What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Converting the cowl from Vans to Sam James

Webb

Webb

Well Known Member
Sponsor
After going through builder's withdrawl, I've decided to do a partial build and give the aluminum mistress a nose job.

I'm converting the standard Vans cowl to the Sam James cowl and plenum.

With 480 hours on the current plane I have a good baseline. When perfectly leaned to best performance, I get 168-170 knots TAS at cruise which is normally between 9,000 and 12,000 feet, WOT, 90 degrees ROP, 2,400 RPM, on an IO360 (180hp) and MT CS prop.

I've been told that approximately 37% of the drag on our planes is due to cooling. With that being said, I will be interesting to see what changes I get in performance by making this conversion. If I don't pick up any speed, I will be mildly disappointed but not completely since I get to fix a few things that bother me about the current cowl......and I also think they look cool.

Performance gains are expected from:

1 - Intake change from snorkle to snout. Current air intake is in the left inlet and will be going to the lower cowl. This is supposed to give me 1 more inch of ram pressure at cruise.

2 - Cooling drag reduction.

My anticipated increase in performance is somewhere around 10 knots. I'll post again once it is finsihed and let everyone know if it was worth it.
 
Last edited:
plan for 5kt increase. that way if it's 10kts you'll be thrilled. but if it's only 2 kts you won't be crushed.

my James cowled -7 at 8000'DA, 24"x2400rpm 8.3gph is 175kts. Runs great and cool at speed. Push everything forward and I can get 185-188kts+

Now the cons: starts getting hot after 10 minutes below 120kts or pattern work. On the ground I boil fuel out of my injector lines. My plenum is too tight to the top of the engine and will be reworking it this winter.

I'll be at OSH most of the week if you want to talk.

.
 
Last edited:
Webb,

Glad to read your post on this subject. I will be most interested in the results.

I am running with direct ram air into the AFP system and on a hot day, opening the ram valve just after take off gives a noticeable boost in power. But that may be due somewhat to an internal K&N air filter when not using the ram air feature. I removed a snorkel type intake earlier this year and closed off the hole in the left intake.

I do see 175 KTAS at 8500' WOT, rpm 2750, with the Catto 3 blade. I will check TAS at 9500' on the way to OSH later this week, just to see how that compares to your present performance.

Cooling drag is an interesting issue. Sonja Englert is on the OSH forum schedule and I will attend to see what she has to say this year.

7/27/2011 - 8:30 AM - 9:45 AM - Forum Pavilion 06 - Map
Engine Cooling Forum - Basics on engine cooling, examples of what works, discussion of aircooling, liquid cooling, tractor, pusher installations, inlet and outlets shapes and locations. If you have an engine cooling issue, come to this forum.
Presented by: Sonja Englert
 
Last edited:
Enjoy the work and appearance

I do not believe you have a wiggle free baseline so whatever you want to see you probably can run a test to see it. The statements that claim the cowl/cooling drag are a major part of the total aircraft drag are misleading in that they can be interpreted to mean the cowl/cooling drag can be reduced and improve performance with no penalties. Simply not true for an air cooled engine. There is speed to be gained in the cooling system but nothing like 37%. I got 4 knots with extensive experimentation - most changes slowed the plane down from the original stock cooling configuration. If you want to go faster get a Hartzell C/S BA prop and run the engine up to 2700 RPM.

Bob Axsom
 
Last edited:
Webb said:
I'm converting the standard Vans cowl to the Sam James cowl and plenum.
Click to expand...

Webb, that's an interesting project. Do you intend to duplicate the dimensions of a James cowl, or do you mean "build a cowl with round holes"? Do you plan to use a prop extension?

1 - Intake change from snorkle to snout. Current air intake is in the left inlet and will be going to the lower cowl. This is supposed to give me 1 more inch of ram pressure at cruise.
Click to expand...

Very unlikely if you look at real numbers. Assume 8000 ft and 175 knots; the maximum available dynamic pressure recovery is:

(0.00339 x 0.7860 x 175^2) = 81.6 lbs per sq ft = 1.14 Hg

A magic increase of 1" by using a pitot-type inlet would suggest the standard Van's cowl has almost no pressure recovery....which is obviously not true, as it would not cool.

On the emperical front, I seem to recall Dan Checkoway having an AFP Y-valve on his -7, one side fed by a pitot inlet, the other by a filtered snorkle from the cowl inlet. As I remember, he reported very little difference in MP when switching back and forth....which makes sense.

