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Thread: Cowl-flap as speed brake?

  1. #21
    Flyfalcons's Avatar
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    Quote Originally Posted by WLIU View Post
    Here is the interaction between cowl flaps and speed brakes on an airplane that has both - When you want to descend fast you close the cowl flaps to keep the engine from overcooling and you extend the speed brakes to increase your drag. If you have ever seen and engine stop from being too cold (think winter extended glides at idle power) then you will want to keep your engine temps well into the green arcs on descents.

    The two controls are used almost always oppositely at the same time.

    Best of luck,

    Wes
    This is the best way to describe why using cowl flaps as speed brakes is a bad idea.
    Ryan Winslow
    EAA 525529
    Stinson 108-1 "Big Red", RV-7 under construction

  2. #22
    bwilson4web's Avatar
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    Thanks Ron,
    Quote Originally Posted by Ron Blum View Post
    Bob: Love the drawings. It looks like the nose is being extended a lot. Have you looked at directional stability? All cross-sectional area ahead of the CG is destabilizing. To put it in perspective, the destabilizing effects of air data nose booms can be easily measured.
    Yes but I'm not at that stage. I may wind up adding a dorsal to the rudder fin after completing getting the exact, firewall-forward configuration.

    The planned, 128 lb engine replaces a 172 lb engine. But I won't know the final placement until I have the BSR (35 lbs), battery, engine (128 lbs), Ivoprop flight adjustable prop, and radiator. With the exact weights (including coolant and hoses,) I'll be able to calculate their final placements using similar weight from the existing cowl to estimate this part. Then I can fabricate the engine mount, and foam-and-fiber glass a cowl and radiator duct. I'll plan several mount locations for the battery so as it serves as movable ballast to get the final CG right at 58".

    Quote Originally Posted by Ron Blum View Post
    The speed brake also looks like it will be ahead of the CG ... another destabilizing surface. You would be surprised what a turned nose gear does to directional stability ... and it's on the center line of the airplane (yes, except the A-10). Perforating the brake will have a tendency to stabilize the surface and not change the (+/-) lift in that area.

    Have you considered splitting the rudder?
    Thanks for tips. I do have a 'loose' rudder but I haven't taken it apart yet to find out what it will take to make it tight. If I have to rebuild the rudder, adding area would be fairly easy at that time. But if I don't have to replace the rudder, a dorsal fin might provide a simpler answer. But funny you mentioned split rudder.

    The original build had streamlined foam over the legs and wheel pants. I had thought about a 'split flairing' around the gear to use as speed brakes. Activated with the brakes, it would be complex but another approach potentially giving more yaw control. But angled in-ward, they might passively add yaw stability.

    Angled slightly inward, 2-3 degrees, the leg flairings should act like 'toe', adding yaw stability. The forward leg would have more 'lift-drag' to turn the nose forward. The trailing leg would be at a lower angle, in a lower drag-lift profile. Located ~48" to either side of the CG, the force differential should help. This will be fairly easy to test.

    Bob Wilson

  3. #23
    Matt Gonitzke's Avatar
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    Quote Originally Posted by bwilson4web View Post
    The original build had streamlined foam over the legs and wheel pants. I had thought about a 'split flairing' around the gear to use as speed brakes. Activated with the brakes, it would be complex but another approach potentially giving more yaw control. But angled in-ward, they might passively add yaw stability.
    Then you'll probably wind up with an unwanted nose-down pitching moment that'll vary with the amount they are open.

  4. #24

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    I believe that EZs use "split" rudders for speed brakes, too. In those cases the two rudders are independent (each can only travel outboard). I did a similar arrangement on my first homebuilt. One potential issue is that pilots that are nervous have a tendency to "secure" themselves by pressing on both rudder pedals ... which puts out the speed brakes. One way around that is to put a spring in the system so that it takes a little more opposing force to add speedbrake input.

    Split surfaces for drag are also used on the Space Shuttle, F-16, A-6, F-22, F-117, etc.

  5. #25
    bwilson4web's Avatar
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    Quote Originally Posted by Matt Gonitzke View Post
    Then you'll probably wind up with an unwanted nose-down pitching moment that'll vary with the amount they are open.
    Agreed and split, flairings would be a complex build.

    Bob Wilson
    Last edited by bwilson4web; 04-30-2012 at 01:48 AM.

  6. #26
    steveinindy's Avatar
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    Then you'll probably wind up with an unwanted nose-down pitching moment that'll vary with the amount they are open.
    Which is exactly why I've been trying to warn Bob against this. If he wants a safe, reliable and predictable speed brake it's going to take a lot of testing (preferably CFD or wind tunnel as opposed to the "cross fingers and deploy" technique) and honestly I wouldn't be putting int on the underside of the aircraft. Something that deploys lateral (think the splitting tail cone speed brake of the BaE 146 or the side mounted ones on the F-86, etc which takes away the issue of inadvertent deployment if someone presses on the rudder pedals as Ron Blum describes) would probably be much less likely to cause a loss of control. The take home point in all of this discussion- which apparently Bob doesn't want to hear (neither did Dan Lloyd want to hear us telling him to take it easy and to not get in a rush; many of us in the hobby know where that got him)-is that what may seem like a good idea on paper or in our heads may not be the best solution to a problem, if a problem exists at all. Most cases of "I can't operate the airplane out of the fields I want to operate out of" are cases of:
    1. Poor pilot technique/lack of skill/inexperience/poor decision making
    2. Unrealistic assessment of the operational performance of an aircraft/pilot selected wrong aircraft for the wrong "mission"

    Heavily screwing with aerodynamics of an aircraft is not the way to solve either of these problems, at least not unless you happen to either be or have access to a very experienced aerodynamicist.

