From: Monty Roberts

To: Rotary motors in aircraft

Sent: Wednesday, December 14, 2005 5:01 PM

Subject: [FlyRotary] Cowl Exit

John,   It really depends on how  much power/heat you are going to be producing.  However, if you have --say 24 sq inch for each of three openings (two for GM cores and one for oil cooler) then you have a total of 72 sq inches for cooling air to come into the cowl.  Now you are going to heat the air which will naturally cause it to expand in volume, and based on your opening size 4.25*11.25 = 47.8 sq inches for this expanded air to get out.   Your exit  area is way too small in  my opinion. 

 

  A "rule of thumb" for exit area that I have seen is no less than 1.2 ratio of exit to intake.  I have seen that ratio up to 1.7. so that would indicate you would need 1.2*72 = 86 to 1.7*72 = 122 sq inches.  Personally, I would lean toward the 122 myself.

 

Here is a rule of thumb I just got off the internet

 

You will hear rules of thumb that the exit openings, because the air is heated and thus presents more volume than the original intake air, should be 2.5 times the area of the intake opening. With good extraction of this heated air, allowed by the fact that it is exhausted into a low pressure area, the exit cross section can be as small as the intake in some cases. In others larger, but I have never seen this exit area have to exceed about 1.5 to twice the area of intake.

 

From a different source  Exit area totals 66.7 square inches which comes to about a 1 to 1.5 ratio inlet to exit area.

 

Notice no indication that anything less than intake area would do the job.

 

 

I used the solid pink foam from a lumberyard (had to glue several planks together).  I taped it with duct tape to prevent the foam from dissolving when I put on the epoxy - work fine for me.

 

Ed A

 

 

Ed,

 

The oft sited rules of thumb are fine. But you have to keep in mind it all depends on your flight conditions. If you size a radiator, inlet and exit for hot day climb, then it will be WAY too big for cruising at 75% power at 12K feet. In this case you use a cowl flap to bring the exit size down. This restricts the flow of air through the radiator thereby reducing momentum drag. It also causes the pressure in the cowl to go up. The air stacks up and you get external diffusion. The high pressure heated and expanded air now squirts out through this restriction and you gain some of your drag back. In this case, it is possible that the exit may be smaller than the inlet (the inlet is sized for WOT climb, not cruise). Remember the engine is putting out way less than rated power because you are (or should be) running LOP at 75% minus the power loss with altitude. So the heat load is way less than what you need to remove at WOT climb on a hot day. You should have some way to vary the exit if you want to get the best out of the airplane in cruise, of course it is more complex. It's all a trade off.

 

I must admit I like the current post following the previous. One can only rebel for just so long ;-).

 

Monty

 

 

Monty, you are rebel!!

Bottom posting it is, ....at least on this thread!

 

Back to square 1!!

 

We had a similar thread back in April!

ROT's about exit/intake ratio where exit HAS to be bigger are gospel. If you want to reduce cooling drag, read up on the following:

(I am cheap enough to just copy my past posts and replies here:))

 

copy from 06-April 2005:

...cut
Note:  the exhaust area requirement is greater than the inlet combination of oil and coolant due to the now considerable hotter air temperature.  Rule of thumb:  Coolant air inlet opening for 200 HP coolant cooled engine ~60 sq. in., oil inlet opening ~30 sq. in. equals a total opening of 90 sq. in.  A good place to start with exhaust opening is 1.4 times the inlets or 126 sq. in.  Close the exit area down with cowl flaps to as little as 80% of the inlet combination at cruise conditions!

...cut

There was an excellent article about that in Kitplanes of Feb.2004, about Brian Schmidtbauer's Mustang II. Though Lyc powered it still goes 250 mph, just about par with his friends RV-4, same engine (Dave Anders of CAFE foundation fame). Of course Dave is part guilty for that Speed demon, as he gave some ideas to Brian about cooling.

I do understand that an air-cooled installation is a different animal, but by the end of the day ( or at the cowl exit...for that matter) everything is air cooled. The interesting thing is that both run a intake/exit ratio of 78%, the intake being bigger - just about confirming the above statement!

Both seem to use exhaust augmentation for the exit.

Their main study material was "NACA reports back to the 1920's and Hoerner's book about Fluid Dynamic Drag" ( by the way that book is still available, but at around a 100 bucks rather pricey, but worth it - says hearsay. Out of my range for now....)

 

I mentioned that on the other list, but I was dismissed as " you can't compare aircooled engines with a rotary".

