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speed / effciency mods in cowling

S

swoeric

Registered User
stillwater ok
So the racer boys and the factories have figured out that there are some efficiencies to be gained in the cowling area.

The idea that if the air going in the cowling does not have equal release behind it...it will pile up and come out the front killing prop performance....high performance planes typically seal well around prop shaft cowling area...

to properly cool the engine we need a certain pressure drop from top side to bottom side of engine and can run a manometer...we can then balance the outlet size of our cowling to produce a true flow rather than too large of a cavity that will create a vacuum the wrong direction...

the factories have shut off the front cowling sizes considerably look at the newer aircraft and carbon cub...


has anybody worked at closing off portions of their cowling and verifying that the flow through the engine is un-impeded and not coming out the front????
 
Look carefully at the Carbon Cub. They use a plenum to direct the air smoothly and evenly over the engine.

You are correct, however in assuming that there are efficiencies to be gained in a "standard" Cub cowling.

MTV
 
One thing I have done is closeup around fly wheel between the cowl and flywheel glass and foam. Also sheet metal around the back of the flywheel contour. It's very tight. I'm not looking for speed but mindless climbs without the concern of CHTs and OT getting out of control at high altitudes. One advantage is I feed the firewall mounted oil cooler with only a 2 inch duct. I get a little warm but once I begin to drop the pressure below upper area, things should get better. CHT's are still a concern but not out of control and I would like to get them lower. I will experiment with a manometer when I get one and establish the base line. I'm looking forward to the end results. Closing off the size of front cowl holes might be an option to reduce cooling drag. Ramps are also a big help. I do not have a lip on the bottom cowl and this might be something I install. My numbers are all in the green but just think I can do better. like low green.
 
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Someone once told me you can't make race car out of a tractor, but knowing we are dealing with some major limitations, and there some super smart people on this forum who can give some great insight in this area . I did some research once about increasing the cruise speed on this type of airframe and the best I came up with was "The Mystery Pacer" Jim Younkins took an old Pacer and rebuilt it, he spent a lot of time on the cowling design, wheel pants, and fairing on almost every component of drag. He added a 160 hp with a constant speed prop and was able to get 150 / 155 mph, I spoke with a guy that worked with Jim on the project and he verified those speeds.

In Johnson Creek this year I was having casual conversation with someone who I consider a leader in the aircraft design business about this very subject and he said that a Pacer was an 140 mph out of the box airplane so he wasn't that impressed by those numbers, at the time I didn't think he was right but i wasn't going to challenge someone with far greater knowledge in this area than I have, but to be honest i thought the Pacer was more of a 120 mph airplane so an increase in 30-35 mph of cruise speed impressed me.

We are a restless group, i don't think we will happy until we land a Super Cub on the moon and bring a pilot safely back to earth :)
 
Utah 12 Driver said:
We are a restless group, i don't think we will happy until we land a Super Cub on the moon and bring a pilot safely back to earth :)
Click to expand...

Heck... I know where we can fake the pictures and you can get your 15 min of fame. It's not too far from you. We could even throw in a short clip of a zoom climb to confirm the blast off. A quick smoke mod to your exhaust would really make it look good.
 
Photo work shop must be a big part time activity in the mid-west :) it will be a little while before we can get a shot of my airplane it's in a total rebuild but should be done "soon"
 
The Pacer is a 120-125 MPH airplane at cruise power of 2450 RPM. Top speed with the throttle to the wall can hit the 140 mark. My 180 fixed 56 pitch runs 125 at 2450, but can hit 155 full power @ 2750 RPM. Our customer with our 180 constant speed STC cruises 140-145 @ 2400 squared. I do not know what it would do full out. I think Yonkins reported numbers was at cruise, not wide open.


Steve
 
Utah 12 Driver said:
Someone once told me you can't make race car out of a tractor, but knowing we are dealing with some major limitations, and there some super smart people on this forum who can give some great insight in this area.



We are a restless group, i don't think we will happy until we land a Super Cub on the moon and bring a pilot safely back to earth :)
Click to expand...


Ha! Too true! Tinker on...

I thought that the airfoil combined with long wings is what made the cub "hit the wall" in airspeed. Thus the short wings to make things go faster. Certainly, there are some draggy parts of the airplane, but they probably help it land slower. People are putting bigger engines on the plane, but not getting much more top end, just climb out performance. Probably the best part of working on balancing the cooling, is helping the hotter compression engines keep their cool. That said, "cooling drag" is significant in most planes.

