Cowl Flaps

[rodsmith]

I'm ready to build cowl flaps. Would like to contact Ron Jones to get more details of his if anyone has his email. Has anyone besides Russ and Ron built cowl flaps?

[alaskabearhawk]

Sent you a PM.

[Battson]

Yeah I did it too.

Very effective at what they do, in my case they are good for at least 20*F or more often 30*F of cooling or heating as desired. And mine are small.

My biggest regret is I was talked out of giving them more control of the airflow. Specifically, I would love the ability to close off airflow more and keep the engine warmer for longer during a long letdown.

Cowl flaps are AWESOME with a big engine. They bring huge flexibility to the landing, descent, and climb phases of the flight regime. If you are constrained by your flight path - say by terrain or weather or ATC, but also trying to take care of your engine temps while flying the safest pattern, they are your best option. Don't let people talk you out of it, if you're using a 6 cylinder engine especially.

[Chris In Milwaukee]

Are your cowl flaps manually operated or electric? Either you or your actuators are quite smooth. :-)

[Battson]

Haha - it's just a steady hand. No electrics there.
They are operated by a white push-pull knob next to the mixture, so I have 4 push-pull controls in a row.
Quite easy to remember, for climb or go around, all 4 knobs go toward the firewall.

[Mark Goldberg]

This is the homebuilt/experimental world. So everyone has the freedom to do what they want. But I have never found cowl flaps needed on my BH with the 540. Mark

[rodsmith]

A lot of people don't get the cooling right the first time, especially with the 540s, and end up adding louvers or a bigger lip on the lower cowl. I see cowl flaps as a tool to optimize cooling. As Battson said, you can make sure you have enough cooling on a hot day, and close the cowl flaps to get better cruise speeds when able or protect the engine in descent. If I had a 240HP engine I might be less inclined to go to the extra work, I'm expecting 280HP or maybe more from mine.
Rod

[whee]

I plan to build cowl flaps similar to Battson's. The TCM IO360 is supposedly sensitive to cooling issues. I don't know that I'll need cowl flaps but I'd rather have them from the beginning.

[S Lathrop]

I am also ready to build my cowl flaps, also. Is there a reason not to build a single flap such as Russ Erb did? I know he had cooling issues even with the flaps.

I built cowl flaps for my son's Cyclone C185. When he got the plane, it did not have flaps and ran hot. I built 2 flaps as Battson has.

[rodsmith]

My thought is one flap the full width of the tunnel. The opening angle would be less for the same area of outlet. I'm thinking there would be less drag when open.

[Battson]

My thought is one flap the full width of the tunnel. The opening angle would be less for the same area of outlet. I'm thinking there would be less drag when open.

With respect, I thought the opposite would be more useful to you, which is why I have two flaps which open at a sharper angle. This was very intentional.
Those homebuilt aircraft with one larger cowl flap at a shallow angle had limited success (in terms of added cooling control) based on my research and review...
Here is why I thought that way, there was some research behind this:

To oversimplify the idea, added drag in that area means added engine cooling. Here's the logic path:

1. a lip or cowl flap at the cowl outlet creates turbulent vortices (added drag),
2. vortices accelerate the airflow in this region,
3. accelerating the airflow creates a lot lower air pressure,
4. lower air pressure creates more suction through the cowl,
5. suction creates extra airflow over the engine,
6. extra airflow cools the engine more effectively.

It's the same reasoning why we add a lip at the cowl outlet if we're not using cowl flaps. Thus the more effective the lip or cowl flap is at creating turbulent vortices (drag), the more cooling effect they create when deployed (if deployable).

This is one major reason why I like cowl flaps - you can choose to deploy them when you need them, retract them when you don't. Otherwise you are creating that cooling drag all the time, whether you need it or not! Of course the other (major) source of cooling drag is airflow through the cowl.

So the counterpoise is, cowl flaps which create very little drag when deployed will create less pressure difference to suck air from the cowl. So, unless you create a much larger opening which lets more air out of the cowling, it's likely the cooling effect will be less. It's hard to say which solution is lower drag overall like you might want in a speed plane, but I can certainly say which one is more effective in a steep climb at low airspeed, less obtrusive visually, and lighter weight by virtue of it's smaller size.

That was my logic at the time, YMMV.

[Battson]

Oh - and the other issue with a full width flap, is how you support the cowling under the engine at the hinge, so it doesn't oscillate at the hinge line as the propwash beats down on it (assuming it's only supported at the sides and at the back, the front will try to oscillate).

[rodsmith]

With respect, I thought the opposite would be more useful to you, which is why I have two flaps which open at a sharper angle. This was very intentional.
Those homebuilt aircraft with one larger cowl flap at a shallow angle had limited success (in terms of added cooling control) based on my research and review...
Here is why I thought that way, there was some research behind this:

To oversimplify the idea, added drag in that area means added engine cooling. Here's the logic path:

1. a lip or cowl flap at the cowl outlet creates turbulent vortices (added drag),
2. vortices accelerate the airflow in this region,
3. accelerating the airflow creates a lot lower air pressure,
4. lower air pressure creates more suction through the cowl,
5. suction creates extra airflow over the engine,
6. extra airflow cools the engine more effectively.

It's the same reasoning why we add a lip at the cowl outlet if we're not using cowl flaps. Thus the more effective the lip or cowl flap is at creating turbulent vortices (drag), the more cooling effect they create when deployed (if deployable).

This is one major reason why I like cowl flaps - you can choose to deploy them when you need them, retract them when you don't. Otherwise you are creating that cooling drag all the time, whether you need it or not! Of course the other (major) source of cooling drag is airflow through the cowl.

So the counterpoise is, cowl flaps which create very little drag when deployed will create less pressure difference to suck air from the cowl. So, unless you create a much larger opening which lets more air out of the cowling, it's likely the cooling effect will be less. It's hard to say which solution is lower drag overall like you might want in a speed plane, but I can certainly say which one is more effective in a steep climb at low airspeed, less obtrusive visually, and lighter weight by virtue of it's smaller size.

That was my logic at the time, YMMV.

Well I didn't get the cowl flaps built back in December, headed off in another direction. I like your design. Would you mind providing dimensions, hinge type, material, any reinforcement you used.

Thanks,
Rod

[Battson]

Hi Rod,

We just used the following to make them:
Regular piano hinge.
Doubled 0.025 AL with a shape to stiffen.
Dimensions are approx.: 5" wide, 8" long, 4" deep when open depending on air pressure. I would have made them 30% bigger, if I had my time over.


They are nothing fancy - simple and effective, in keeping with the whole Bearhawk mantra.

[rodsmith]

Thanks Battson!

[whee]

My plan has changed a little since December and I imagine it will change again. I'm planning to install my cowl flaps in the sides of my cowl like Cub Crafters does. Antisplat has some cool actuator operated flaps that look pretty nice but are spendy for what they are; I'll probly build something similar.

[whee]

Battson, You still thinking 30% larger would be good? I'm currently fabricating my cowl flaps and have come up the following dimensions: 8.25" long x 6.5" wide and opens 4.5" That's about 30% larger than yours.

The anti-splat flaps were smaller than I wanted so I've opted to build my own. I will be installing them on the sides of the cowl similar to a Carbon Cub. I've also made my opening at the tunnel quite tight and it is partially blocked by a muffler. This should allow the engine to reach the appropriate engine temps during cruise.