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Cowl flap advice

NYTOM

Well Known Member
Reading through the archive's it seems a cowl flap can be a good investment towards to control of cylinder head temps. And I see some have installed two. My question is if one cowl flap is good, is two cowl flaps better? Would rather take the time now to install a second flap if it's going to make a readonable difference. We have about 40 days over 90 here and the heat can be brutal.
 
The first thing to ask is if you have your baffles and seals working properly to begin with. And this includes addressing the zero fin depth areas on the front of #2 and the back of #3 (remedied with bypass ducts). Generally, the standard Vans cowl provides plenty of differential pressure to cool effectively. My 200 HP -8 has a stock cowl and cools just fine in 110+ temps. I did have to take it easy leaving Arizona one day, but the OAT was 121...

Next thing to look at is ignition advance. If you are EI, do you know what the timing is doing at TO power? If you have a CPI for example, you can retard timing (from data plate values) and help your CHT right off the bat with no loss of power.

A cowl flap can be a good thing if its used to pinch down the exit at altitude to reduce drag and increase temps, but if you're adding one to increase exit area, that's probably a bandaid fix for another problem.
 
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Reading through the archive's it seems a cowl flap can be a good investment towards to control of cylinder head temps. And I see some have installed two. My question is if one cowl flap is good, is two cowl flaps better? Would rather take the time now to install a second flap if it's going to make a readonable difference. We have about 40 days over 90 here and the heat can be brutal.

Have you flown the airplane? Since I adjusted the cylinder blockers properly during the first year or so of flight, CHT's have never been a problem on my -6.

Where I'm going is that I wouldn't spend the time and money on a flap or flaps unless you have flown the airplane and have tweaked things for proper cooling.

But to answer your question, yes, two would be more effective than one IF you need the additional airflow.
 
Not looking to solve any current problem, just want to do it as best as I can the first time. Have read all I can on baffles and problems and hope to avoid the usual stuff. Already planning fabricating bypass ducting on the problem areas and hope my sealing is more than sufficient. It's just so darn hot here most of the year that I wanted to get some insight on cowl flaps and if having them would be a wise decision. Have looked at many baffle installation on RV 's and it's amazing how different they can be. I guess it's all in the fine detail.
 
Not looking to solve any current problem, just want to do it as best as I can the first time.

Making an RV the best it can be means different things to different people.....

To one it means not doing any more build effort, spend more money, or add any additional operating complexity to the airplane beyond what is necessary.

To another it means installing every gadget possible.

Since it is rare I have inspected an RV that the engine baffling system couldn't be improved (regardless of what the owner thought), my recommendation would be to not install cowl flaps until you have determined that you are unable to resolve a cooling problem any other way.
 
I understand your desire to head off as many "problems" as possible during construction, but I think it would be best to embrace the notion of build, then test, then adjust. I speak from experience here.

During construction I had 2 friends whose engine oil temps were lower than they wanted, even during mild temperatures so I thought I would head off that problem during the build and just install the louvered oil shutter. My thinking was that there was no real downside to having it. Well, from day one my problems were too much heat rather than not enough. In the process of trying to fix the problem, I wound up taking the oil shutters off because I realized that in the open position they still cut off some of the air flow across the cooling fins of the oil cooler and I needed all of cooling I could get. It still wasn't enough. I then resorted to installing a larger oil cooler, then installing two cowl flaps. Both oil temperature problems and cylinder temperature problems were fixed, but only after testing, getting lots of advice and implementing EVERY suggestion that implied I had screwed up the baffles and/or installation.

Every build is different. With all of the choices we have, we have to acknowledge that, even when done as well as humanly possible, the result of the choices will yield differences in performance, many of which are hard to predict until you test it. Both my friends who had too-cool running engines came over and helped address whatever faults they could think of with my installation. Ultimately, they acknowledged that on my plane, with my engine, ignition, compression ratio, nickel cylinders, etc, etc, etc, my plane ran much hotter than theirs.