135hp Pacer spec.
125MPH cruise

160hp Tri-Pacer spec.
SL 124MPH
7000ft 134MPH

150/155 knt Pacer sounds significant to me. But, that wing drag is a lot to overcome and it sure does lift well.
 
Fortysix12 said:
One thing I have done is closeup around fly wheel between the cowl and flywheel glass and foam. Also sheet metal around the back of the flywheel contour. It's very tight. I'm not looking for speed but mindless climbs without the concern of CHTs and OT getting out of control at high altitudes. One advantage is I feed the firewall mounted oil cooler with only a 2 inch duct. I get a little warm but once I begin to drop the pressure below upper area, things should get better. CHT's are still a concern but not out of control and I would like to get them lower. I will experiment with a manometer when I get one and establish the base line. I'm looking forward to the end results. Closing off the size of front cowl holes might be an option to reduce cooling drag. Ramps are also a big help. I do not have a lip on the bottom cowl and this might be something I install. My numbers are all in the green but just think I can do better. like low green.
Click to expand...

I'm trying to work in the same direction, Fortysix12. My '12 won't go any faster but I want the ability to climb for a very long time on a hot day without concern for the CHTs, and I'm not there yet. #4 always goes above 400 degrees in that scenario. I can manage it but there's definitely room for improvement. Could you post a picture of the work you did up front around the flywheel, and your ramps?

I've blocked off the #2 cylinder a bit with a crude ramp but it seems to me that although it does help #4 somewhat it also raises the CHT of #2 to a greater degree---so in other words it's a quickly diminishing return.

Thanks.
 
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I heard that Yonkin only used 2 positions on his throttle, Wide open and closed.

Jason
 
The guy that owns the pacer named "Miss Pearl" also claims 150 mph cruise speeds. Highly modified fairings, cowling, etc.

Steve
 
Jim Younkin had a fixed pitch prop, I believe a 61" pitch and Jason is correct that he flew it with the throttle to the firewall all the time. He came up with some very ingenious modifications that made the airplane way more efficient but never really finished. From conversations with Jim his next step was a cowl flap to eliminate cooling drag. Frank Sperandeo copied a lot of the mods on Miss Pearl from Jim Younkin's Mystery Pacer.
 
aviationinfo said:
I'm trying to work in the same direction, Fortysix12. My '12 won't go any faster but I want the ability to climb for a very long time on a hot day without concern for the CHTs, and I'm not there yet. #4 always goes above 400 degrees in that scenario. I can manage it but there's definitely room for improvement. Could you post a picture of the work you did up front around the flywheel, and your ramps?

I've blocked off the #2 cylinder a bit with a crude ramp but it seems to me that although it does help #4 somewhat it also raises the CHT of #2 to a greater degree---so in other words it's a quickly diminishing return.

Thanks.
Click to expand...

If you climb full throttle you don't have to worry about CHT's.
 
Search on CHT.... lots of info, there. My efforts were primarily designed to improve cooling, but reduction of inlet area might be effective for drag, once all the cooling mods were taken care of. It didn't help anything in the early stages..
In the formula: x + OAT= CHT I moved x from 390 down to 310.... what is your x?
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Most of the drag on a cub is the frontal area of the wing....profile drag...
 
About Jim Younkin and the Mystery Pacer, you are correct he ran a 74" Prop (Sensenich) fixed at 63 at 6500 feet he reported 23" of manifold pressure at 2600 rpm and true out at 155 mph. I have the article from Sport Aviation March 1989. I would "try" to post the article but it's 7 pages and by the time this topic will be long dead.

Points worth mentioning he talks about cooling drag. He starts by talking about preventing the air from spilling out around the spinner, plus really tightening up all the baffling. He moved the oil cooler from the lower portion of the cowling, and pulled in everything on the bottom of the engine and pulled up the lower cowling to reduce profile drag. In addition to that he said that he paid a lot of attention to the way the cooling air left the cowling. As far as exhaust he had special exhaust made that ran back through a recessed channel in the bottom of the fuselage.