I don't think there is a great difference on the amout of cooling necessary, as the efficiency of both engines are fairly close. So some heat goes out the exhaust and the rest has to be cooled. For sure you need different ducting, but the amount of heat energy should be about the same and you want to get rid of it with the least drag, either way.

 

Schmidtbauer mentiones the " rule of thumb" - exit about 150% of inlet. By fine tuning the ductwork he got rid of up to 30% of the total drag, just by eliminating most of the cooling drag.

Anders beat John Harmon in his Harmon Rocket, by over 1000 points (CAFE system, Harmon 1316,45 - Anders 2381,24)

Harmon around 300 hp, Anders 200hp (6mph faster than Harmon - 250+ mph for the RV-4)

 

 

So much to "thumb-rules", and "not doing ones homework, because I don't like formulas, etc."

I believe until there is a solid FWF- instalation for every popular airframe, you will HAVE to do your homework, or give away efficiency in a big way!!

 

There are so many guys on this list that know their ways around formulas, being on the list to share info and trying to help on every corner, you don't even have to dig in that hard, just ask someone who enjoys formulas!

 

Back lurking,

 

Thomas J.

 

PS: I have 3 pages of that article scanned, if anyone wants/needs them I can forward it. =00-300Kb each....

 

end of copy

 

 

 

I got to direct replies - here the copies to complete my answer:)

 

reply 1:

 

 

Al,

 

I did mean "intake"!

On both their planes, the Mustang II and the =V-4 the intake area is biiger than the exit!!

As mentioned, if you need I can email the scans =f the mag! You also might want to read up on: http://www.cafefound=tion.org/aprs/RV-4.pdf

 

Thomas J.

----- Original Message -----

From: Al =ietzen

To: Rotary motors in =ircraft

Sent: Wednesday, April 06, 2005 =1:29 PM

Subject: [FlyRotary] Re: rule =f thumb and RV-3 sizes-

..cut

Note:  the exhaust area requirement is greater than the inlet combination =f oil and coolant due to the now considerable hotter air =emperature.  Rule of thumb:  Coolant air inlet opening for 200 HP coolant cooled =ngine ~60 sq. in., oil inlet opening ~30 sq. in. equals a total opening of =0 sq. in.  A good place to start with exhaust opening is 1.4 times =he inlets or 126 sq. in.  Close the exit area down with cowl flaps to as =ittle as 80% of the inlet combination at cruise conditions!

 

...cut

 

I do understand that an air-cooled installation is a different animal, =ut by the end of the day ( or at the cowl exit...for that matter) everything =s air cooled. The interesting thing is that both run a intake/exit ratio =f 78%, the intake (I think you meant ‘exhaust) being bigger - just =bout confirming the above statement!

 

Schmidtbauer mentiones the " rule of thumb" - exit about 150% of =nlet.

 

I think =or our purposes, the 1.5 ratio is more applicable than 1.28. It can be =ower IF you have well shaped entrance and exit ducting.  Typically we focus =n entry ducting, and then have rapid expansion at the exit from the =ore – sudden expansion pressure loss; losing all momentum and then, =sually a rather sudden acceleration out a fairly small opening.  =efinitely less than optimum.

 

I don't think there is a great difference on the amout of cooling =ecessary, as the efficiency of both engines are fairly close. So some heat =oes out the exhaust and the rest has to be cooled. For sure you need =ifferent ducting, but the amount of heat energy should be about the same and =ou want to get rid of it with the least drag, either way.

 

You’re right.  The biggest difference is the higher rejection temp, =nd larger delta T of the air on the air-cooled engines allows for a lower air =low rate.

 

Al

reply 2:

 

Hi Barry,

I agree that augementation plays a serious role in these two planes.
It was never mentioned or opposed or other, they just found the right
combination/optimization of various areas in their system to make it work
contrary to the "rule of thumb" of 150% (or whatever number) the exit area
hat to be bigger than the intake. Obviously this ROthumb does not care too
much about drag reduction.

Thomas J.

> Thomas J,
>
> The key line in that article followed the size of the exhaust: "is
> augmented by exhaust flow." Exhaust augmenters really work according to
> Contact magazine and others. This article is just more evidence of that.
> Apparently the augmenter is so effective that it allows what would
> ordinarily be a restrictive outlet on the back of the heat exchanger.
>
> Barry Gardner
> Wheaton, IL

 

OKAY, end of post:

 

To recollect:

 

If anyone cares for send me your email and I forward the scanned pages of the KItplane report.

Also check out http://www.cafefound=tion.org/aprs/RV-4.pdf on Anders RV-4!!

 

Thomas J.