Cooling with J-3 style cowls. (Pietenpols, Cubs, Biplanes, etc)


Several people have been discussing these types of cowls for their Corvair powered planes. Clearly they work on 65 hp Cubs, why not on a Corvair powered Piet? Well, they can and do, but there are subtile points to the design of these things that are just as critical as enclosed cowls. I am quite sure that Piper learned these by trial and error before standardizing the “J-3 Eyebrows” that people tend to use on 65 hp Continentals. As we go over this, keep in mind that when Piper went to 90 Hp engines in Cubs after the war, they switched to regular pressure cowls. Every Super Cub has one of these, and there is little argument against the proof  that they work better, even on slow planes. However, if your heart is set on a J-3 style cowl, please read the following notes to avoid harming your engine.

Above, Frank Metcalfe’s plane at sun n fun. This installation works. Look at how large the eyebrows are in relation to other examples. If a 65 hp Piet and a 100 hp Piet are both climbing at 55 mph (They will have a very different rate of climb) it makes sense that the 100 hp plane will need more cooling air. Yet, I see too many Corvair powered planes with eyebrows that are smaller than the ones on a J-3. Think that over.

Above, our Pietenpol in 1999. This system worked great, but the two versions I made before it didn’t. TLAR (that looks about right) does not apply here, evaluation and testing does. In a phone call today, a Piet builder told me that a set of Corvair eyebrow scoop drawings are circulation on the net. What is the first question to ask? Have they ever flown and been proven on high hp Corvairs in hot weather? He was not so sure. If you want advice on down parkas, I am not your guy. If you need advice on cooling systems that work in hot weather, ask the guy in Florida.

Above, several details, some visible, some not. Notice that the scoops extend downward. They capture air that would run under the front cylinders at high angle of attack and ruin cooling. Notice the rounded nose bowl and spinner. If you want to have a flat plate as big as an end table, you will need to have much larger scoops to make up for this. Note also that the alternator is in the back. If you have a front one, it will work, but again the scoop must be bigger. (Dan Weseman has just finished testing his rear alternator, and it is the only one I endorse) The most critical part of this whole equation can not be seen: under the cowl there is a 3.5″ diameter hose connecting the two sides together. Without this, you are hurting the engine. You have a choice: connect the two sides, or use scoops 50% bigger. If you copy the size here, and then use a very blunt cowl and no transfer hose, than you are not doing anything positive for yourself, my reputation as an engine instructor, or the Corvair movement.

OK, now we get to the big quiz: Would you rather spend an hour reading something that requires a little thinking, or would you rather fry the heads on your engine, spend $1,000 or so, and loose half a seasons flying doing a rebuild? Right now you are thinking that 100% of the builders thinking of J-3 cowls are going to choose the first option, but you are not right……

This link :

goes straight to a 16 page story ( it has pictures, it is only about 5,000 words) about how I had to rebuild Gardiner Mason’s engine several years ago after he used TLAR to design a very blunt cowl. I like Gardiner, so he just bought the parts, I did the work, then wrote the story. KNOW THIS: I am not ever going to assist anyone for free to rebuild another engine that cooked it because: “I saw that story but I didn’t have time to read it because 1) It was sooo long 2) The big game was on 3) I didn’t think it applied to me because my plane is gray not red.” All future rebuilds will be done at the shop labor rate that Lockwood Aviation charges (the US importer of Rotax engines) Just read the story, learn something, save yourself a $1,000 in damage and preserve a little of my sanity. Please.

Above is Gardiner’s plane, the focus of the 16 page story. It does not look like it has a J-3 cowling, but it functions like one. This is a blunt cowl. This means you need bigger scoops. It isn’t hard to put some effort into making the front end smaller. It doesn’t just help cooling, the plane will be faster and look better. On a Pietenpol, a plane with a bad cowl and poor windshields actually has less elevator and rudder feel.  If the entire fuselage is bathed in a foot thick boundary layer of very turbulent air, you can feel this when it gets back to the tail.

 So, you were planning on reading the 16 page story after watching Dancing with the Stars and the Celine Dion concert? Keep in mind that this website has story tracking on it, and I will tell if 600 people read this but only 150 click on the link to the Gardiner story. Lucky for all the Celine Dion fans it doesn’t keep track of who didn’t read what. Just how many people didn’t. Don’t worry, I will still be able to tell who read it by reading the Pietenpol archives and seeing who writes in asking what to do after their engine severely overheats…….

