reports/1938/naca-report-612.pdf
http://www.pilotfriend.com/aero_engines/images/j5.jpg
http://www.tpub.com/content/construction/14264/css/14264_213.htm
http://www.lnengineering.com/pinz.html
(the naca reports have moved, but are still available at http://naca.central.cranfield.ac.uk/
Quote from http://www.allstar.fiu.edu/AERO/Propulsion1.htm

[preivers]

part two: The cooling airflow.
According to NACA researches between 1920 and 1940, the air in the front of a cowl is almost stagnant, albeit very turbulent. Propeller action is of hardly any influence, except for creating extra pulsations.
It is this turbulence that scrubs the fins of the engine so to speak, and in doing so takes care of cooling of the exposed engine front side.
The cooling airflow is set up by a pressure difference between the front, and the rear of the engine. The cooling available is dependant on this pressure difference. Remember that the required cooling depends on the engine power. To quote precisely: Cooling=Power^0.8 for 4-strokes, and 1.8x power for 2strokes.
At low airspeeds, there is no high pressure zone in front of the engine, so all flow depends on the cowl exit being located in a low pressure zone which extracts the air from the cowl. This low pressure can only be achieved by the propellor airflow on the cowl outside. A small air deflector in front of an air exit opening will help very much in creating a low pressure zone that extracts air from the cowl. For best flow, this extracted air should merge with the free air flow witout any, or with the least possible direction change.
At a certain mass of flow through the engine, and a given air temperature rise trough the fins, the amount of heat that is taken away from the engine is fixed. It can only be improved by optimizing the airflow through the engine fins, and changing the pressure difference between cowl entrance, and cowl exit.

This picture shows good air exit example. Yje large louvres in front extract the engine cooling air, The small louvres in the rear fuselage extract the air from the canister tunnel.


Possible system(s). I use my tow plane as an example, because the 35cc MVVS engine has quite small cooling fins, yet must pull very hard when towing a glider. This means a reduced airspeed, whilst maximum power is asked for. I can hardly think of a better test platform. The cylinder is very exposed to the free airstream, even with cowl fitted. Without baffling, power sags slightly when the engine gets hot. With the added baffle, power is constant without any signs of sagging

engine without baffles:


Baffle lay-out test:


Lower baffle:


Assembled, and airstream views:
Upper baffle, side view


Lower baffle, side view


Lower baffle, air entry


Upper baffle, air entry


Rear air exit


The rear exit of the air can be improved to avoid stagnant air. Now the cooling air is forced to flow around the cylinder, and exit in the hot exhaust port region, where it is pushed out . I will try to showthe differences.
new blank


new blank shaped


Test fitted, front view


Test fitted, rear view


total weight of the 1mm alu sheet metal baffles is only 75 grams. Of course that could be improved upon. The system in part 4 only weighs 30 grams, and even that can be further reduced.
Further improvement would be possible by reducing the intake area, and carefully shaping the intake duct to reduce airflow speed. This would result in increased air density, and better cooling at lower losses, as NACA reports on radial engines have shown. Since the system as shown gives good results, no such plans are made at the moment. Speed racing model planes however are normally equipped with such a system.

Flat-four baffling: An elementary way to do things: a simple airdam baffle which prevents air from bypassing the engine.



More examples to check out:
3W comments on engine temperatures:
Operating temperature
The normal operating temperature range for your engine is 176-212 Fahrenheit (80-100 Celsius).
If the temperature of the engine rises above 230 degrees Fahrenheit the engine is in an overheated
condition where sealed ball bearings and needle bearings could be damaged. Further, you risk
an elongation of the heated engine parts, and / or having an oil film build up, which could lead to
a seized piston or pistons. Following the instructions and recommendations in this manual will
keep the engine operating comfortably within the normal temperature range.
Baffling
Deflecting of the air (baffling) to and over the cylinder(s) is highly recommended for engine cooling.
The idea is to get all of the cool air that is coming through the air intake opening(s) to hit the middle
of the cylinder(s) directly, and then be forced over the cylinder(s), creating turbulent air moving
through the cylinder(s) fins. The freely flowing, but directed and turbulent air between the fins
provides the maximum cooling for an air cooled engine.
http://www.3w-modellmotoren.com/mounting_tips/details/Mustang_121.htm
http://www.3w-modellmotoren.com/mounting_tips/details/Mustang_120.htm
http://www.3w-modellmotoren.com/mounting_tips/details/Mustang_216.htm
_________________
PRME - Pe Reivers Model Engines
Dealer for MVVS, Mejzlik, MTW
http://www.mvvs.nl
http://www.prme.nl
http://www.mvvs-nl.com

[preivers]

Part three: more pictures:















_________________
PRME - Pe Reivers Model Engines
Dealer for MVVS, Mejzlik, MTW
http://www.mvvs.nl
http://www.prme.nl
http://www.mvvs-nl.com

[pe reivers]

part-4: making a simple baffle of corrugated cardboard or Depron.
step 1
draw the baffle outline and the engine outline on the cardboard. Keep a about 5mm space around the engine. Notice the tabs, that are later bent to force the flow to stay between the fins.
step 2
cut tab bend lines, and bend the tabs to loosely fit the cylinder. Secure the folding lines as needed.
step 3
fit the baffle outline as required for exhaust manifold etc.
step 4
fit the baffle in the cowl or to the firewall. If fitted to the latter, at may make a neat extra cowl support.
Time to make: 30 minutes
weight: 30 grams (corrugated heavy duty cardboard)
Step 5
Make a device for air extraction behind the baffle. Louvres are very effective, so are air deflectors. Just an opening in the cowl can be very counterproductive, and may not work at all. See http://www.prme.nl/forum/viewtopic.php?t=148



_________________
PRME - Pe Reivers Model Engines
Dealer for MVVS, Mejzlik, MTW
http://www.mvvs.nl
http://www.prme.nl
http://www.mvvs-nl.com