Build to the best of your abilities. Then have lots of experienced eyes look for issues. Then test PROPERLY. Don't just fly off the hours. And folks who imply that you didn't build it right if you have problems just got lucky with their build or are forgetting or minimizing things they needed to address. Yes, use their suggestions to check if you in fact did screw up, but filter out the implications that "if it isn't perfect right out of the box, you suck as a builder".

You will also need to figure out both who knows what they're talking about AND does that advice work with my priorities. I have another friend who does stunningly beautiful work try to convince me that I needed to paint everything before initial assembly. He was speaking from experience in how to MOST EFFICIENTLY make an airplane beautiful. I ignored his advice because I knew that me building an airplane wasn't about efficiency in the build (7 years!) and also that I knew of planes with problems that should have been addressed but weren't because the owners didn't want to screw up the paint job. I didn't want to be that guy. Just embrace the concept that this is a hobby and is supposed to suck up all your time:D

In the case of cowl flaps, I don't see a downside to installing them during the build, except of course cost, weight and complexity but remember that I didn't see the downside of the oil shutter and wound up needing to remove it, so me not seeing too much of a downside should not give you much comfort:D. I wound up installing them after I knew I had a problem and implemented every other suggestion on how to fix the problem.

And of course, I only discovered the effective solution AFTER the plane was painted:eek:
 
...my recommendation would be to not install cowl flaps until you have determined that you are unable to resolve a cooling problem any other way.

+1. After 400 hrs of reasonably cool engine temps, I decided to seal up the baffles as best I could during some maintenance. I thought I had done a good job initially as I only had to lower the nose occasionally to keep the CHTs below 400 after a quick turn during the summer months in Texas. $14 at NAPA for red RTV and 3 hours later, temps are now consistently 20 degrees cooler.
 
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Like they say, there's no substitute for experience and this is exactly the type of reply I was hoping for. Thank you gentlemen. I will proceed as you suggested.
You can learn so much from others experience here, all you have to do is ask. :D
 
I've installed one flap and love it! No worries about rising temps on extra long taxies around Texas airports on hot summer days.
That's the perfect use-case.

Reading through the archive's it seems a cowl flap can be a good investment towards to control of cylinder head temps. And I see some have installed two. My question is if one cowl flap is good, is two cowl flaps better? Would rather take the time now to install a second flap if it's going to make a readonable difference. We have about 40 days over 90 here and the heat can be brutal.
 
Like they say, there's no substitute for experience and this is exactly the type of reply I was hoping for. Thank you gentlemen. I will proceed as you suggested.
You can learn so much from others experience here, all you have to do is ask. :D

...Well tom, I agree that baffling, timing, mixture and all the normal possible causes for excessive heat should be addressed, and corrected if possible first. One would assume this to be the case prior to any further modification. I also wholeheartedly agree that there is no substitute for experience, especially in the aviation world. Cowl flaps have been in use on virtually every commercially manufactured general aviation, as well as military aircraft throughout the world since the conception of the cowl itself. Definitely not a revolutionary concept by any means. They are without any doubt the most ideal method of obtaining large gains in cooling airflow, as in a climb, taxiing, operating in elevated air temperatures etc. All of this advantage and control at your fingertips with no penalty in cruise speed whatsoever. The "Easy Cool Flaps" are a very simple, quick installation, that really doesn't add much complexity or maintenance concerns to your aircraft. They increase the utility, reliability, and lifespan of your engine, by keeping the temperatures down when it is working hard and the demands are the greatest. They also add a considerable increase in comfort level, knowing you don't have to worry about excessive climb out temperatures, the need to push the nose over or pull off the power off to control them. You shouldn't be required to fly with any special maneuvers or odd practices to keep the temperatures in check! You wouldn't accept this in your car every time you wanted to go somewhere or came to a hill? Why have the power if you can't use it? Thanks, Allan..:D
 
Agree with most of the above with the subtle, but important distinction that cowl flaps are generally an engineered solution which provide both large exit airflow for high demand situations AND a small exit for drag reduction at cruise. The RV cowl is a compromise between these two and favors excess cooling at the expense of drag reduction. This is evidence with those that pinch off the exits and see speed increases and proper temps, and those like me who have the stock exit size, but see our CHT "too cool" at cruise.

