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Simple speed/cooling fixes when finishing under the cowl

hecilopter

Well Known Member
I'm helping a buddy finish up his -7A and remembered a few simple tricks that I used and picked up from other's advice/forums etc. and thought I would reiterate them here while on my mind in case anyone else is finishing up :)

1. Don't forget to fiberglass the inlets on the upper cowl closed either on the inside or outside. This keeps air from leaking under the inlets and causing drag/cooling problems.

2. Put a thick washer between the baffling and the #3 cylinder in the back. This lets air circulate all the way around the cylinder and keeps it 10-15 degrees cooler.

3. If mounting the oil cooler on the baffle behind #4, put a supporting piece of angle attached from the baffle close to the oil cooler and run diagonally across to one of the engine case bolts. This strengthens the baffling and keeps it from cracking due to vibration.

I'm sure other people have some other tricks, but these are simple to do and result in a stonger/cooler/faster airplane. I'm very surprised they haven't found their way into the construction manual. I have been amazed in looking at some other planes that the upper inlets have not been closed off. It isn't in the instructions, but it makes a BIG difference cooling/speed wise.

Tailwinds!
 
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hecilopter said:
1. Don't forget to fiberglass the inlets on the upper cowl closed either on the inside or outside. This keeps air from leaking under the inlets and causing drag/cooling problems.

!

I don't understand why this would do anything for drag/cooling. Would the pressure be the same on both sides of the upper cowl ramp. Or maybe I don't understand what is being closed off here.

Kent
 
kentb said:
I don't understand why this would do anything for drag/cooling. Would the pressure be the same on both sides of the upper cowl ramp. Or maybe I don't understand what is being closed off here.
You incur a drag penalty for every bit of air that comes in the inlets, so you want all of it to do something useful, like go through the cylinder fins, or through the oil cooler. If you don't close off on end of the ramp that is fibreglassed to the upper cowl, the area under that ramp creates a large leak from the area above the cylinders to the area ahead of the front baffles, which is connected to the area below the cylinders.
 
kentb said:
I don't understand why this would do anything for drag/cooling. Would the pressure be the same on both sides of the upper cowl ramp. Or maybe I don't understand what is being closed off here.

Kent
See this thread I wrote for the answer.
 
kitplanes article on cooling

I'm at work so I don't have it in front of me, but there's an interesting article in this months Kitplanes about how wierd things happen under the cowl to the air flow. Completely oposite of what you'd expect. I'm doing my phase one flights and working through cooling issues now, did the ramp seal off, I think I'll try the washer behind #3 next. I wish I had the time ,$, patients, to do the instrumentation discussed in the KP article. Fascinating.
 
Anit reversion cones

I forgot to mention about how I helped the situation with the reversion of the air at the inlets on my -6. When I made my small 2 3/4" inlets, they open up very gradually to an oval shape to keep the air attached along the walls of the inserts/diffusers, then blend in smoothly with the inner plenum with a clearance between the walls of the plenum and inserts. Guess what I'm trying to say is they are designed for anti-reversion to prevent some of the air from backing up or moving forward back out of the cowl inlets. It's designed kinda like some of the exhausts headers that were made years back that had an anti-reversion cone in the primary tubes at the exhaust port to prevent reversion of the exhaust from entering the combustion chambers.

Just got back from flying a 3 hour trip to and from a fly-in event in Cookson, OK today and the ambient air was 70F. I cruised at 7,500 ASL x 24" x 2,450rpm and the CHT's were 312, 312, 312, 308 and the oil at 180-185F I'm still to cold and need to make some revisions to get the CHT's up.
I took Allen Barrett with Barrett Pression Engines from Tulsa, OK for a ride today and he was quite amazed with the performance and how well the engine cooled even at full power climbing hard and slow at 110mph. He said I'm running to cold which I'm aware of.

The airflow problems inside the cowling or plenum can be improved in many airplanes with a little homework and experimenting with different mods. One thing I did learn if making changes is to make one mod change at a time,
not 3 or 4 changes because you don't know what did what. I guess that's why they call these RV's Experimentals.
 
Why is too cold a problem?

