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Oil Cooler Diffuser

ChiefPilot

Well Known Member
A while back, scsmith shared the concept of a diffuser on the oil cooler to smooth the airflow and thus make the cooler more efficient (thread here). I liked the idea, and decided to give it a go except with aluminum. I sized it such that the outlet is approximately 1.5x the inlet, and this is the result :


48104194941_925977a39a_z.jpg

48104312127_f9363eb33e_z.jpg


I'd never really had issue with warm oil temps, except when doing sustained climbs at Vy to > 10,000' or when doing extended ...umm... formation maneuvering with another aircraft at high power settings and relatively low airspeeds. Both of those arenas are no longer an issue.

Objectively, flight testing comparing under conditions as close as possible, a full power Vy climb hit 230º oil temps well before hitting 10,000' while the same test got to 10,000' with temps at only 204º. There's more flight test info, but that was enough to convince me this was a good idea. Thanks Steve!
 
Great. I'm glad it works, and thanks for the useful data.

What this really does is increase the area at the exit of the flow path of the cooler, so that the exit pressure is set at a larger area. This means that at the smaller area where the cooler itself is, the pressure is lower and the velocity is higher i.e. more flow through the cooler. It is equivalent to putting the heat exchanger at the throat of a venturi.
 
A while back, scsmith shared the concept of a diffuser on the oil cooler to smooth the airflow and thus make the cooler more efficient (thread here). I liked the idea, and decided to give it a go except with aluminum. I sized it such that the outlet is approximately 1.5x the inlet, and this is the result :


48104194941_925977a39a_z.jpg

48104312127_f9363eb33e_z.jpg


I'd never really had issue with warm oil temps, except when doing sustained climbs at Vy to > 10,000' or when doing extended ...umm... formation maneuvering with another aircraft at high power settings and relatively low airspeeds. Both of those arenas are no longer an issue.

Objectively, flight testing comparing under conditions as close as possible, a full power Vy climb hit 230? oil temps well before hitting 10,000' while the same test got to 10,000' with temps at only 204?. There's more flight test info, but that was enough to convince me this was a good idea. Thanks Steve!

Just curious if you have done more testing and confirmed the positive effect?

I made one for me IO390 engine and have only one flight but it did not seem to make any difference. Of course it is hard to do an objective testing.
 
Just curious if you have done more testing and confirmed the positive effect?

I made one for me IO390 engine and have only one flight but it did not seem to make any difference. Of course it is hard to do an objective testing.


Nope, did not do additional testing beyond what I did initially as that testing confirmed that the diffuser objectively improved the situation. Tests included the climb as mentioned above, time to cool after reaching 235º, and a couple of others. All of them showed an obvious and statistically significant improvement vs. no diffuser.

Anecdotally, I used to have to call a knock-it-off during mock dogfights due to high oil temps but haven't done that since installing the diffuser. That's difficult to quantify since it's a dynamic environment with lots of high alpha/low speed/high power settings for varied amounts of time. But it definitely eliminates the oil temp issues during those kind of flights, so it's not coming off any time soon :)
 
Nope, did not do additional testing beyond what I did initially as that testing confirmed that the diffuser objectively improved the situation. Tests included the climb as mentioned above, time to cool after reaching 235º, and a couple of others. All of them showed an obvious and statistically significant improvement vs. no diffuser.

Anecdotally, I used to have to call a knock-it-off during mock dogfights due to high oil temps but haven't done that since installing the diffuser. That's difficult to quantify since it's a dynamic environment with lots of high alpha/low speed/high power settings for varied amounts of time. But it definitely eliminates the oil temp issues during those kind of flights, so it's not coming off any time soon :)

Thank you for responding.
I made a similar diffuser with fiberglass and had hoped to get as positive of a result but it was no change to the extend I could tell so I end up removing it.
 
I have the opposite problem...even in the hot summer in AL I have tape covering my oil cooler to get oil temp up to 180
 
I assume you have (I)O-360 engine with your cooler mounted behind #4 cylinder. My RV7A also ran similarly, in the heat of N. CA, 180 would be the max oil temp I would see but the 390s run much hotter on oil temp but cooler on CHT.
 
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