FWIW, the next one of these I do will take oil cooler air from inside one inlet and combustion air from the other.
 
Webb, in the September issue of KitPlanes I have an article regarding my engine swap, including some tips regarding fitting the SJ cowl and plenum.

Remember, you have some fixed points you need to pick up with an existing installation, rather than a completely new FwF installation.

It would be cool if you did some very accurate bench marking before you made the swap so you can report back as to how much of a gain you received afterward.

(I just received my issue, so they should be hitting the mail soon and the news stand shortly after that.)
 
Dave Anders pushed his RV4 past 250 mph with control of cooling air way before Sam James came along. Dave did a great deal of research on cooling drag. His notes are posed on the CAFE site. This might be a good place to start for anyone that wants to reduce cooling drag.
 
gereed75 said:
Dave added considerable horspower on the way to those speed increases.
Click to expand...

Yes, and many other things......... My point was, he has done the research on cooling and is offered to the public with these notes. It sure beats starting from scratch.

And it is a lot more involved than just making a cover for the top of the motor and adding round inlets........... JUST READ THE NOTES they are free.
 
Bob Axsom said:
I do not believe you have a wiggle free baseline so whatever you want to see you probably can run a test to see it.........

.....There is speed to be gained in the cooling system but nothing like 37%.

Bob Axsom
Click to expand...

Bob - You are definately the experimenter and have produced some awesome results from it.. I truly must tip my hat to you.

Please note that I didn't say I would gain 37% but instead it was estimated that was the percentage of drag that came from cooling an airplane (as in what was happening under the cowl once the air entered the cowl. Please please please do not hold me to that exact number. I said estimated and am referring to some material I read once upon a time. I would feel lucky if I could eliminate any of that drag. Who knows, I might even make it worse (just my luck!!!).

As far as a baseline, I used TAS in cruise and have traveled at the baseline speed many times so I feel fairly comfortable to use it since it has been repeated many times.

Who knows what the result will be.......
 
DanH said:
Webb, that's an interesting project. Do you intend to duplicate the dimensions of a James cowl, or do you mean "build a cowl with round holes"? Do you plan to use a prop extension?


A magic increase of 1" by using a pitot-type inlet would suggest the standard Van's cowl has almost no pressure recovery....which is obviously not true, as it would not cool.
Click to expand...

Dan - The cowl and plenum are both from Sam James so there will not be a duplication. If I possessed the glass skills that you do, I might consider tackling a duplication project or modification to round holes.

And yes, I am using a 2.5" extension (same ones as on the Red Bull racers).

Since I have a forward facing throttle body, the extension is needed for the air intake since the SJ cowl is longer than the standard Vans cowl. The estimated (note the word estimated) 1 inch increase in manifold pressure (not pressure recovery) is from a ram type of effect instead of the air entering the left inlet and making turns to get to the throttle body. The air will entering what I called the snout on the bottle of the cowl. Whether this is true or not, I am going to write down the MP at what altitude that I am getting on the way to OSH and add it to this thread so when the conversion is complete, the answer will be known.

See you at the beer call and I'll buy you a brew. We can pick a number of what the change will be and bet a buck on it. Closest man wins.
 
Last edited:
Webb, at the Wenatchee race last month, I had 1" more MP with my Bower RamAir system running WOT and 2570rpm 200mph+ when compared to another RV-7 with the Vans Snorkel in the left inlet.
 
DanH said:
You mean like the nonsense about inlet/outlet ratios?
Click to expand...
Yes, Dave's RV4 has been in service for over 2000 hours and he holds the current non challenged CAFE record.

So until you break his record, it may not be nonsense.
 
Webb said:
The cowl and plenum are both from Sam James...
Click to expand...

Ahhh, I assumed you were planning a nosectomy with a sabre saw. Matters not; a straight cowl swap (Vans to James) will be of even wider interest.

Obviously cooling isn't just about cruise. Van takes pride in balanced performance. I'll bet the gang would really like to see three comparisons...

1. A long climb at gross, say 110 knots IAS from field elevation to 8000, starting out hot.
2. Standard cruise up high.
3. Ultimate top speed down low.

A recording EFIS/EMS would be nice, but a clipboard would do and it would be easy to gather it all quickly.

