RC Aircraft Proving Grounds - BME Engine Baffling

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Baffles
by: Alan Walters

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I thought I would report back with my experience with baffling.

Engine is BME 110, just back from service. Now has 10 flights. Prop MSC 26x10. Airframe is H9 Extra 330, about 26.5 lbs. Still on LB ashless at 32:1. Running very rich, with burbles on mid range but comes on smooth at full throttle up-lines.

My first attempt, I just installed a straight piece of balsa inside the front of the cowl up to the cooling fins. Installed onboard temp gauge without memory feature, with the sensor in between the next to last fin and last fin next to spark plug. Ground temperatures taken after landing with the onboard temp gauge, were 235°-266°. Ambient air temperature was 55 degrees. Altitude approximately 450 feet ASL. Relative Humidity ~50%

I installed additional baffles on the sides and back of the engine so that now there is very little bypass space except over the fins. Kind of like Chip Hyde’s 4 cylinder DA 200 Ultimate Bi-Plane. At least I think it is Chip's, but not quite as elegant, albeit functional.

I installed a new/different onboard temp gauge that has a memory feature for recording highest and lowest temps so I could get the actual maximum in-flight' temperatures. I placed the pickup in the same place on the engine as the previous flights with the partial baffling.

Highest temps recorded were in the air at 204°, 209° and 207° while flying the 2007 Advanced IMAC sequence. By the time I taxied back to the pits, the running temps were down in the 165°-175° range a surprising 18-20% lower than the 'in air' temps. Ambient air temp was 68°. Altitude approximately 450 ASL. Relative Humidity ~50%

(I do not know the math formula for cooling/heat dissipation rates and so my calculated, in air, temperature assumptions are based on linear progressions derived form the maximum and minimum air temperature reading of the second series of test flights)

What I can extrapolate from this experiment:

1. The highest temps are in the air, so reading temperatures on the ground is not reliable data for determining maximum in air temperatures.

2. Although my first series of flights did not use a temp gauge with memory, I assume that these in-flight temps were in excess of the 266 deg. F max on ground temp, perhaps by as much as 20% (based on the second set of flights max temp and ground temps.) This could calculate to be as much as 319 deg in flight. (1.2x266=319)

3. The difference between maximum temps with full baffling and maximum temp with partial baffling was 57 deg. and min temp variance was 31 deg. Worst/best case scenario is that all temps are down at least 26 deg. (235 best/coolest partial baffle vs 209 worst/hottest full baffle). The difference between coolest full baffle temperature and hottest partial baffle temperature is 62 deg. A 23% temperature reduction. If point #2 is a valid assumption, maximum temps could have been reduced by as much as 115 deg. or 35%. (calculated max in air temp 319 - 204 = 115)

4. The cooler, full baffle temperatures were obtained with the ambient air temperature 13 deg. higher than the previous day's flight tests, with partial baffling. Had the ambient temperature been the same the results could have been even better.

5. The total range of maximum temperatures with full baffling was very consistent at only 5 degrees variation, (204-209 in air), while the range of temps with partial baffling was 31 deg. (235-266 on the ground).

6. Baffling WORKS. More baffling WORKS 'more better'

Alan

The following is from a RCU member (Albatross) in Mexico City, Mexico
This plot is from a data logger I installed on my Edge H9 33%.
Some of you will find this information useful, at least those BME 110 engines.
The readings on the ground are clearly a high temperature trend, look at those cylinder heads temperatures rising up 260 F.
Within a few minutes of flying the engine cylinder temperatures are already down to 170's.F
Note at the end of the flying session the temperatures are gradually rising up to 210's F.

This is a "winter flying" in Mexico, comfortable 70 F @ 5750 feet above sea level.

What did I learn?
- The time invested on custom building the engine baffles is worth it.

- After landing the cylinder temperatures are never going to represent the actual, in flight, cylinder temperature. You need a data logger to know the efficiency of your baffle system.

- Now that I know the baffles will cool down this engine, I can work the needles to obtain the best performance from this engine, eliminating the heat factor thus minimizing engine damage and dead stick landings.

Note: The graph shows with great detail the progress of both cylinder temperatures.

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