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  • #16
    I originally planned on actuating my cowl flaps with cables or pushrods. Turned out the only place I could fit my oil cooler negated that possibility. Will be using the Acutronix P16 linear actuators. That was the one they recommended, has been used successfully in that application.

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    • #17
      Originally posted by rodsmith View Post
      I originally planned on actuating my cowl flaps with cables or pushrods. Turned out the only place I could fit my oil cooler negated that possibility. Will be using the Acutronix P16 linear actuators. That was the one they recommended, has been used successfully in that application.
      I want to do the same with the P16. I'm trying to figure how to only use one though, I want to use the resistive position setting via a LAC board but it won't drive two actuators.
      Dave B.
      Plane Grips Co.
      www.planegrips.com

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      • #18
        I too am using the P16. They should work pretty good. I’ve been wondering if I should use a momentary switch or not. Partial open cowl flaps may be of use sometimes and would be easier to manipulate with a momentary switch but I’m thinking I’ll use a regular on-off-on switch.



        Roughly modeled the doors in SolidWorks to determine linkage dimensions.

        Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.

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        • #19
          If I was going to do it, which currently I am not planning too, I think I would use louvers with a sliding shutoff. In other words you have a plate with slots cut in it that slides to open or close the louvers.

          One big cowl flap would weaken the structure, and the flap would be quite flexible making smooth actuation difficult. I like Jonathons' and Daves' CF better.

          I am hoping not to want or need cowl flaps, but you never know. I will start out without them, and if I can't live without them, I will make some. I thought the same with a skylight, and I am already adding a skylight.

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          • #20
            As an alternative, has anyone ever heard of dumping air out from inside the baffling? Size the bottom exhaust for worst case hot weather/high power, and have a flap(s) that spill air out from the inside of the baffling. It might take a much smaller flap area to dump slightly pressurized air out of the cowling, than using negative pressure to suck more air through the engine with a normal exhaust cowl flap.

            The flap would be used to keep the engine warmer during low power/low temp.

            Just a thought.

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            • Bdflies
              Bdflies commented
              Editing a comment
              I completed understand what you're saying. Never heard of it. In general terms, it makes sense and I can't see why it wouldn't work. Only issue I can imagine is locating the dump valve. I could imagine a danger of diverting too much airflow through a cylinder, or two. Wouldn't be good for 4 to be nice and cool, while 1 or 2 are hot. Just a thought.

              Bill
              Last edited by Bdflies; 12-13-2018, 05:22 PM.

          • #21
            Originally posted by whee View Post
            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.
            I have enlarged my cowl flaps during the rebuild. The way I did it, they are not opening a larger area at the choke-point, when full open. This was a mistake in hindsight. They don't make things much cooler when they are full open, so I guess that's a function of the choke point, not how much they protrude out into the air stream.
            They do restrict the opening a lot when closed fully, which was my first priority. This has definitely made things hotter when closed, much hotter. This is a considerable benefit when cruising at high altitude in winter, particularly when running lean of peak.]

            About the idea of "partially open" cowl flaps. I would suggest you always want them full open or full closed, from our experience.
            Either the engine is running too hot and you want them full open, or it's not and you can leave them closed. There is no reason to control the engine temps to within a few degrees.
            Last edited by Battson; 12-13-2018, 03:40 PM.

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            • #22
              Ok, thanks Battson.
              Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.

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              • #23
                BDflies;
                I have read several threads of oil cooler installs changing CHT's as well. The baffling forms a manifold big enough to function as a true manifold with equal pressure. I would probably put the flap at the rear baffling, opposite side of the oil cooler.

                I have never read that the oil cooler makes one CHT hotter than the other. They are offset one side, so I believe it doesn't matter.

                At the moment, I am trying to figure out why this won't work, and is easier to implement.

                Size the exhaust so it works well with the inlet on a hot day at high power. Bypass some of the cooling air on a cold day/long letdown.

                Not there yet, but I am seriously thinking about it.

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                • #24
                  I would love to have the problem of too cold cht! I've just finished installing the round lip at the bottom edge of the firewall and look forward to testing it.

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                  • #25
                    I wish I had more experience with light aviation, but sometimes being unencumbered with convention is actually an advantage. Other than the last year, I haven't thought much about what we are doing here since I was 16. But I thought a lot about it then.

                    I am sure every thing we can think about with cowl flaps has been tried before. Variable inlets, variable exhausts, variable valves in the cooling manifold (plenum). If I was going to guess, I would say I am in the lower 10% of experience with this.

                    If you have a variable inlet, you will change pressure with a fixed exhaust.
                    If you have a fixed inlet, but variable exhaust, you will have pressure change as well.
                    In both cases, you might have an overpressure situation sometimes were cooling air is wasted by pushing out cowling and spilling air.

                    If you have a fixed inlet, and fixed exhaust, of proper size, they should be in balance. By spilling air from the baffling, or plenum, you change the volume of air going through the cylinders, but not the total volume of air going through the inlet or exhaust. By my logic, the volume doesn't change.