I hope this helps
 
I remember (when rebuilding) thinking that that huge leading edge would be a tough thing to push through the air. I don't have a degree in aeronautics, so someone tell me why the leading edge is not as sharp as the trailing edge??
 
Tim and Tim,

Thx for the explanation, it makes sense. Sorry for the thread creep here. I wonder if it's realistic to expect these O-320s to have an unrestricted full-throttle climb with leaning, and not have a rear CHT creep over 400? Could I just be leaning too much? Sooner or later during the climb the % power is low enough for peak leaning.
 
I believe on most super cubs the limiting factor on speed is the prop. You can only go so fast before you red-line the engine. That said, there are a lot of things I'd look at cleaning up before I spent time on the cooling drag on a Cub.

NimpoCub, If it was, it would stall very violently at a very low angle of attack. And at low speeds a teardrop shape with a rounded leading edge is actually more efficient. A sharp leading edge is mainly a benefit when you get up to Mach .95 or higher. At which point drag is the least of your problems in a Cub. ;)

Phil
 
kaspar2.jpgIf the leading edge was sharp, the stall would be very nasty, and at a lower angle of attack. You could, however, configure it as a (Wittold)Kaspar airfoil, and do vertical landings utilizing vortex lift....lotsa drag, though....the sharp leading edge might not buy you much speed in cruise, though, if you kept the airfoil at 16% thickness...drop it down to 12 %, and that might work better...
 

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Im glad several of you had thought into this.. its always nice to see a good discussion...I had no hopes of making aq cub or similar plane much faster...with all of the effort it takes we could just buy a faster plane.....My intentions were finding out how we can verify they that the air goin through the cowling was one way...just as you mentioned sealing off around the spinner.....it seams that this would be the easy quick fix area on the plane to ensure max efficiency.... for instance i removed and blocked off some of the holes in the back of my baffling when i got rid of the generator, i also blocked off some just below the no 1 and 2 valve covers...i saw a reduced static rpm of 200....I believe it was due to over pressuring the top of the engine and causing a back up behind the prop......

what should the pressure in the top and bottom of the cowling be so that we know all of the air through the prop is being moved on and not wasting energy being compressed
 
At 100mph indicated, the top pressure should be + 4.2 inches, and the bottom just about zero.

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The most effective mod is to block the downflow at the front of the engine= 27 degF
 
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Tim, the carb thing you're talking about may be about the "economizer" valve equipped carbs.

All 0-320 dash number carbs for gravity fuel systems should theoretically flow the same amount of fuel when wide open. You need "X" amount of fuel to make HP. The economizer does not add more fuel when at full throttle, it reduces the amount of fuel at partial throttle near the 75% power range. If I remember corectly it has something to do with adding a bit more back pressure suction to the float bowl which reduces the fuel flow through the main jet. It's not actually a valve but an extra hole in the throat of the carb that when the throttle plate moves past it the port goes from the high to low pressure side of the Venturi. What happens with economizer carbs is that the temps go up as you pull the power back before they start to go down. A non economized carb should have a constant temp reduction as you pull the throttle back. Ever notice why some planes you have to pull the mix out a lot and some only a little... That's quite often due to the difference between the economizer and standard carbs.

You never heard about engine temp problems on stock cubs with stock cowlings and stock baffling and stock engines. The farther from stock you get the more issues I seem to hear about. Cubs ran fine for 40 years and many of them well past TBO with no issues before people stared putting 4 channel engine analyzers in.

Jason
 
Jason, I burned up five cylinders before I installed a lot of instrumentation to try to find out what the issue was. Once you put the same mods in the cowl that most other aircraft have, it cools just fine. If you didn't have those mods, and you were flying up north and at low altitudes, then it wouldn't be much of a problem, either. But try flying in the high desert, in the summer, and ffffft....$$$$...I agree that a cruise climb at reduced power, is problematic...better to go full throttle.
 
aviationinfo said:
Tim and Tim,

Thx for the explanation, it makes sense. Sorry for the thread creep here. I wonder if it's realistic to expect these O-320s to have an unrestricted full-throttle climb with leaning, and not have a rear CHT creep over 400? Could I just be leaning too much? Sooner or later during the climb the % power is low enough for peak leaning.
Click to expand...

Fuel cools an air cooled engine as does oil and air flowing over it.
 
Younkin's Pacer

For what it's worth, here's the article in PDF form.
 

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