So in essence, your cowl flap adds exit area to a cowl design that is already too large for the cruise configuration. Yes it works, but to say there is no speed penalty requires the qualification: "...compared to a stock RV cowl..."
 
During the build?

As others have mentioned, the standard RV cowling works pretty well for most people with proper attention to sealing. One thing I have noticed on many RVs that I have inspected is that the little tab on the baffle which is supposed to be bent to be very close to the cylinder base of #3 is often left wide open. Even if the tab is bent properly, that whole area should be sealed with RTV and often isn't.

I modified my cowl during build a la Dan Horton's "Shrinking Exit" by increasing inlet size and decreasing exit area. I was concerned that I would have excessive CHTs and oil temps during taxi and climb so I took drastic measures to increase airflow through an oversize cooler and I cut in very large cowl flaps. As it turns out, I have to choke off most of the air to the oil cooler just to get into the green and don't need the cowl flaps for taxi and climb even on 100 degree days. Now that the weather has cooled to about 90 degrees (much cooler than that today), my hottest cylinder is about 340 down low with high power set. So - I definitely have excess cooling capacity. My intent is to shrink and extend my exit scoop further which will enclose my exhaust pipe and add some augmentation to the mix. My hope is that I'll reach a point where I do need the cowl flaps for ground ops and climb but not for cruise.

As pertains to this thread, I wouldn't do cowl flaps during build on a standard installation. If attention to detail is employed, it'll cool ok. If it runs hot anyway, then the addition of cowl flaps is a viable method of adding mass airflow for cooling.

Ed Holyoke
 
Agree with most of the above with the subtle, but important distinction that cowl flaps are generally an engineered solution which provide both large exit airflow for high demand situations AND a small exit for drag reduction at cruise. The RV cowl is a compromise between these two and favors excess cooling at the expense of drag reduction. This is evidence with those that pinch off the exits and see speed increases and proper temps, and those like me who have the stock exit size, but see our CHT "too cool" at cruise.

So in essence, your cowl flap adds exit area to a cowl design that is already too large for the cruise configuration. Yes it works, but to say there is no speed penalty requires the qualification: "...compared to a stock RV cowl..."

...I am aware and in agreement with most of the above. This being said, it doesn't change the fact that many people are experiencing very high CHTs on climb-out on hot days. They are finding it necessary to compromise their flight path to accommodate temperature issues. This is not necessary and to us is unacceptable! The cowl flaps only address this climb-out issue, and cruise speed is an altogether different subject. We have looked at cruise airflow as well, and are working on a product package to address this also. Our prototype is in testing now and showing very favorable results. Our 9-A is seeing cruise speeds of 205 to 210 true at altitude, with fuel burn of 7 GPH with a stock 0-320 160 HP. We hope to offer this package for purchase in a couple of months. Thanks, Allan...:D
 
...I am aware and in agreement with most of the above. This being said, it doesn't change the fact that many people are experiencing very high CHTs on climb-out on hot days. They are finding it necessary to compromise their flight path to accommodate temperature issues. This is not necessary and to us is unacceptable! The cowl flaps only address this climb-out issue, and cruise speed is an altogether different subject. We have looked at cruise airflow as well, and are working on a product package to address this also. Our prototype is in testing now and showing very favorable results. Our 9-A is seeing cruise speeds of 205 to 210 true at altitude, with fuel burn of 7 GPH with a stock 0-320 160 HP. We hope to offer this package for purchase in a couple of months. Thanks, Allan...:D

And some of the rest of us have tried to to convey that there are many ways to resolve over heating problems.