I run down at those temps all the time...sometimes down in the 290's CHT range when doing IFR training, well LOP and slowed to "thinking speed"

Frank
 
To cold to hot.

frankh said:
I run down at those temps all the time...sometimes down in the 290's CHT range when doing IFR training, well LOP and slowed to "thinking speed"

Frank

From talking to many engine builders I've learned that some of the clearances in the engine are best at the 350-380F CHT range. Some have told me to never run under 300F CHT. I started a thread about this issue a couple of months ago and got every answer in the world as to what is correct or ideal. I'm only going by what experienced shops have told me after tearing engines apart with history behind them with CHT's running a trend of continous cold, warm, or hot from the past. I don't know if there is an ideal temp just as long as we don't go past 380F CHT's. That's about all I know for now.
 
frankh said:
How is that Rocket kit coming along?

Frank 7a
Man, I am so overworked right now at the office I can't afford time to fart.
At least I'm flying every weekend. I'll get the EVO going this summer after
a mega deployment at the office is finished. Thanks for asking.
 
Pictures of cowl inlets on my RV-6

erich weaver said:
Hi Alan

any chance of seeing some pictures of your inlets?

erich weaver

Hi Erich, here are a few pictures. None of the pictures show how they fit inside the plenum because I have no way to take a picture inside the plenum. The sides of the diffusers where they get larger into the plenum are around 3/4" smaller than the plenum to help with reversion of the air flowing forward. What I have found out is that you need the air flow to be high at the point of entry then to slow it down. Keeping the air through the first few inches laminar and attached I think is the key. Im not an engineer or a NASA rocket dude, but testing various designs over many hours in the air in actual flight is what I've found to be the best testing.
Note that the left and right inlet diffusers are not the same shape or length!!! That is because of the spacing / setback between cylinders 1 & 2. Number 2 cylinder is further back than number 1 cylinder so provisions need to be made with the inlets to direct the air.
Both front cyl heads have 3-D contoured diverters in front of the heads as well to direct the air smoothly over the heads and they are not touching the fins, but have a large gap at the back to conseal the reversion of air to direct it back down in front of the cyl fins.. Sorry but I have no pictures of these.

When I get the cowl back off I'll try to take pictures or check it out at Oshkosh 07 as I will be there.

dsc07624bk3.jpg

dsc07628fm9.jpg

dsc07627xe4.jpg

dsc07635xw7.jpg
 
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Sweet!!

Beautiful work Alan,
What did you use as a mold to shape the inlets over? I'm curious as to whether or not we can use West system 105 with carbon. If not, which resins did you use?

Secondly, would you care to divulge max straight and level speeds at 7500 and 75% cruise speeds. If not, I can understand.

Thanks,
 
Coca Cola Cup

pierre smith said:
Beautiful work Alan,
What did you use as a mold to shape the inlets over? I'm curious as to whether or not we can use West system 105 with carbon. If not, which resins did you use?

Secondly, would you care to divulge max straight and level speeds at 7500 and 75% cruise speeds. If not, I can understand.

Thanks,

Thanks Pierre, Picture of my mold. :D Coca Cola plastic cup. I started with that then I made the rest of the shape from acetate plastic cut in strips and shaped like a tulip and taped together. I use West system 105 on all of my glass work. I know it's not the correct stuff to use, temp range wise in the cowl, but I've not had any problems. My cowl inlets are 99% S and E glass with a lot of glass balls mixed in the resin at the front area. The carbon fiber is only on the front for looks.

I have not done the 3 leg tests at 7,500 x 75%. I've made a few trips lately to fly-in events and just haven't done it. If my airspeed indicator is correct, :confused: last Sat I flew at 7,500 MSL x 2,450x 23" and was indicating 190-191mph. This was not full throttle either. Full throttle that day at 7,500 did show 24.2 " MAP. OAT was 65F Degrees. I'm not sure what the pressure ALT was, but this should be around the 213-215 mph TAS roughly maybe. When I made my trip Sat to Cookson, OK I was seeing 242 mph ground speed with a quarterly tail wind. When I flew back the opposite direction my ground speed with the same wind conditions I was showing 189 ground speed. I know this is not a 3 leg test, but that averages 215.5 TAS at roughly 75%. There are a lot of RV's that are rigged much cleaner and lighter than mine and at higher alltitudes it's maybe a little faster than some of the others in my area but not much.
dsc08098af4.jpg
 
cooling

On point two you mention putting a washer between the No.3 cylinder and the baffeling - do you have a picture or could you explain this better? Thanks
 
Sorry... I still don't get it...

walter said:
See this thread I wrote for the answer.

