to plenum or not

Dear Cummunity, Happy new Year all,

Im near the first flight and some things are in mi mind so I will apreciate your help.

I have a Brand new TMXIO-360, is there a reason to not put a plenum before the breake in finish?

If we dont want a lot of heat in this process, may be the plenum temperatures will add some glassing posibilities in our cylinders?

RE: Plenum ???

Hi

I did the plenum over my cylinders based on advice from Robbie Attaway and have found some interesting things.

Robbie Attaway (he is the best and really helped) info here:

http://www.attawayair.com/New RV6 QB.htm

Robbies Plenum:



My Plenum



First depending on how you build it ... (fiberglass or metal and shape)
Second the air flow under the lid
Third the ease of engine inspections.........

I built mine in metal and I believe (????) with out alot of pressure testing a solid fit to keep the air from leaking.
I am not sure about the airflow. I do know that if you have a baffle mounted oil cooler you will want a smooth flow of air to that location at least some folks smarter than me here at VAF have stated such..
Finally, it is a bit of a pain to install/uninstall/install/uninstall......during times when testing the engine.

So with all this said would I do it again. I just don't know for sure but I am a real believer now of thinking long and hard before you leave the path established by Van's. Build it like he and the company suggests and you just won't go wrong.

I hope others with more experience and info will weigh in and help you out in your quest to build the best airplane possibe.

HSANTIBANEZ said:

Dear Cummunity, Happy new Year all,

Im near the first flight and some things are in mi mind so I will apreciate your help.

I have a Brand new TMXIO-360, is there a reason to not put a plenum before the breake in finish?

If we don't want a lot of heat in this process, may be the plenum temperatures will add some glassing posibilities in our cylinders?

Click to expand...

Do baffles correctly.

Baffles really make servicing so much easier and there hasn't been a real conclusive speed benefit to a plenum if your baffles are done correctly. It's one area that should be carefully done, all openings sealed with no air loss anywhere. Pay close attention to the crosswise baffle behind the spinner, that it seals well and prevents air from coming out of the cowl/spinner gap. creating drag.

I had baffles on both my -6A and now my -10 and both airplanes exceed Van's cruise and top speed numbers by a few MPH.

Best,

i dont get plenums

Why would you build a top of a cooling air flow area just under a top of the already there cooling air flow area (the cowl top)? seems redundant.

plenum

If you want to race you need a plenum. If you don't care about speed might as well build a Pietenpol and enjoy the scenery. The performance improvement from a plenum has been well documented by San James, Dave Anders, Chris Zavatson and others.

Not necessarily.

A friend of mine has a -10 with 40 more horsepower than I do and a Sam James cowl and plenum and is 6 knots slower.

Too many other factors come into play.

Best,

I put one in for engine breakin. No problems and temps are all under 400.

billnaz said:

Why would you build a top of a cooling air flow area just under a top of the already there cooling air flow area (the cowl top)? seems redundant.

Click to expand...

I agree, building a box within a box doesn't make sense to me. Never mind the additional weight, complexity, and time spent taking the plenum on and off for inspections and service. A well thought out set of baffles for me thanks.

John Clark ATP, CFI
FAAST Team Representative
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA

I think the basic idea behind a plenum is to make the airflow more efficient so that you can reduce the size of the air inlets. This reduction in air inlet size will improve the aerodynamics of the plane and that is what results in an increase in speed.

Just adding a plenum does not make your plane faster, but it allows you to do things to your plane that will.

plenum

The typical homebuilt airplane sacrifices 5 to 30 m/h from the firewall foreward. There is no way that a conventional baffle system can be made as efficient as a plenum. The round inlet really serves just one prupose-the junction between nose cowl and plenum can easily be sealed for zero leakage. This can be done with the rectangular inlet with rounded ends, but it iks very difficult. One example of perfection with the rectangle inlet is Cory Birds Symmetry. If you take two absolutely identical non plenum airplanes and put a plenum on one and optimize the inlet/outlet, the plenum airplane will be faster. Or more fuel efficient. The access issue is hogwash. Allen head capscrews and a power driver make this a non issue. If access is such a big deal then all homebuilt airplanes should have an ugly set of cowl doors that can be opened without tools.

The real problem.

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,

I am with you Pierre

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.

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

penguin said:

After several years of prevarication...
Pete

Click to expand...

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

Your own brew

penguin said:

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

Click to expand...

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

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.