One extreme would be to sell the airplane and buy one that doesn't overheat.
The other would be to zero in on the root cause of the problem and fix it.

Adding a cowl flap (or two) to a cooling system design that the major majority of the same airplanes cool well without, is another method of solving the problem, but not the best way in my opinion.
 
...I am aware and in agreement with most of the above. This being said, it doesn't change the fact that many people are experiencing very high CHTs on climb-out on hot days...

We are in agreement. The standard Vans cowl is a compromise and there are going to be a few outliers that have issues. That, and I think their continuing baffle treatment of #2 and #3/5 is borderline irresponsible...

That said, many of the people who "think" their baffles are good are in error. There is a lot more to it than just aiming some air in the general direction and hoping for the best. Every molecule of air that comes out the exit needs to have gone through a cooling fin first or its a waste.

I'm doing a true cowl flap on the Rocket to address the significant cooling needs of the engine at high power/low speed flight as well as sealing things up to keep the heat IN when I'm high and LOP. I'll be interested to see what you come up with.
 
My take on cowl flaps

I sure appreciate everyone's point of view on this. I have done everything I could to get temps down on my two RV's except for putting in a solid plenum and cicular inlets. Here in Phoenix, I was unable to keep below 400 degrees on climbout in both the RV-4 and RV-6a. Addition of electric cowl flaps solved the problem on both airplanes.

The RV-4 used to cool fine with the O-320 in it, but when we went to an O-360 with 9:1 compression cylinders, it runs real hot on climbout without use of cowl flaps.

Sure would be nice to know what I was doing wrong with the baffling so I could fix it. Baffling fits good all the way around, Liberal use of RTV everywhere I could. My no 2 cylinder has small fins in front so no need to do a bypass around it.
 
...My no 2 cylinder has small fins in front so no need to do a bypass around it...

Can you define "small fins"? There is some variation between manufacturers, but the deepest I've seen so far (on the 360/540) is only about 1/8 inch - not nearly enough. And how about the rear of your #3 - same?
 
Two cowl flaps installed. Adds significant operating (temp) flexibility. Does cost money, weight, noise, and drag when open. Am I'm just about trained to connect the power to the actuators when reinstalling the cowling now after a year.

One issue with the EZ Cool devices. The fasteners need Loctite, and I felt like the actuator needs insulation. I strapped a small piece of foil backed cloth on each one of mine.
 
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Front fin on no 2 is about 1/8 in deep. Used washer in back of no 3 cylinder. All four cylinders run about same temp.
 
And some of the rest of us have tried to to convey that there are many ways to resolve over heating problems.

One extreme would be to sell the airplane and buy one that doesn't overheat.
The other would be to zero in on the root cause of the problem and fix it.

Adding a cowl flap (or two) to a cooling system design that the major majority of the same airplanes cool well without, is another method of solving the problem, but not the best way in my opinion.

... The majority of these aircraft do not operate in the extreme heat like Ca.Tex. Az. etc. It would stand to reason that most or many don't have the problem in cooler climates. In our testing I took off from Lake Havasu City AZ. 117 degree day, 800 foot elevation, pretty much an ideal test location for heating issues. I rotated at max power to 70 kts. 2400 ft. pr min, held that speed to 12k ft. Temps never passed 380. I closed the flaps pushed the nose over to cruise configuration and a true airspeed exactly on my VNE. at less than 7 GPH. I don't know of another RV that can do this without cowl flaps. Whats not to like? I like you deal with, and talk to a lot of RV owners as well as other extremely high performance aircraft owners/builders. Most report back big reductions in climb temperatures, usually in the 50 degree range with no penalty in speed or efficiency. We currently have over 800 aircraft out there with "Easy Cool Flaps" and have yet to receive a performance report from an unhappy customer. This speaks volumes in itself, and unlike "many suppliers" we offer a 100% guarantee of satisfaction on all products and purchases! I wish everybody in this business operated this way, as it would make the flying and owning an aircraft much more enjoyable. Thanks, Allan..:D:D
 