[QUOTE}The estimated (note the word estimated) 1 inch increase in manifold pressure (not pressure recovery) is from a ram type of effect instead of the air entering the left inlet and making turns to get to the throttle body.[/QUOTE]

"Ram" is pressure recovery. You will lose some pressure through the rather small filter used with a Vans snorkle, and yes, the duct may further increase loss. However, let's be sure we compare apples to apples; a proper comparison will require a filter in the new pitot-style intake. James offers a filtered intake. If you go the Bower route, MP will be poor when running filtered; all you get is hot, low pressure air from inside the lower cowl. If you do decide to go with a filterless pitot inlet, at least remove the snorkle filter for one comparison test flight.

BTW, a lot of snorkles are not well sealed at and under the intake ramp, and thus leak away pressure into the lower cowl.

See you at the beer call and I'll buy you a brew. We can pick a number of what the change will be and bet a buck on it. Closest man wins.
Click to expand...

Works for me!
 
Increase HP - Decrease Drag - Fly Faster

From Dave Anders notes - IO360 tweaked to produce 235 HP and many drag improvements (est 27% drag reduction) all add up to a speed of 265+ mph.

Just shows what determination and know-how can do. Klaus Savier did similar things with his VEZ, another very determined guy. For all out efficiency, the canards still hold the top spot.

http://www.fuelventure.org/documents/biography.html
 
This reminds why the big letters on the bulkhead say "EXPERIMENTAL".

DanH said:
Obviously cooling isn't just about cruise. Van takes pride in balanced performance. I'll bet the gang would really like to see three comparisons...

1. A long climb at gross, say 110 knots IAS from field elevation to 8000, starting out hot.
2. Standard cruise up high.
3. Ultimate top speed down low.
QUOTE]

I'll have to jot down some numbers for comparison on the way to OSH so I will have specifics.

#1 - Typically use 110 knots and lean accordingly to keep CHT's around 390.
#2 - Will have to write these down since I don't remember specifics but seems they runs somewhere around 350-360.
#3- Haven't a clue.

DanH said:
However, let's be sure we compare apples to apples; a proper comparison will require a filter in the new pitot-style intake. James offers a filtered intake. If you go the Bower route, MP will be poor when running filtered; all you get is hot, low pressure air from inside the lower cowl. If you do decide to go with a filterless pitot inlet, at least remove the snorkle filter for one comparison test flight.
QUOTE]

Comparison will be from filtered air to filtered air (not doing the Bower type). I also am interested in the MP change also.

Unlike the majority of all SJ installations which take place at the time of initial build, mine will be taking a 500 hr airplane and doing the converson on a known performer. Since this is all I am doing and not changing the prop or hanging a bigger engine, I think it is fair to say........"we'll find out".

Is a Sam James cowl/plenum faster than a Vans cowl? If so, how much? I don't know but I think so. Will the pitot type filtered air have a higher MP than the snorkle type? Again, I think so. Time will tell.

I would like to know if we have any conversions out there and what gains they have seen.

BTW - The parts have arrived. The extension by Sabre is truly a beautiful piece of work. It is 1 solid piece of metal and is very impressive. The cowl comes from Will James and the plenum from Sam James and the workmanship is outstanding on both. Both of these guys (and the guy from Sabre) are first class and will take the the time to answer any questions you might have when you call them.

Again, see you at the beer call. My buck is on 10 knots.
Click to expand...
Click to expand...
 
Webb said:
Unlike the majority of all SJ installations which take place at the time of initial build, mine will be taking a 500 hr airplane and doing the converson on a known performer. Since this is all I am doing and not changing the prop or hanging a bigger engine, I think it is fair to say........"we'll find out".
Click to expand...

It's a beautiful experiment. Can't encourage you too much....seriously document everything you can about current performance. You'll kick yourself later if you don't.
 
DanH said:
It's a beautiful experiment. Can't encourage you too much....seriously document everything you can about current performance. You'll kick yourself later if you don't.
Click to expand...

Thanks for the kind words and you can bet your sweet bippy that I'll have documentation on the performance changes. With so little conversions that I am aware of, I haven't been able to find out how accurate the claims of speed gain are. Hey....after all, I've got a buck on it with you and I hate to lose.

One thing about it. I am saving the old cowl in case it is a bust. I should be able to use most of the baffling by cutting it down and will replace the oil cooler mount section. My goal is a fast turn on the replacement so I'm not down for several weeks.

Have you picked a number yet?
 
Webb, do you have CHT and oil temp data as well? Seems that if you increase your TAS by 5 knots and increase CHTs by 40 degrees F (just an example) it may not be worth it.
 