Below is a sample from the 16 page story:

” Here is the major cooling issue of a propeller-driven aircraft that many builders don’t understand. When the plane is climbing at a 10 degree angle of attack, the blade roots near the cooling inlets have a 20 degree difference in their angle of attack between the effective angle of the ascending and the descending blades. They pump very different amounts of cooling air into each side of the engine. This is not theory, it is fact. Get into a light plane, fly to a safe altitude, slow it down to its best angle of climb speed and set it to full power. Notice how much rudder you have to put in to hold the aircraft heading. You may have been told that this was some swirling slipstream or “P” factor. Discard those ideas. A strand of yarn behind an engine on a test stand will show you the air even at zero airspeed doesn’t corkscrew much, and “P” factor does not apply to aircraft in steady flight like a continuous climb on one heading. What is going on is far more simple; the ascending and descending blades are making very different amounts of thrust. You feel it in the rudder pedals, the engine feels it in differential cooling.”

Above, the Pietenpol of Kurt Shipman. This plane won the Bronze Lindbergh trophy at Oshkosh. Before anyone starts saying how nice work Kurt does, keep in mind that there were 24,500 other planes at Oshkosh that year, and the judges were able to find four others that they thought were ‘better’ than Kurt’s. So he beat out 24,495 other planes…..that still isn’t first place. Nice try Kurt, maybe you can do better on your second home built.

Seriously, I flew in this plane and it is phenomenally well done. Look at the cowl. This is a typical pressure cowl. This is built off the nose bowl mold that Piet builder/flyer Shad Bell made. Rounded edges and reduced area like this make a huge difference. If you don’t have a committed feeling about cowling design, I highly encourage you to look at this set up. It works very well, and has a front alternator. Either system you choose, use all the available data that has been flight proven in hot weather, do a good job, and don’t alter the details without good reason.-ww

5 Replies to “Cooling with J-3 style cowls. (Pietenpols, Cubs, Biplanes, etc)”

  1. So p-factor is the asymmetric thrust caused by the descending blade having a higher angle of attack and airspeed than the ascending blade when the aircraft is at a positive angle of attack. You describe this phenomenon, but say it is not “P” factor since ““P” factor does not apply to aircraft in steady flight like a continuous climb on one heading;” have you confused p-factor with gyroscopic precession?

    1. Russell, it isn’t gyro precession, because in a steady climb, you need constant rudder input, but once established there is no continuing gyroscopic force the rudder is addressing.

      1. That was my point. The claim that “”P” factor doesn’t apply in steady flight” is false, but the claim gyroscope precession isn’t a factor would be true, or the claim that spiraling slipstream is negligible would also be true.

        P-factor is the asymmetric thrust caused by one side of the prop disc having a higher AoA than the other, which is exactly that you said was causing the need for rudder input and asymmetric cooling, right after you made the claim that it is not P-factor.

        This post incorrectly defines P-factor. I was hoping you could add a correction and was also wondering what you mistook P-factor for.

      2. Russel, you are misreading the excerpt in the story and taking it out of context, my point is that what what people are taught “P-Factor” is, some type of swirling slipstream, is not the primary, or even a major factor at play. The entire article is about asymmetric thrust, caused by angle of attack differences on accending and decending blades, and the difference is greatest when the plane is climbing. My point is that the common definition of P factor isn’t what is actually going on. You are actually agreeing with me but are getting off track missing that the complete article speaks of the public misconception of the term.

      3. I have yet to meet someone that mistook P-factor for spiraling slipstream. If indeed it is a common misconception as you state, then you are contributing to it’s continuation by misusing the term. P-factor is asymmetric thrust.

        I do agree with the rest of the article connecting asymmetric thrust to asymmetric cooling, but you’ve muddled spiraling slipstream and P-factor together. This was originally brought to my attention by a pilot working on his own experimental who showed it to me when he was trying to figure out what you were meaning and couldn’t because while he knew the correct definitions he was assuming you knew more than he did therfore he must be missing something.

        If someone doesn’t know the definition of P-factor then they will have the incorrect definition locked into their vocabulary. If they know enough to recognize this error they may assume you don’t know what you are talking about and go elsewhere for information, which would be a shame as you do have good pointers for homebuilders.

        I want you to continue to be a resource in the homebuilder community and was pointing out an error that confused at least one homebuilder out there.

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