Not wanting to drag up old discussions, but I see a lot of folks chasing the "gotta stay below 400 deg F" at all times or ??
I personnaly have no issues letting mine go to 410-415 for short durations of a few minutes but rarely see that even in the hottest weather when heat soaked. That is still way below continuous operating specs let alone red line.
I have never had the need to climb out at 70kts. I see no advantage to that when best rate is significantly higher. It would be rare you would ever need to do that in an RV. We climb so well, lower the nose, cool things off if you need to.

I see a lot of folks adding complexity and cost to their machines based on poor understanding, or misinformation.

I have "fixed" two machines "cylinder cooling issues" with timing adjustments.

So, define what is a problem to your own definition, understand it, then address it as needed. If you "need" cowl flaps, or just think they are the answer, know that you are adding some weight, complexity, and cost. If that is justifiable to you, that is all that matters.
However, if I see an RV with cowl flaps, the first thing I am going to ask myself is "what else is wrong here". Maybe nothing, but I am a betting man....
 
Ok, I will make the bypasses and see what happens.

Do you have any pics or can you point to any posts that show these ducts for the cylinders? I've seen them referred to a few times in posts, but haven't seen exactly what kind of duct you guys are referring to.
 
Thanks Michael, makes sense. I thought the reference was to a fiberglass duct mounted to the top cowl, which is why I couldn't find much info on it.
 
Whole lot of debate over nothing.

If a fella wants to cover all bases during construction, rather than waiting until after test and break-in to judge temperatures, well, why not?

The Vans mantra is balanced performance, an airplane with a wide operating range that does all things pretty well. A builder can make choices which add to the range (go faster, stall slower, etc), and remain in accord with the mantra if his modification does not subtract from performance elsewhere (for example, go faster, stall faster).

Here the subject is cooling. The ideal approach, when incorporating changes early in the build, is to widen the cooling range at both ends.

How? Assume the builder wishes to use something like Nimmo's doors. Determine the additional exit area provided by one door. Section the stock cowl exit so as to remove fixed exit area equal to that one door. Now install two doors.

The result is one door area less than stock in cold cruise, and one door area more than stock in hot climb. That should be a useful range addition given stock inlets, baffles, etc. Frankly, if it won't cool with one additional door area, yes, there are other issues.

The concept can be applied multiple ways, and with system mods the cooling range can be extended even further.

RV-8 stock exit. Compare with below. (Yes, the pipes had to be modified):



Don't really need doors. Instead build swappable exit panels for hot and cold seasons:



Very small fixed exit (on right, above), installed for winter:



Even less fixed exit area....



....with low drag variable exit area:

 
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I live in West Texas and did install an Anti-Splat Cowl Flap. I do not need it most of the time but it sure is nice to have when I do. It adds very little weight and is easy to install. Better than opening up the exit area and creating more constant drag in my opinion!
 
Missing pictures wanted

If Dan or anyone else has these missing fotobucket pictures , I?d very much like to see them. I?m fighting high temps and have done the basics of plugging holes in baffling. I have resorted to leaving the close out panel for the nose gear off in order to get enough air through the cowl. I need to do more and was about to resort to a cowl flap.
Thanks,
Eric

Whole lot of debate over nothing.

If a fella wants to cover all bases during construction, rather than waiting until after test and break-in to judge temperatures, well, why not?

The Vans mantra is balanced performance, an airplane with a wide operating range that does all things pretty well. A builder can make choices which add to the range (go faster, stall slower, etc), and remain in accord with the mantra if his modification does not subtract from performance elsewhere (for example, go faster, stall faster).

Here the subject is cooling. The ideal approach, when incorporating changes early in the build, is to widen the cooling range at both ends.

