to plenum or not

[pierre smith]

QUOTE...."optimize the inlet/outlet"....is the difficulty, not putting on the plenum. Most guys do not do the 'optimize the inlet/outlet' because they don't know how, but rather, put a plenum on like my buddy with his 300 HP -10.

There's been ongoing discussions on here about the "Shrinking exit" by Dan Horton and others....and more discusions on variable inlets/exits/cowl flaps and so on. Optimizing inlets and outlets gets into a whole 'nuther discussion, and yes, doing that properly will yield increased airspeed but it's a difficult science that's been discussed to death on here.

I stand by what I said, "If you simply put on a plenum, your speed won't increase."

Best,

[GusBiz]

This is the field of aerodynamics.

It is complex and unique to the situation you are facing for YOUR aircraft.

There is no product, applicance, method or rule base that when used and applied will work 100% of the time.

The reason for this is because aerodynamics is cumulative in its effects.

If something is slightly different 100 mms down stream it can invalidate everything else that is standard beyond that point and destroy the designed intention even 150 mm beyond that point.

Said in a simple manner....

Its like measurements, you build a set of shelves and if you measure from one support, to the next support, to the next, what ever error you are making on each support, adds up till later on, down stream, the whole thing is no longer level and looks like a child put it together, even if you were only .5 % out with every measurement. 10 supports later and your 5% out

You have to alter and test and measure and test and alter again and then measure again.

If that sounds tough, it gets worst. What you were doing in the first test could be perfectly correct if you had combined it with a different shape down stream and at a different velocity.

There is a reason the professionals spend millions of dollars on just making a shape.

Its tough work and I am not sure I am willing to do all the experimentation, still learning everyday about theory and experimentation techniques. All for what is a max 10 knot increase.

I now know why everyone says just build it lite, because if you don't its darn hard to increase performance any other way.

[penguin]

My vote is for no plenum. On my 6A the issue is getting the air out of the cowling. After several years of prevarication I fitted louvres to the bottom of the cowl 6 months ago and straight away saw upto a 40*F drop in CHT and a 10* drop in oil temperature. I think I could also close down the air inlets and go faster. I have worked hard on the baffling to ensure there are minimal air leaks - but these results say we need more exit area. I would suggest a plenum is wasted effort.

Pete

[Don]

Originally Posted by penguin

After several years of prevarication...
Pete

Uh, perhaps you meant procrastination or perhaps perseveration? And then again, maybe not. ;-)

[GusBiz]

Originally Posted by penguin

I have worked hard on the baffling to ensure there are minimal air leaks - but these results say we need more exit area. I would suggest a plenum is wasted effort.

Pete

This only true if cooling is your main concern.

If drag reduction is the issue then reducing the air intake is not the issue, it is the ratio of intake to exit. Believe it or not it is best to have larger intakes to smaller outlets to increase the velocity of the exiting air mass.

So you can either optimise for climb cooling and have big exits or optimise for cruise drag reduction and have small exits. Or you can have cowl flaps which is a WHOLE other post.

Luvers have been found by NASA to cool well but increase drag considerably. ie good climb temp reduction bad crusie idea, but hey if its what you want then it was exactly the right decision.

....I guess that was my point. You have to find your own secret sauce

[Vlad]

Hernan plenum is one of those things with little or no sense in tightly cowled RVs. There were good explanations before. I would say it's almost the same as full glass panel with triple redundancy in day VFR carbed airplane. It's more about tinkering and experimenting you will find many diehards on both sides. I did mine, enjoyed the process, installed a hundred nutplates . Was it needed? No. Would I do it again? Sure but better. Now I have "too efficient" cooling. Oil cooler completely blocked oil barely can reach green temp arc during winter flying.
Your choice you can not go wrong with any.