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Thread: Cowl-flap as speed brake?

  1. #11
    bwilson4web's Avatar
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    This might help explain what I was thinking about:

    IF I decide I need a speed brake, my cowl flap will be a little longer and extend beyond 60 degrees.

    The whole idea is to increase drag so the plane will descend at a steeper angle. But this is not something I have to do in the first version other than to make sure the linkage supports a larger extension. If the plane needs more drag, a longer replacement and/or a perforated plate (aka., SDB dive bomber) will work.

    Bob Wilson

  2. #12
    steveinindy's Avatar
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    Bob, seriously....one has to be very careful with putting speed brakes, etc on aircraft because if you get it wrong you're not going to live long enough to sort it out. A strong enough pitch down (such as might occur in a mis-sized or over-extended speed brake) and you could be rendered unconscious or severely incapacitated as it doesn't take that much negative G to red someone out.


    There's a very good reason this sort of thing on commercially built aircraft is extensively wind tunnel tested before they ever try it in flight. I have a hard copy of the wind tunnel test reports for the SBD's dive brakes (you'd be amazed the crap you stumble across from time to time). It's in excess of 100 pages long and is more boring than a French novel....just on the dive brakes. I say this as someone who does not want to hear of another friend (and as pilots, we're all brothers and sisters or at least friends regardless of whether always see eye-to-eye or have ever met) getting hurt or killed.

    If you need to get down quickly (such as an ATC request), either use my favorite phrase ("unable") or request a 360 descent to give yourself more "room". A lot of pilots for some reason seem to forget that there's nothing wrong with making a 360 for separation or descent. The idea of "direct to" seems to override even basic piloting skills or non-linear thinking as a problem solving technique in this day and age of GPS and the magenta line of doom. Since we're not trying to avoid enemy fire while putting a bomb onto a Jap flat-top, I can't see any need for a "dive brake" on a light piston engine aircraft. I really don't mean this the way it's going to sound but there's really no polite way to put this: It's far better to learn how to properly fly the airplane as it was design than it is to go seeking engineering solutions to what is most likely a human factors problem. That's not to mention that it's a lot more fun because you get to fly while doing it.
    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.



  3. #13

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    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

  4. #14
    bwilson4web's Avatar
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    Hi,

    This is the aircraft, a Viking Dragonfly:

    The center of gravity is ~58" from the prop or roughly between the seat-back and instrument panel.

    The new engine configuration:


    So the proposed cowl flap / speed brake will hinge just behind the radiator. In normal flight, it will be parallel to the air stream. In climb, slightly down to increase radiator cooling at climb speed, ~80-90 mph.

    When over-extended down, there will be two forces:
    • drag - significantly increased, it will have a moment arm of about 14"
    • radiator drag - maximum airflow will further add to the drag with a shorter moment arm, ~7"
    • lift - only from any static pressure increase, it has moment arm of about 35-40" and opposite to the drag force
    Without flight measurements, it is too soon to know how these forces will balance but having opposite signs, there is a chance they may be trim neutral. Flight testing and careful metrics will find out.

    I'm going with a water cooled engine with a thermostat. During engine warm-up and descents, the thermostat closes to preserve engine block heat. Even with the cowl flap / speed brake maximum open and maximum cooling air through the radiator, the thermostat controls how much, if any, goes to the engine block.

    I learned to fly at Hyde Field that has power lines to the North and trees to the South. The nearest airport with hangars is Hartselle AL with trees to the South and the town to the North. Emergency landing fields are often worse with fences, trees, power lines, and all sorts of unexpected things to fly into on the way down. The cowl flap / speed brake provides another mechanism to steepen the approach in addition to a slip.

    The operational scenario is:
    • downwind - 80 mph, clear traffic, cowl to climb position.
    • crosswind - 75 mph, clear traffic, monitor obstacles and keep well above
    • final - cowl flap / speed brake to maximum, add power, trim 65-70 mph, clear obstacles
    • approach to flair - adjust power to reach numbers at 65 mph
    • wheels landing - pull power . . . brakes . . . turn-off . . . close cowl flaps
    They are a safety feature to give additional, approach angle control to augment a slip if needed.

    Bob Wilson
    Last edited by bwilson4web; 04-29-2012 at 03:42 AM.

  5. #15
    steveinindy's Avatar
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    The cowl flap / speed brake provides another mechanism to steepen the approach in addition to a slip.
    They are a safety feature to give additional, approach angle control to augment a slip if needed.
    Assuming you do it correctly, otherwise it turns what should be an orderly glide to approach into a plummeting spine-splintering slam to the ground. That's assuming you don't pitch the nose over and end up with your engine in your lap.

    I'm getting you are set on doing this and I've said all I can as to why there is a better than even chance that it will work out as nicely as you think it will.. If you don't mind my asking, can you PM me your address as I have a release form I would like you to sign before you pursue this any further.
    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.



  6. #16

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    Now that I've seen the plane, I'm wondering why you'd worry about a speed brake for descents - that thing looks ready to slip like nobody's business!

    Bring the throttle back, crank it sideways and follow the elevator down!
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

  7. #17

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    Today's "high-performance" airplanes are like old fashioned muscle cars... old fashioned airframes (1930's technology) with big engines. They are typically very draggy and don't need speed brakes for approach control or landing. Small speed brakes are sometimes helpful to reduce cooling problems with those big engines.

    With rising gas prices, we're going to see a shift towards more efficient airframes. The "high-performance" airplanes of tomorrow will be very motor-glider like. Performance will come from efficiency rather than high power. These clean machines will need large speed brakes (not little precise flight devises) for approach control and to reduce floating in ground effect. In the future, big speed brake, more than big engines, will indicate a high performance airplane.

    PS - Once you get used to them, speed brakes are pretty awesome. They tame even the most slippery airframe and give you exact control of your approach.

  8. #18
    bwilson4web's Avatar
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    Thanks,
    Quote Originally Posted by SBaircraft View Post
    Today's "high-performance" airplanes are like old fashioned muscle cars... old fashioned airframes (1930's technology) with big engines. They are typically very draggy and don't need speed brakes for approach control or landing. . . .
    My old, straight-wing, Cherokee 140 fit that model, 150 hp, fixed pitch prop. With full flaps, the Cherokee 140 came down quite nicely and the cowl mounted, landing light stayed on the numbers. It also illuminated unlit obstacles on approach and a few night takeoffs. But slips at night pointed the landing light off the runway numbers at a time when other visual clues are 'in the dark.' In particular, I'm remembering two night slips, Tuscaloosa and a grass strip near the Finger Lakes, where a slip at night was needed on final. During the slip it was equivalent to not having a landing light.

    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.

    Quote Originally Posted by SBaircraft View Post
    . . . Once you get used to them, speed brakes are pretty awesome. They tame even the most slippery airframe and give you exact control of your approach.
    I have to add cowl flaps for the radiator anyway so implementation really comes down to length and how far down the linkage pushes the cowl flap / speed brake. It seemed like such a natural solution to a slippery airframe, I thought I'd ask if there were other examples . . . apparently not.

    Bob Wilson

  9. #19
    bwilson4web's Avatar
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    Quote Originally Posted by steveinindy View Post
    Assuming you do it correctly, . . .
    Not a problem, I am.

    Quote Originally Posted by steveinindy View Post
    . . . If you don't mind my asking, can you PM me your address as I have a release form I would like you to sign before you pursue this any further.
    <SIGH>Take this up with the moderators, I have.

    Bob Wilson

  10. #20

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    Directional Stability

    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.

    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?

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