How? Assume the builder wishes to use something like Nimmo's doors. Determine the additional exit area provided by one door. Section the stock cowl exit so as to remove fixed exit area equal to that one door. Now install two doors.

The result is one door area less than stock in cold cruise, and one door area more than stock in hot climb. That should be a useful range addition given stock inlets, baffles, etc. Frankly, if it won't cool with one additional door area, yes, there are other issues.

The concept can be applied multiple ways, and with system mods the cooling range can be extended even further.

RV-8 stock exit. Compare with below. (Yes, the pipes had to be modified):



Don't really need doors. Instead build swappable exit panels for hot and cold seasons:



Very small fixed exit (on right, above), installed for winter:



Even less fixed exit area....



....with low drag variable exit area:

 
When I put an upper gear leg fairing on our 6A, closing the slot in the lower cowl better, temps went down and the plane got faster. My take on that was that air was entering through the slot reducing differential pressure.

Ed Holyoke



If Dan or anyone else has these missing fotobucket pictures , I?d very much like to see them. I?m fighting high temps and have done the basics of plugging holes in baffling. I have resorted to leaving the close out panel for the nose gear off in order to get enough air through the cowl. I need to do more and was about to resort to a cowl flap.
Thanks,
Eric
 
Hot Jugs

Drove me crazy having to fly my high performance RV-7 like a fat lazy Cessna 172 to keep the jugs cool on a hot day. What is the point of having performance for a continuous 1500 fpm climb, yet slog along at 300-400 fpm to keep the jugs cool?

Put in two antisplat cowl flaps that solved it good. (not crazy anymore).
(yes, I did all the cowling, baffle things to no avail)
 
Ed,
I already had the slot well closed out. So, I built a manometer system to have some facts about the pressure differential between the upper deck and the lower deck and discovered I didn?t have the inches of water pressure differential for cooling that Lycoming required. When I removed the slot close out, the pressure differential got better. Good enough to climb to pattern altitude before having to reduce power to control engine CHT temps. Thanks for the suggestion anyway. I?m still hoping to see DanH?s fotos.

Eric Schlanser

When I put an upper gear leg fairing on our 6A, closing the slot in the lower cowl better, temps went down and the plane got faster. My take on that was that air was entering through the slot reducing differential pressure.

Ed Holyoke



If Dan or anyone else has these missing fotobucket pictures , I?d very much like to see them. I?m fighting high temps and have done the basics of plugging holes in baffling. I have resorted to leaving the close out panel for the nose gear off in order to get enough air through the cowl. I need to do more and was about to resort to a cowl flap.
Thanks,
Eric
 
I recently installed Antisplat cowl flaps (2 of them) and even on the hottest Florida day, the CHT's will not ride over 390. It even helps keep my oil temp down when wiating for IFR release...
 
Howdy Eric,

I just started doing some manometer testing too. It looks like I've got about 9" differential. How about yours? I built some cowl flaps into mine and I can raise the differential by 5 or 6 more inches.

Ed Holyoke

Ed,
I already had the slot well closed out. So, I built a manometer system to have some facts about the pressure differential between the upper deck and the lower deck and discovered I didn?t have the inches of water pressure differential for cooling that Lycoming required. When I removed the slot close out, the pressure differential got better. Good enough to climb to pattern altitude before having to reduce power to control engine CHT temps. Thanks for the suggestion anyway. I?m still hoping to see DanH?s fotos.

Eric Schlanser
 
I just started doing some manometer testing too. It looks like I've got about 9" differential. How about yours? I built some cowl flaps into mine and I can raise the differential by 5 or 6 more inches.

No comparison can be made without comparable altitude and airspeed.

To record cowl pressure information which can actually be compared to others, all participants fly the same altitude (current small database was taken at 3500PA), and record cowl pressures on one leg of NTPS-style TAS triangle. Fly it at 100, 125, 150, and 175 indicated, then plot the data with true airspeed (from the triangle) on the X-axis and pressures on the Y.
 
