exhaust exit fairing dimensions

[DanH]

Originally Posted by rvmills

...I'm hesitant to place anything in the flow path that might block, slow, or cause separation within, the exit flow that we are trying so hard to reaccelerate to free stream velocity.

The side locations may be less of an impact than the center-mounted location…air is earlier in its acceleration (I believe), and there is more room to flow around the cowl flap side walls and actuator. Down in the tunnel section, seems that it would be more critical to keep flow smooth and fast.

Steve Smith was kind enough to offer some guidance in that regard. If I understood correctly, when an enclosed volume at some pressure higher than ambient is offered a simple exit, the acceleration of air is practically all within the exit itself, or very close to it. As a side note, with a stock RV 6, 7, or 9, the sharp edge of the firewall probably causes some flow loss (top illustration).

I'm using an exit bell. In this case, the air is accelerating over a longer distance, as it passes down the converging duct. Although there may be some advantage in getting the airflow nicely organized (for lack of a better phrase), the bell itself has a lot of surface area, and the additional skin friction may slow the flow as compared to an exit with less skin area (middle).

The addition of a radius at the base of the firewall may be the best compromise, and in fact it's the standard on an RV-8 (bottom).

Notice I've used a lot of "may" and "probably" words in the above descriptions. All design is a matter of making intelligent compromises. Nobody knows precisely, for sure, what is best here...but given an aerodynamics issue I'd bet on Steve. The bell has some other attributes.

Returning to additional variable exit area, certainly everywhere inside the cowl has flow, in considerable volume. However, we're probably well advised to not worry too much about flow in the slower areas, and worry about it a lot in the high velocity areas. I suspect that adding flow impediment (a door mechanism) in the outboard rear corners of the lower cowl wouldn't actually be much of an impediment.

[DanH]

Originally Posted by BillL

Couple of questions - 1. are you using internal curtains like Alan Judy and Bob? And do you think they provide some help?

I have doubts Bill. Yep, the textbooks all illustrate theoretical cooling flows with converging ducts, but (1) note previous comments about duct skin friction offsetting flow gains, (2) a big converging duct under the engine is chock full of stuff...wires, pipes, hoses, all flow impediments, and (3) regular maintenance becomes more difficult.

Assuming AJ has the same heat rejection to head/cylinders as the rest of us (may not be true) then he is using waaaay less air than that Lycoming cooling flow chart with those 2-1/8" inlets.

We all probably have roughly the same total heat rejection for any given HP (oil to air and cylinder head/cylinder to air combined), but the parallel valve heads appear to have less fin area, more widely spaced. They also tend to run higher CHT for a given HP. I think parallel valve heat transfer from the heads and barrels is somewhat lower than from an angle valve.

Nothing wrong with small inlets, if you have the space for proper diffuser shapes behind them. Might even flow more than you think at first glance. Have you read Chris Zavatson's work?

http://n91cz.com/

[BillL]

Originally Posted by DanH

<snip> Have you read Chris Zavatson's work?

http://n91cz.com/

Yes, read several papers and squeezing for detail. Is there a particular part to which you refer?

Still, a little disappointing that the information provided by Lyc is lacking details about fraction to oil and conditions.

Curtains - looking under the engine had me wondering if they worked at all. May be a good heat shield. (if needed)

Why has no one (on VAF) but DanH used the prop spinner seal? That seems to be a key sealing area to manage.

[DanH]

Originally Posted by BillL

Yes, read several papers and squeezing for detail. Is there a particular part to which you refer?

I was just thinking about the good diffuser photos Chris posted. The paper you sent me was excellent.

Why has no one (on VAF) but DanH used the prop spinner seal? That seems to be a key sealing area to manage.

Actually there are several. For example, Tom Martin's Rocket is reputed to be the fastest in SARL. He seals his propshaft opening with a foam ring.

The fast Continentals simply run the front baffle wall below the propshaft, as they don't have an alternator and starter down there.

[Bubblehead]

Originally Posted by SHIPCHIEF

I like the idea of side exit for cooling air, as used on Grumman Bearcats and Focke-Wulf 190a. My Thorp T-18 uses this as well.

I noticed at Oshkosh this year that a lot of Glassair IIIs have a louver on the right side. Here's a picture https://www.google.com/url?sa=i&rct=...09090324619167

I attended a forum on engine cooling and bought the book from a lady engineer. I'll post her name when I get home from work. She said the sides were a great location and cited the T-18 as a great way to move the hot air out with minimal drag. In looking at the T-18's it looks like they have quite a bit more square inches of exit area on the sides than we have on the bottom.

[glenn654]

For those who don't already know, go to the CAFE Foundation's site to find three articles from the Miss. State aerodynamics research papers.

Lots of good information and well worth the time and effort.

Glenn Wilkinson

[rmartingt]

Originally Posted by BillL

Why has no one (on VAF) but DanH used the prop spinner seal? That seems to be a key sealing area to manage.

Are there any pictures of this? I read about it a while back but never found any...

[DanH]

Originally Posted by rmartingt

Are there any pictures of this? I read about it a while back but never found any...

Been experimenting with this one 350+ hours now. A fine use for Van's black baffle rubber...

When new:





Lower cowl pressure pushes it against the ring bear carrier. Wears a smooth strip on the paint, and a feather edge on the seal.

[rvmills]

Originally Posted by BillL

Hey Bob,
I think you are like our old friend Bob Axsom, posting at 3AM! Beautiful artistry with the digital flow lines there!.

Couple of questions - 1. are you using internal curtains like Alan Judy and Bob? And do you think they provide some help?

2. what did you use for that slick inside coating of your cowl.

edit: Oh - - why is it no one but DanH has the flow blocked behind the spinner?