    I apologize if I'm a bit too pointed in my comments Bob. I just don't ever want a conversation I have with another builder to ever referenced directly or indirectly in another NTSB report so long as I live. Given that you seem to take any criticism as flippant or quashing a great idea I have this dreadful feeling that I'm seeing history repeat itself. You might not like my speaking up and recommending that you reconsider and you may not even be reading this, but in good conscience, I can't abide with remaining silent when I see someone engaging in questionable activities simply because I have a conscience that will not allow it.

    Agreed and split, flairings would be a complex build.
    Bob, if you'd like to see the diagram for the flight spoilers I have for the larger of my two designs which could be relatively easily modified to be used as a laterally deploying speed brake, I'm happy to share the basic design with you since it might solve the problem of needing to slow down without the issue of a pitching moment. They also include a fail-safe mechanism so that if the deployment cable fails, they stay retracted instead of popping open. I see the need for speed brakes in certain sorts of operations and my only beef is with where you want to put them. If I can help with a simple and possibly safer alternative, I have no problem doing so.
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



  7. #27

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    What brought my question up was working on the landing light installation. One thought was to have the passenger side light pointed along the approach angle and the pilot side pointed down and offset based upon measured slip angle. But based upon my night flying experience, I want both lights pointing along the approach path. Ideally, pointed at both sides of the numbers with modest overlap in the center. The radiator cowl flap at speed-brake angle keeps the lights on the approach path while steepening the approach angle.
    Ah. I don't fly at night and so hadn't considered that.

    The split rudder idea sounds like a winner.
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

  8. #28

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    This post is laughing at the "lighter" side (yes, bad pun intended)

    ... As for the landing light, here are a couple suggestions.

    1) Mount it on a 2-axis swivel; it could turn into the relative wind all the time. (... please don't think about this too hard; it doesn't work in a crosswind)

    2) I thought the only reason for a landing light is to be able to turn it off if you don't like what you see.

  9. #29
    bwilson4web's Avatar
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    Quote Originally Posted by Ron Blum View Post
    ... As for the landing light, here are a couple suggestions.

    1) Mount it on a 2-axis swivel; it could turn into the relative wind all the time. (... please don't think about this too hard; it doesn't work in a crosswind)

    2) I thought the only reason for a landing light is to be able to turn it off if you don't like what you see.
    LOL!

    My plane, N19WT, is conventional geared so the landing lights on the ground would blind anyone on final while trying to taxi to the run-up area and do nothing for taxi-way visibility. So I'm working on a custom, eight LED configuration that will have two modes: anti-collision or taxi-mode controlled by a TI MSP430 controller.

    There will be four on the top and four on the bottom of the fuselage. The fuselage top LEDs, just behind the cabin, will be angled 90 degrees apart, forward/back and left-right. The bottom LEDs will be 90 degrees apart handling front left and right quadrants and rear left and right quadrants.

    In taxi-mode, the bottom, front quadrant LEDs will be bright and at 1,000 L throw enough light to see the taxiway out to the edges. This augments the red and green LEDs on the canard tips. The four top and two rear LEDs will have reduced intensity, just enough to help others see the plane on the ground. Forward taxi visibility is still a problem regardless of ambient light but the new engine, two vertical cylinders, is much narrower than the opposed, horizontal, four cylinder VW giving more forward visibility.

    In anti-collision mode, the LEDs will cycle alternating top and bottom and around the plane. Each LED is rated at 1,000 lumens which exceeds the FAA 400 L requirement by a factor of two. Best of all, the average load will be ~11 watts.

    The two landing lights will be 11 watts, 1,000 L each. With the Freshnel lenses, they will be awesome. I've already been testing the LEDs and lenses and they are painfully bright:



    Last night, I cut the first, cone reflector out of aluminum sheet and clamped it. I still have to rivet it; cut and mount the Freshnel lense; mount a LED, and; fire it up to check the thermal characteristics. Initially, I'll use a junkbox LM319 to regulate the LED, 11V supply. Then I'll take it to Moontown or a dark, rural area after 'moon set' and measure the effective distance.

    The new night light configuration will use:
    • less power for the LED navigation lights, 0.26A, versus 2.3 A for the Grimes
    • replace the front facing, xenons, .68 A. with 1 A. at 11 V., 360 degree collision lights that on the ground also serve as taxi lights
    • adding two landing lights, focused and overlapping either side of the runway using ~2 A at 11V.
    LED power is sensitive to voltage so I'll be using 92% efficient, auto-grade, DC-to-DC converters to step-down the aircraft, 12V bus to the well regulated, 11V, 0.9A each LED requires. The canard tip mounted, LED landing lights will each use a low-dropout, linear, ~0.5V, to handle the (I**2)R losses and avoid any parallel load issues.

    Night VFR has a lot of advantages but the aircraft lights need to be done carefully and with attention to detail. The only thing not included is a tail-light but a stock LED will work when I get to that part of the rebuild.

    Bob Wilson

  10. #30

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    Sounds like a great plan ... LEDs are changing the certification world, too. Ironically, the LED power gain (or loss depending on how you look at it) has also produced a "not-thought-about", undesirable side effect for those airplanes approved for FIKI ... the lenses are no longer anti-iced. Not an issue for your design.

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