To Plenum, Or Not........

[Toobuilder]

Pulling air all the way around the fins on the head seems like the way to go based on my limited experience. My next engine installation will look a lot more like the air cooled VW and Porsche baffling than the typical "throw some air in the general direction" Lycoming installation. There's a lot of volume moving through an RV cowl, but not as much as should be going through the fins.

[airguy]

Originally Posted by N54SG

If I could offer my experience/advice.
#1 Be happy you have as much room in your RV cowl to build your plenum. I am jealous.
#2 Sure baffle seals work, but inefficient is an understatement.
#3 Cooler than normal CHT's is a bad thing. As said before, you're paying for that with speed.
I have 400 cubic inches cooled by less than 20 square inches of inlet area. Two 3.5" inlets. At race power (3,000 rpm) during the air races, I barely get over 400 CHT. Standard baffles could never do this.

Here is my current baffle/plenum I am running.





Read about it being built here:
http://www.n54sg.com/baffle-kit-diffusers-and-plenum/

This is bang on the money - we pay for cubic feet of air in the inlet (via drag) and then a lot of gets wasted by not putting it where it needs to be. Take every cubic foot you have and put it where it should be - and don't buy any cubic feet you don't need.

[Toobuilder]

Has anyone done any tests to determine how deep into the inlets it takes for the relative velocity to reach minimum? In other words, if the relative velocity is near cruise speed right at the inlet lip, how far down the pipe does it take to reach max pressure/min velocity?

If it is only an inch or two before it stuffs up, then the only critical part of the duct is that first part which manages the aerodynamic part - after that, you are dealing with a pressure vessel and the shape of the divergent duct should become less critical. I'm thinking specifically about the "6 degree maximum divergence rule" so often used when discussing divergent ducts.

[DanH]

Originally Posted by Toobuilder

Has anyone done any tests to determine how deep into the inlets it takes for the relative velocity to reach minimum? In other words, if the relative velocity is near cruise speed right at the inlet lip, how far down the pipe does it take to reach max pressure/min velocity?

If it is only an inch or two before it stuffs up, then the only critical part of the duct is that first part which manages the aerodynamic part - after that, you are dealing with a pressure vessel and the shape of the divergent duct should become less critical. I'm thinking specifically about the "6 degree maximum divergence rule" so often used when discussing divergent ducts.

The short answer is that the velocity reduction can take place before reaching the inlet plane (low Vi/Vo, large diameter), or well back inside the inlet, in the divergent duct (high Vi/Vo, small diameter). The shape of a duct (if any) inside a low Vi/Vo inlet isn't critical. The shape of the diffuser inside a high Vi/Vo inlet is apparently quite critical. Either can result in good inlet performance. In my own (so far unproven) opinion, the low Vi/Vo inlet probably works better at low speed and high AOA, while the high Vi/Vo inlet probably results in less total drag (including exterior form drag) at high speed.

Note the physical space requirement. Good high Vi/Vo inlets require space (length) aft of the opening for good diffusers, which pretty much dictates the need for a prop extension. See the Lancair photos posted above, or look at the nice shapes on Chris Z's diffusers:

http://www.n91cz.net/cooling/Cowl_report_4-12-2005.pdf

For my own airplane I wanted a 390 with a Hartzell BA, a vibration-surveyed combination with no restrictions. The big motor and heavy prop made a prop extension undesirable, thus the low Vi/Vo inlets, for which the ductwork (if any) can be quite stubby.

Consider the case of a midsize inlet with ducts connecting to the plenum....ducts with no particular diffuser shape, maybe some misalignment, maybe a sudden turn upward and volume expansion right at the cylinder face. Think it might not ring the bell for inlet performance, i.e. low conversion of Q to static?

[JonJay]

Can anyone comment on the differences between inlet styles and how angle of attack may affect their ability to cool.

[Toobuilder]

Originally Posted by DanH

The short answer is that the velocity reduction can take place before reaching the inlet plane (low Vi/Vo, large diameter), or well back inside the inlet, in the divergent duct (high Vi/Vo, small diameter)...

Makes sense.

Therefore, the most effective cooling in a standard length RV inlet would likely be achieved with tightly shrouded fins on the cylinders and head - with small openings only at the top and bottom. In other words, closely duplicate the typical "bottom" shrouding on the top as well. If there is little relative velocity and the upper cowl is essentially a pressure vessel, then the real cooling path is vertical, just like a VW bug. Shrouding would direct air down and around much more of the fin area than is typically seen. Should remove a lot more heat.

[N54SG]

The inlets I made for my cowling and plenum (posted above) are also divergent.

3.5" at the inlet, expanding to 4" over a 2.5" depth. Then 4" SCEET tube to connect to the main diffusers. A lower divergent angle would probably be better, but the space (or length in this situation) is limited, hence the prop extension.

[hydroguy2]

Originally Posted by RV10inOz

?.....but to be fair, you are operating at a different MO than the normal cruise. It would be interesting to see what you get operating at a DA of say 8-9K and WOT and 10-20LOP.
By the way your % power is wrong, some engine settings are not entered right. It should be showing about 79% power......

Ok David, I had a few minutes before work today and gathered some new numbers. This is a normal cruise set up for me, actually if I'm going somewhere I will be higher to clear the mountains. But for 8000DA this is about right. I don't know how LOP I am, as I just lean the ol fashioned way. Pull until rough and then smooth it out. My engine gets rough, then just seems to quit soon after.

WOT, 2450rpm, 9gph (when I'm at 11500 I'm <8gph)


I haven't looked into my Dynon %power settings yet, but think it is fairly close in the above pic, based on the formula you gave me.

[Wesael]

Originally Posted by hydroguy2

WOT, 2450rpm, 9gph (when I'm at 11500 I'm <8gph)
.

Some nice looking numbers right there.

[pierre smith]

Does the D-180 not have a "Lean" mode? With my D-100, push the "menu" button (second from right) and then "more" and you'll see a "Lean" button. Pushing this allows you to see your EGT climb to peak and then the big numbers disappear and a -15 or so. will appear, indicating degrees lean-of-peak.

To exit this mode, return to the "Lean" button and simply press it again.

Best,