Bill,

No 3am posting here...our clocks are 2 hours apart. It was a bit late tho! On your Q's:

1. I don't have internal curtains like Alan, or any baffles like Bob Axsom. Those techniques would seem to help isolate upper and lower cowl plenum areas, prevent reverse flow around the engine, and help direct flow towards the exit. However, it is art and science, as Bob found out. Some baffle configurations slowed his plane, and his final configuration appears to have increased his speed. It was a lot of trial and error, and placement and design of the baffles did not appear to me to be intuitive at all. John Huft's super-fast 8 didn't have zone separation baffles (as Bob called them) but did have ducts (fences) on the lower cowl that curved towards the exit opening. As I reshaped my lower cowl and exit, I had numerous conversations with Steve about the project, and as Dan said below, he expressed concern about skin friction. It's why I'm trying to keep things as uncluttered as possible. It's very difficult to say which method is best.

2. The inside needed a lot of work. I ended up sanding it clean (even removed a little material in some really thick spots, then smoothing with Superfil, and painting with primer and paint left over from a neighbor's Bearhawk build. The tape is Van's heat resistant tape, with fiberfrax underneath.

As Dan said, Tom Martin also sealed his cowl at the prop shaft. He glassed-in the front opening to within 1/4", then sealed it with a foam strip. Jason Rovey sealed his gap using Dan's method. Maybe each of them will post their results here.

Originally Posted by DanH

Steve Smith was kind enough to offer some guidance in that regard. If I understood correctly, when an enclosed volume at some pressure higher than ambient is offered a simple exit, the acceleration of air is practically all within the exit itself, or very close to it. As a side note, with a stock RV 6, 7, or 9, the sharp edge of the firewall probably causes some flow loss (top illustration).

I'm using an exit bell. In this case, the air is accelerating over a longer distance, as it passes down the converging duct. Although there may be some advantage in getting the airflow nicely organized (for lack of a better phrase), the bell itself has a lot of surface area, and the additional skin friction may slow the flow as compared to an exit with less skin area (middle).

The addition of a radius at the base of the firewall may be the best compromise, and in fact it's the standard on an RV-8 (bottom).

Notice I've used a lot of "may" and "probably" words in the above descriptions. All design is a matter of making intelligent compromises. Nobody knows precisely, for sure, what is best here...but given an aerodynamics issue I'd bet on Steve. The bell has some other attributes.

Returning to additional variable exit area, certainly everywhere inside the cowl has flow, in considerable volume. However, we're probably well advised to not worry too much about flow in the slower areas, and worry about it a lot in the high velocity areas. I suspect that adding flow impediment (a door mechanism) in the outboard rear corners of the lower cowl wouldn't actually be much of an impediment.

Very much on the same page with you here Dan.

I chose the radius for simplicity of fabrication and ease of installation in a very crowded firewall area. The friction of the bell's surface was considered, but I would think that the bit of extra friction might be countered (in terms of pros and cons) by moving air towards the exit that otherwise might find its way to the firewall and all the clutter back there. Is that what you mean by other attributes?

I do think that the air accelerates quickly at the exit, and there is far less movement further upstream. Steve and I were texting about the placement of the louvers or cowl flap(s), and he concurs that the cowl flap side-walls and actuators may have little impact in the side positions of the bottom of the cowl, but may be impacting in the center of the tunnel. How much is unsure. Are you aware of any flow studies within a typical cowl? I believe quite a bit of study has been done on differential pressures, but I would be very interested in flow visualization within a cowl. I believe you and Steve are right about the flow action being focused around the exit...or at leakage spots like around the prop.

A related question...if the flow is less in the corners, does that make the cowl flap less effective there than in the tunnel?

In my case, I want to add cooling capacity with minimal impact to the race configuration. Louvers may do that, but the flexibility of the cowl flap adds the ability to maintain improved cruise performance while adding cooling for climbs and formation work. I'm contemplating a pair of louvers that match the size and hole pattern in the EZ Cool flap, adding nut plates to each, and making them interchangeable for comparison testing to a flat plate plug.

Hey Allan...want to join in a comparison test?!

Great discussion!

Cheers,
Bob

[DanH]

Originally Posted by rvmills

Maybe each of them will post their results here.

Minor side note...the shaft seal, like many other features discussed here, is an individual component of a system. Added at random, individual components may not show any significant change by themselves.

The friction of the bell's surface was considered, but I would think that the bit of extra friction might be countered (in terms of pros and cons) by moving air towards the exit that otherwise might find its way to the firewall and all the clutter back there. Is that what you mean by other attributes?

The primary additional attribute is its effect on the oil cooler duct exit.

A related question...if the flow is less in the corners, does that make the cowl flap less effective there than in the tunnel?

Nope. Don't confuse velocity with pressure. Air moves from here to there because of (1) pressure differential, (2) inertia, (3) entrainment, or (3) all of the above.



My guess is that lower cowl internal flow with a single center exit looks like the example on the left. The primary driver is the difference in pressure between the enclosed cowl volume and the outside air. Air moves from high to low, which sets up a flow to the exit. Air in the rear corners is entrained along "A" and circulates, doing nothing very useful (same thing happens in the wheelwells of cars). There is some energy loss. You could wall off those dead spaces and eliminate the circulation (a big duct extending from the underside of the engine, as previously discussed), but skin friction from the walls would also be an energy loss.

Two additional small exits offer new ports from high to low, so the air goes there too. Velocity at all exits will decrease.

I'm contemplating a pair of louvers that match the size and hole pattern in the EZ Cool flap, adding nut plates to each, and making them interchangeable for comparison testing to a flat plate plug.

My man!