Ron Lee said:
Webb, do you have CHT and oil temp data as well? Seems that if you increase your TAS by 5 knots and increase CHTs by 40 degrees F (just an example) it may not be worth it.
Click to expand...

I'm going to write down the exact numbers from the engine monitor on the way to OSH so I will have a good baseline instead of approximate numbers from memory.

Will it be worth it....unknown at this time. With so few conversions and little data found, the results will be interesting.

Numbers will be posted later in week when I fly to KY to visit family for a few days prior to OSH. More to come...........
 
Hey Dan....that dollar is burning a hole in my pocket.....

Finished the intake with the conical filter. Note the alternate air source that is kept closed by springs.

 
Bob Axsom said:
Webb, when you record your performance data be sure to include the outside air temperature.

Bob Axsom
Click to expand...

Thanks Bob. Great suggestion. As much fun as I'm having, I really want this to be well documented.

So far I plan to measure ALT, OAT, TAS, AS indicated, CHTs, MP, RPM, oil temp, EGTs, and will do so at different altitudes several times. Will also climb out at 110 knots and record CHT, however there is some variability since I control the mixture but will do as I normally do.

Am I missing anything?
 
Consider flying at a pressure altitude since this will yield density altitude along with OAT.
 
Dan - I absolutely must take my hat off to you since you are the glass man and I had forgotten how much I hate to do glass work. Especially since it is a 100 degrees in Mississippi right now.

Here is the result of the last few days work.



BTW - for inquiring minds, here is a comparison with the Vans and SJ top cowl sections put together.

 
Cowl at paint shop and finish date expected next Tuesday.

Finished oil cooler 30 degree mount that goes behind cylinder #4 tonight. Decided to not mount on firewall.

Had to make several new baffle pieces since the inlets are different and oil cooler is lower. lucky enough to use old pieces as patterns for fitting on engine.

Spacer installed on prop.

Front leg fairing cut into 3 pieces, shaped in correct curve on bottom and reglassed. Also at paint shop.

Air filter tube completed and ready to install. Need cowl back for alignment.

Will re-install baffling and fit plenum with plate nuts, put new plugs in, secure plug wires, seal air gaps with RTV, and check case bolts since access is easy this weekend.

Anticipate airborne on Labor day weekend (or maybe before).

Turns out that this project was more than I expected. Took plane down after OSH. Not all bad. Had mags rebuilt since they had 498 hours on them. Also knocking out some other things that are due soon at annual time.

Come to think of it, since it will finish in Sept, I think I'll just go ahead and complete the annual the following weekend. A month early but why not since everything else will be done under the cowl.
 
Webb said:
......Turns out that this project was more than I expected. ........
Click to expand...

:D ha, I remembering thinking your schedule was optimistic, but "wow that Webb must be a real worker."

Can't wait to see the performance/cooling results
 
The blooming cowl was finished 2 weeks ago. It's the other little things that pop up that are slowing me down. Like the intersection fairing. I couldn't send the cowl off to the paint shop until the fairing matched the curve on the bottom of the cowl. The Van's type is flat. The SJ is curved right at that spot.

fairing.jpg


photo1ed.jpg



Oh - it's also amazing how much more you can get down if you are.....single.
 
Last edited:
Look at what came out of the paint shop today!!!

bottomlfb.jpg


toprw.jpg
 
Last edited:
Cool...

....Only at a site like VAF would I find a newly painted cowl sexy. I could stare at those curves all day! <sigh>
 
Trying to fit the SJ plenum to the baffles was a real pain. I finally cut the lip all the way around the plenum and installed angles on the sides and back and it lays perfectly on the angles instead of trying to fit it inside the baffles.

Many thanks to Mississippi Jim for giving me an example of how he installed his. In addition, the baffles are so strong you can grab hold of them and shake the plane.

Also had to scrap putting the oil cooler on the back baffle. The motor mount kept getting in the way.

baffle.jpg
 
Last edited:
Good job Webb

Webb,
I must say that your attitude was much better this weekend than last! We thought for a while that we were going to have to ban you from the airport! I'm glad to see that you have a good solution, and that you are back to your old happy self. Your work looks great, and yes, it IS much stronger! BTW, you were some pretty good entertainment for a while! I even learned some new cuss words in the process of observing you.
 
Don't let Jim fool you. I was just repeating the words I had learned from him!
 