Couldn't find the thread, but I went to your web site and found the following picture.

http://www.rv7-a.com/145_4552.jpg

This looks like the way that I did mine and I don't see that the sides were sealed off. Just a nice smooth flow from the top inlet to the top of the cowl.

Is there another picture that I can look at.

Kent
 
Please explain the addition of the washer in the No. 3 cylinder baffle area. Do you have a picture or diagram?
 
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I think there

is a pic somewhere on this site for the washer trick. If you look on the back side of cylinder #3, right where the rear baffling and side baffling meet and are riveted together, there is a #8 screw attaching the baffling to the #3 cylinder. Take this screw out, put a thick washer between the baffling and the cylinder, and replace the screw. It does make a difference. Here is a link I found to show how to glass in the inlet ramps...

click here

Good luck!
 
Not sure that this will do any good for me....

walter said:
I guess it would be nice if I posted the link to the thread, eh?

http://www.vansairforce.com/community/showthread.php?t=8209
I took the cowl off last night and the baffle seal fits tightly along the entire outboard edge of the cowl. The only place that does not have a tight seal is the inboard edge of the inlet. Just behind the screws that hold the upper to lower cowl halves together the baffle seal in this area is just a layer and air pressure in this area will not make the seal any better. I looks like there would be about 1/8 inch gap that is about 2 inches long.
As an experiment I will seal one of my upper ramps and report if I see a temp. drop.

Kent
 
Nice work Pierre!

I too have a SJ cowl and I like your intake inserts primarily bacause they can be tried without cutting up the cowl.

Now your 2 3/4 inlets are small because you have a seperate inlet to your oil cooler correct?

I have a relatively hot #4 cylinder (I had a little casting flashing that I removed which helped some) but I have the oil coller take off right behind this pot. It maybe the air resistance thru the coil is low, thus accelerating the air and dropping the static pressure...Therefore low pressure available to cool the #4 cylinder.

I may try the additional baffle to wrap partly round the top of the head...I don't know why it would work but I'm told it does and its easy to make...Just have to find the time..:)

Assuming I get this fixed I will then try a smaller opening.

Incidently, if the cooling openings need to be around 3" then it would seem appropriate to reduce the size of the combustion air intake...i think Kent Pacer did this for his o320..Might be worth a try, i.e reduce the size until it affects the manifold pressure then open back up a bit.

Oh to have the time..:)

Frank
 
Hmmmm

frankh said:
I too have a SJ cowl and I like your intake inserts primarily bacause they can be tried without cutting up the cowl.

Now your 2 3/4 inlets are small because you have a seperate inlet to your oil cooler correct?

I have a relatively hot #4 cylinder (I had a little casting flashing that I removed which helped some) but I have the oil coller take off right behind this pot. It maybe the air resistance thru the coil is low, thus accelerating the air and dropping the static pressure...Therefore low pressure available to cool the #4 cylinder.

I may try the additional baffle to wrap partly round the top of the head...I don't know why it would work but I'm told it does and its easy to make...Just have to find the time..:)

Assuming I get this fixed I will then try a smaller opening.

Incidently, if the cooling openings need to be around 3" then it would seem appropriate to reduce the size of the combustion air intake...i think Kent Pacer did this for his o320..Might be worth a try, i.e reduce the size until it affects the manifold pressure then open back up a bit.

Oh to have the time..:)

Frank

YES my oil cooler is not cooled by the cowl inlets. My oil cooler is taking air at another location on the right lower cowl in a precise location. This area on the lower cowl can be done right or wrong to make things worse, it takes a lot of testing to find the right spot and angles for a NACA duct.
The cowl inlets are VERY critical at getting the air to work inside the cowling. There are not two RV's or should I say maybe a few that are close to being the same with good airflow management inside the cowl. Spam Can airplanes are a joke. Go to RENO at look at what the racers are doing with air management if one can look. Air entry at the cowl and what happens to it aft is very critical.