                    Like I said, I am pretty sure everything we can think of has been done many times. We are just relearning it.

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                    • #26
                      Originally posted by svyolo View Post
                      As an alternative, has anyone ever heard of dumping air out from inside the baffling? Size the bottom exhaust for worst case hot weather/high power, and have a flap(s) that spill air out from the inside of the baffling. It might take a much smaller flap area to dump slightly pressurized air out of the cowling, than using negative pressure to suck more air through the engine with a normal exhaust cowl flap.

                      The flap would be used to keep the engine warmer during low power/low temp.

                      Just a thought.
                      My impression regarding dumping air..... Air that enters/exits the cowling is drag. So, bypassing air from the high pressure side of the cowl to the low pressure side is simple, but in-efficient compared with restricting airflow thru the cowl. (like closing a cowl flap, or downsizing the air inlet)

                      As an example, At OSH last year I was checking out Bob B's Patrol. We talked about his cabin heat/arb heat set up. Bob's is the first I've seen that does not dump hot air overboard. Bob said something like "It's a wives tale that the heated air has to be dumped overboard. That Scat is good up to 500F. I don't dump air overboard because its less flow thru the cowl, and decreases drag."

                      I also believe that a very draggy part of the fuselage is the belly. Evidence is that belly pods typically don't cause an airspeed penalty.

                      So, my impression is airflow dumps out the bottom is entering the slip stream to an already draggy area. Dumping air out the side of the cowl to a clean area I'm thinking might make it a little more dirty. Cub have Cheeks. Cubs are slow. But these are feelings & impressions I have and not based on science.
                      Brooks Cone
                      Southeast Michigan
                      Patrol #303, Kit build

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                      • #27
                        Originally posted by Bcone1381 View Post

                        My impression regarding dumping air..... Air that enters/exits the cowling is drag. So, bypassing air from the high pressure side of the cowl to the low pressure side is simple, but in-efficient compared with restricting airflow thru the cowl. (like closing a cowl flap, or downsizing the air inlet)

                        As an example, At OSH last year I was checking out Bob B's Patrol. We talked about his cabin heat/arb heat set up. Bob's is the first I've seen that does not dump hot air overboard. Bob said something like "It's a wives tale that the heated air has to be dumped overboard. That Scat is good up to 500F. I don't dump air overboard because its less flow thru the cowl, and decreases drag."

                        I also believe that a very draggy part of the fuselage is the belly. Evidence is that belly pods typically don't cause an airspeed penalty.

                        So, my impression is airflow dumps out the bottom is entering the slip stream to an already draggy area. Dumping air out the side of the cowl to a clean area I'm thinking might make it a little more dirty. Cub have Cheeks. Cubs are slow. But these are feelings & impressions I have and not based on science.
                        It's been a few years since I read the selection of research papers I found on aircraft engine cooling. The gist was that the belly of an airplane is a high pressure zone thus reducing the pressure difference (Dp) between the inlet and the outlet. The sides of the cowl are not a high pressure zone so the Dp will be greater. The reason most aircraft manufactures didn't put the cooling air exit on the sides was for aesthetics.

                        Additionally, some 180/185 owners that have installed belly pods have reported that their engine runs hotter with the pod installed. They determined the pod caused a localized increase of pressure right at the front of the pod. This location is also where the cowling outlet is so the pod is causing an additional loss of Dp.

                        Since I'll be installing a pod, flying in cold temps during the winter and hot in the summer, and plan to operate LOP a once size fits all approach to engine cooling wasn't going to work for me. We'll see if what I came up with works.
                        Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.

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                        • #28
                          I can honestly say I have never given this any thought in the past. With a fixed inlet (no cowl flaps on the inlet side), the amount of air entering is fixed, unless too much pressure is created and it spills back out the front. Exhaust(air) cowl flaps allow more air/less pressure air to exit from the cowling.

                          I was proposing dumping air out of the baffling, sort of an internal cowl flap. The amount of air entering and exiting the cowl would be fixed. The only difference would be the flap on the cowling would bypass or "bleed" air from engine cooling.

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                          • #29
                            Originally posted by whee View Post

                            It's been a few years since I read the selection of research papers I found on aircraft engine cooling. The gist was that the belly of an airplane is a high pressure zone thus reducing the pressure difference (Dp) between the inlet and the outlet.
                            Yes I remember reading this logic too. Also the heat wants to rise, further worsening the belly cowl exit location.
                            On pusher aircraft, the exit vents are often found on the top of the cowling for this reason. Although pushers can have cooling problems for a different reason.

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                            • #30
                              Some of the PA-32s had exhaust from the top, if I recall. Aside from a disaster if there was an oil incident, any reason why one wouldn’t/shouldn’t up-cool?
                              Christopher Owens
                              Bearhawk 4-Place Scratch Built, Plans 991
                              Bearhawk Patrol Scratch Built, Plans P313
                              Germantown, Wisconsin, USA

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