Here is a picture (slightly out of focus - sorry) of the plenum that FINALLY sits tighter than a drum on top of the ledger put on the baffles. Many many thanks to Mississippi Jim for the example he did on his plane.

For those that ever try to install a SJ plenum, throw the #$^$^$ instructions away and the first thing you do is completely cut the edge lip off the plenum and then install 3/4" angle on the existing baffles and cut the excess baffle off. The plenum gets flexible and can easily be screwed into platenuts on the angles. You still have to build new ramps. Since you are using an electric screwdriver, who cares if there are a few extra screws holding it down to ensure an airtight seal.

Although I didn't make Labor Day for going up, it will be this weekend!!

topgl.jpg


Also, here is a pic of one of the new ramps. Notice the old RTV line from the original baffle line.

cornerp.jpg
 
Last edited:
Brian,
I can speak for Webb since we're such good friends. He does have hair left, but he was close, believe me. He finally moved my band saw and belt sander and beer to his hangar to keep from walking back and forth, and that eased his nerves some. After that, it started coming together for him. Once the light bulb came on he was a different man! He's gonna wind up with a good one for sure!
Jim
 
I had to steal some of his beer because he kept emptying my fridge!!!

Seriously, Jim loaned me some of his tools so I didn't have to bring mine from home and saved me a thousand steps between the two hangers. A true friend.
 
Last edited:
N32WW went airborne today.

Have a couple of squawks to work on. Cylinder #1 was 20 degrees cooler than 3 and 4 and cylinder #2 was about 10 degrees. Will be putting in a shield to cover the front of those cylinders just like there was before to bring those temps up.

Overall, cylinders ran about 20 degrees cooler. Oil temp was much higher but I put a new temp sensor in since the old one was bouncing around 2 degrees on temps all the time. The new one was bouncing 10 degrees and I don't believe what I'm getting is accurate so I'll put the old one in to check.

Since the oil cooler is on the firewall, it was predicted to be higher but not as much as what was shown (70 more). When you look at surface area, the scat tube is 3" and using A=piXr2, the area comes out just over 7 inches. The back end of the cooler has about 3 times that area and I think that is part of the cause. Will make a back pressure recover piece to equal the incoming surface area to overcome that pressure.

ASI was more but until I get the temps situated, won't be testing it for the change in speed at altitude until the oil temp is corrected.

nosexa.jpg
 
Last edited:
AND?.... inquiring minds want to know any speed comparisons

EDIT: just saw your post about no speed until cooler corrected
 
Drum roll please.....

Averages

At 7,000 feet I picked up just a smidge over 1" of manifold pressure. Also saw a speed increase of 6 knots.

At 9,000 feet, I picked up 0.4" of manifold pressure and an increase of 7 knots.

Dan Horton - I owe you a buck because 7 knots is 0.75 mph short of 9 mph.

I'm glad to have done the conversion for improving some things I didn't like about the old, but it was a heck of a lot of work. Knowing what I know now, probably wouldn't go through all the effort.

The only real problem I've encountered is the higher oil temps on climb out due to the lower plenum. On the way to KLEX, temps settled down at 190 degrees (214 on climb out). Issue is the air was nice and cool at altitude and when it gets to be summer, it will be too hot. Setting the plenum up higher would probably have eliminated this. Fix will be installation of a 13 row cooler on the firewall feed by 4" sceet hose and a damper in case it gets too cool during winter.
 
Last edited:
good job Webb. 6-7kts is actually quite good.

Nice talking with you tonight. Good luck on the internal mods. I'll be watching for more numbers, before I tackle my stuff.
 
Webb, those are excellent numbers! Please take my buck and buy yourself a beer ;)

If you get an extra moment, how about measuring the inlet and exit areas of the stock cowl and the James cowl? I'd like to compare your results with some theory.
 
Dan - sold my old cowl so someone else will have to supply on the Vans size.

Can do the new. If memory serves correctly, the inlets are 5.5" in diameter but I will recheck. Will also measure the exit. From memory, the exit is slightly larger but I'll have to measure.
 
inlet rings are 5.125" OD, I think the ID is closer to 4.375". I'll be a the hangar tomorrow and will get some numbers. I would like to have some one else look at my set up too. My inlet/outlet ratio might be part of my cooling issue.....I have 4 pipes sticking in the outlet.

today at 11500(12500 DA) I was WOT 21.5" 2470rpm 10gph(rop) was 181KTAS.
 
You must log in or register to reply here.
Back
Top