Manometer tests

Sorry Ed, but I?m away from my test records. The Lycoming number for my O320 is 5.5 inches at sea level in standard atmosphere conditions at 105 mph. I just tried with each test to duplicate the conditions I had on the day I did the test. Probably at 3-4000ft msl at a range of speeds and power. I think it started with about 2 inches and got up to 4 inches with very limited testing. It is enough to fly in cool (<70 deg) conditions or with reduced power in warm (>70 deg) conditions. But not enough to climb above pattern altitude at full power at Vy.
Eric
 
Howdy Eric,

As warm as you are running, gotta ask if you've looked at timing and fuel flow and sealed everything up. At first blush, your cowl pressure numbers look low which kinda jibes with your temp control issues, but might not be all of it.

Like Dan, I've got a modified inlet to exit area cowl, so my numbers are going to be different. I've only done a couple of tests, one at 8K and a very brief look at 3500'. I haven't done any testing at lower airspeeds. The low altitude run was at about 165kt TAS and I saw about 9". I couldn't spare much time to be scientific. I was talking to approach, getting ATIS, and looking for traffic. Both at high and low altitudes I was able to increase the pressure differential substantially by deploying cowl flaps. Once again, I didn't really get a lot of data, just enough to see that it really works.

I always used to cruise climb at about 130 - 140kts groundspeed with our fixed pitch O-320 powered 6A. I was able to get more RPM (power) that way while packing more air into the cowl. She would still climb at about 700 - 800fpm and cooled a lot better. When your temps are marginal, lower nose is better. Just have to be sure of obstacle clearance.

I've got the cowl flaps on my new airplane, but hardly ever use them and I still mostly cruise climb. It's a habit, I guess. When the winds and fires let up around here I'll try and get some more accurate data.

Ed Holyoke
 
18 kts

I closed my very large cowl flaps at cruise and gained 18 knots. I also got a substantial pitch down. I don't usually open them at cruise, just sometimes forget to close them right away. That and forget to turn off the boost pump. ;-(

Ed Holyoke

has anyone opened the cowl flap at cruise speeds? Not just in the climb??
 
I just wondered if its safe to open in cruise if you choose to make a hard climb and you put the cowl flap out in that situation. Thanks
 
Yeah, it's safe as far as not causing drastic aerodynamic effects. Depending on the cowl flap - how it's built, how it's actuated - you might be able to bend or break the cowl flap or mechanism, but it's not going to bring the airplane down. I get a significant pitch-up and a bunch of drag at cruise speeds, but it doesn't come anywhere near affecting the controlability of the airplane. If you are having trouble controlling CHTs in climb, crack the cowl flap open if you've got one. If you only have to open it a little to get the cylinders cooled off, there is no reason to fling it wide open. If you really do need it wide open, inspect it closely afterwards and you'll have your answer. My homemade cowl flaps don't have any problem holding up to the highest speeds I've thrown at them. Another thing is that, if you really are making a hard climb, you won't be at cruise speed for long. Lower airspeeds will reduce the aerodynamic loads on the flaps.

Ed Holyoke

I just wondered if its safe to open in cruise if you choose to make a hard climb and you put the cowl flap out in that situation. Thanks
 
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A "cowl flap" need not be a flap, or extend very far into the airstream, or require much mechanical power to open or close.

Original implementation was powered with the smallest Firgelli linear actuator(L12). It was powerful enough given the linkage design, but developed internal freeplay over time, assumed due to temperature. It's now powered with the familiar trim servo.

