Hi Rob
Im not as technical as you, but if any help, when we were test flying the Grob 109 with the 2400 Stemme engine, we had to carry out a pre STC approval series of flight tests. One was relating to engine and oil cooling. So we used a standard ASI with the pressure in the engine and the static behind the baffles. We were comparing the indicated airspeed with the “baffle one” The results we got were as follows 1. Using the original baffles the ASI at slow speed was around 5Kts faster at higher speeds (100kts) the two came together 2. When we had finished the new baffles, we were getting a 15 Kt difference at slow speed and again they came together at high speed. In practice it didn’t have a lot of effect on the CHT’s with the Grob. However a similar effort to make the baffles “as tight as a fish’s bum” on our Dimona had a really big effect on the CHT’s. We didn’t use the ASI test as were very happy with the results. I have recently installed a MGL 4 ch CHT (nice colours) which confirms our work. I hope that is some help Rob. Best regards Ian W From: [email protected] On Behalf Of Rob Thompson Sent: Monday, 20 July 2020 12:17 p.m. To: [email protected] Subject: [DOG mailing list] Cooling airflow and Mixture relationship G'day Everyone, As many of you will know I am OCD about sealing every tiny air leak in the Limbach engine bay compartment with silicone, aluminium tape and rubber cowl seals. I have previously rigged up a water manometer in the cockpit with the tubes running to the top and bottom of the engine. It told me I was getting very good pressure difference but I didn't take much notice of the actual measurements. I think it was somewhere around 30cm. As well as the ram air going into the front of the engine the lower cowl is designed to "suck". We fitted a big scoop type bottom cowl which seems to suck pretty well.... particularly with the baffles tightly sealed with silicon. What I am wondering now is..... What is the effect of this extra suction at the back of the engine on the carbs and mixture? Maybe it is not much different to flying at altitude and the SU carbs altitude compensating ability can deal with it. Any fluid dynamics physics experts in this Dog group who can enlighten me? regards Rob Rob Thompson 0429 493 828