It's kinda like a wing on your RV.... If you bring the angle of attack to high it will stall.......... the same thing happens inside the cowling. You do not want the air to stall inside the cowling or plenum!!!! It needs to be High pressure and not stall,,,,with NO reversion. The reversion of the air is a problem if the inlets and diffusers are not designed right or the angles are to severe, but with the airflow needs to cool the engine and an oil cooler, the inlets need to be sized correctly, but they need to be laminar with no reversion of the airflow or the air is going to be turbulent and a mess inside the plenum / upper cowling.

One advantage of a TIGHT sealed plenum is to reduce this. With a stock BIG Van's plenum / cowl that is not sealed well will be very turbulent and the air is going all over the dang place turning circles and going back/forth like a tornado.

Think of it like this:::::: Air in needs a path to go, but it needs to be fast at the cowl / diffuser inlet with the correct shape and size. The air then needs to be slowed down after the inlets by enlarging the chamber / volume, but one must keep the air laminar with the correct entry angle, over a very long path,, the longer the better. :rop extension and extending the cowl inlets is one key. Use only the amount of air that is needed on cowl inlets that is neede and NO more.

Take a garden house for example, one that is 1/2" diameter and see how far the water flows out with the faucent wide open...... Now go get a 3/4" garden house and see how far it shoots out in front of you. After doing this you will see the velocity and distance that the 1/2" garden hose will produce. It's all about pressure and volume, but were dealing with air keeping it laminar / attached to the walls of the diffuser inlets and slowing that air down with no turbulent air. If the air inside the cowl / plenum is turbulent or stalling like a wing on an airplane,,,,, you will have very unstable air inside the cowl and it will make turns or stall and have no direction to cool the cylinders..

Cooling an air cooled engine and getting speed from cooling drag reductions is something to think about when working with the air inside the cowling.

I need to run now, but if anyone has any questions feel free to ask. I've tested a lot of things over the years of trying this and that and I will tell you the bad and the good at what worked and didn't. Later,,,, AJ
 
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Ok here are the results...

kentb said:
I took the cowl off last night and the baffle seal fits tightly along the entire outboard edge of the cowl. The only place that does not have a tight seal is the inboard edge of the inlet. Just behind the screws that hold the upper to lower cowl halves together the baffle seal in this area is just a layer and air pressure in this area will not make the seal any better. I looks like there would be about 1/8 inch gap that is about 2 inches long.
As an experiment I will seal one of my upper ramps and report if I see a temp. drop.

Kent
I only sealed the ramp on the inboard side of the left side of the cowl (effects cylinder #2 and #4). The conditions were not identical between the before and after runs, so I am looking at the difference between temp change on #1/#3 and #2/#4.

In climb/takeoff:

before: #1....#2....#3....#4
..........405....407....412....386
after:
..........399....395....395....377

#2 dropped 6 degrees and #4 dropped 8 degrees

In cruse

before: #1....#2....#3....#4
..........334....335....344....317
after:
..........314....316....323....301

#2 dropped 1 degrees and #4 dropped 5 degrees

Not real impressive, but as I said before my baffle seal fit very well, still it is an improvement and I think that I may go ahead and seal the other side ramp. This make me think more about changing to a full plenum setup. Maybe next winters project.

Kent
 
Impressive

kentb said:
Not real impressive, but as I said before my baffle seal fit very well, still it is an improvement and I think that I may go ahead and seal the other side ramp. This make me think more about changing to a full plenum setup. Maybe next winters project. Kent
I'm impressed. Don't sell yourself short, that's a good eye, a good catch. A small change effected a significant and measurable drop in CHT temp.

YES to the sealed plenum. Not only for the reduced leakage between the baffle and cowl, the the sealed clamped connection from the cowl inlet to the plenum. Round is easier to deal with with ducts and clamps. If you stick with the stock rectangle shape cowl inlets, its more of a challenge to seal. The plenum will increase efficiency so you can reduce the inlet area, thus picking up speed, ie less cooling drag.
 
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