The lower cowl itself required the addition of reinforcement ribs, as shrinking the exit raises lower cowl pressure, and the entire cowl tries to blow itself into a round balloon shape, and this system requires cutting a pretty good size hole. The deformation didn't allow full closure, as seen here in the first video. Note the gap increase at 180 knots as compared to 120. It's a direct response to increased cowl pressure.

https://youtu.be/nA5PY7PYBsU

BTW, while there, note the disturbed flow when open, just aft of the rear slot, and just in front of the tailpipe. That's the disadvantage of anything extending into the airstream. As a flap gets larger and/or the open angle gets more acute, the disturbance gets larger. Pure drag. Everything goes smooth, including the tufts in the primary exit, as the auxiliary exits close and primary exit velocity rises. A slot exit like this one would produce less drag when open, thus there is less airspeed delta, open vs closed. The pitch trim change is very small.

A center hinged door means one of the two auxiliary exits is a recessed slot.

This video taken after internal cowl reinforcement, to check for full closure. Didn't bother with tufts, as the first video told me all I needed to know on that score:

https://youtu.be/aIBXAE2Ezn4

Folks who are adding Nimmo's flaps while retaining a full size stock exit don't need to worry much about cowl deformation due to internal pressure. On the other hand, I've never videoed one at 180 knots.
 
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Thats very interesting, Its amazing the cameras don't blow off at those speeds too. I put the EZ cowl flap in today hope to try it out this weekend. Thanks G
 
Slightly off topic

Dan I took your advice and Im adding a SS heat shield with fiberfax backing in a installation that looks very similar to yours. I think I obtained the right thickness SS but just wanted to ask what yours is. In the video yours looks nice and snug with no visible movement at all. That stuff is so thin I was afraid of some kind of distortion at speed. :)
 
Dan I took your advice and Im adding a SS heat shield with fiberfax backing in a installation that looks very similar to yours. I think I obtained the right thickness SS but just wanted to ask what yours is. In the video yours looks nice and snug with no visible movement at all. That stuff is so thin I was afraid of some kind of distortion at speed. :)

The stainless panel in the video is a polished SS replacement for the standard Vans aluminum 867A-1 exit ramp. It is typical firewall material, about 0.018" thick, not a foil overlay as one would do on a 6-7-9 or 10. There is also a lot of aluminum tape stuck to it in the video, for tufts, and measurement probe hoses and wires.

A foil overlay on the firewall is fine at 0.002". Some folks are using as thick as 0.005" on firewalls and belly panels, for appearance.
 
A foil overlay on the firewall is fine at 0.002". Some folks are using as thick as 0.005" on firewalls and belly panels, for appearance.

Some folks <ahem> use as thick as .008. Still easy to cut with snips and thick enough to roll the edges slightly. Doesn't deform much at all when riveted.
 
As others have mentioned, the standard RV cowling works pretty well for most people with proper attention to sealing. One thing I have noticed on many RVs that I have inspected is that the little tab on the baffle which is supposed to be bent to be very close to the cylinder base of #3 is often left wide open. Even if the tab is bent properly, that whole area should be sealed with RTV and often isn't.

I modified my cowl during build a la Dan Horton's "Shrinking Exit" by increasing inlet size and decreasing exit area. I was concerned that I would have excessive CHTs and oil temps during taxi and climb so I took drastic measures to increase airflow through an oversize cooler and I cut in very large cowl flaps. As it turns out, I have to choke off most of the air to the oil cooler just to get into the green and don't need the cowl flaps for taxi and climb even on 100 degree days. Now that the weather has cooled to about 90 degrees (much cooler than that today), my hottest cylinder is about 340 down low with high power set. So - I definitely have excess cooling capacity. My intent is to shrink and extend my exit scoop further which will enclose my exhaust pipe and add some augmentation to the mix. My hope is that I'll reach a point where I do need the cowl flaps for ground ops and climb but not for cruise.

As pertains to this thread, I wouldn't do cowl flaps during build on a standard installation. If attention to detail is employed, it'll cool ok. If it runs hot anyway, then the addition of cowl flaps is a viable method of adding mass airflow for cooling.

Ed Holyoke

sorry for the thread revival.

is the cirlced bit the tab you are talking about?

I'm experiencing high CHT #3

wmwPI50.jpg
 
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