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Thread: Cowl-flap as speed brake?

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    bwilson4web's Avatar
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    Cowl-flap as speed brake?

    Hi,

    Has anyone seen examples of an over-sized, over-extendable, cowl flap that also doubles as a speed brake?

    As I was driving into work yesterday, it occurred to me that an oversized cowl-flap might work as a speed brake. Hinged at the front, the rear could have a linkage so it could be pushed down, normal to the direction of flight, to increase drag and steepen the approach. I was remembering a belly-board, speed brake seen on a Velocity. I tried a Google search but no hits, yet.

    Speculation on my part, an over-extended, cowl flap might add some up-pitch to the plane. My thinking is to contact the owner(s) to see if they have any pilot reports on other flight characteristic and lessons learned about the cabin control and latching mechanism.

    Thanks,
    Bob Wilson

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    I know that the cowl flaps on the Corsair (F4U) were a source of drag on the aircraft.
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

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    steveinindy's Avatar
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    I'd be concerned about the pitch up, especially if it was used during landing in a tailwheel.

    Why, if you need speedbrakes/spoilers, not just put them on top of the wing? That way when you can use them as both a way to slow down and after touchdown to maximize braking effectiveness by disrupting lift by spoiling the airflow.

    The only drawback is getting them sized correctly for use during flight so that you don't kill too much of your lift.....
    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.



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    Quote Originally Posted by bwilson4web View Post
    Hi,Has anyone seen examples of an over-sized, over-extendable, cowl flap that also doubles as a speed brake?As I was driving into work yesterday, it occurred to me that an oversized cowl-flap might work as a speed brake.
    Wouldn't you be compromising the primary function of controlling engine temps?

  5. #5

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    Cowl flaps have to be located and sized to manage engine temps. Speed brakes create both flat plate drag and often also destroy wing lift. The two engineering requirements really do not overlap in practice except in odd cases.

    I own a set of Precise Flight speed brakes and know of some other types of speed brakes. Putting them on top of the wing provides the maximum effectiveness as they destroy wing lift and create a little more drag due to the slightly higher air velocity in that location. Putting them on the fuselage of a prop plane is less efficient as the air is already disturbed by the prop. Translated that means they must be larger and heavier. And they have to be located aft for stability and other practical reasons which starts to get into CG issues.

    Wing mounted brakes must be sized so that if they malfunction and only one deploys the airplane is still controllable.

    I will note that wing mounted speed boards have to be located to avoid creating aileron flutter.

    Most pilots get no real utility from speed brakes. Unless you are hauling skydivers, you don't need to come downhill that fast. Pops the passengers ears and makes them ill. Better to use the 1 mile for every 1000' of descent rule and plan to start down earlier. And if your speed brakes are not a manual linkage (heavier) you generally do not want to deploy them to slow down as you cross the numbers. They generally do not deploy exactly synchronized so you will have the airplane wiggle a little just as you are flaring. Tried all that.

    If you want to know what it is like to come down at 4000'+ per minute, go buy a right seat ride at the local skydiving center.

    The better speed brake is a constant speed prop and full RPM. Another plus on the list of reasons to use a constant speed prop.

    Best of luck,

    Wes
    N78PS

  6. #6
    bwilson4web's Avatar
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    Quote Originally Posted by WLIU View Post
    . . .
    The better speed brake is a constant speed prop and full RPM. Another plus on the list of reasons to use a constant speed prop.
    . . .
    I am going with an inflight-adjustable prop with a constant speed controller. I hadn't considered this and it makes a lot of sense.

    If flight testing shows a speed brake is needed, it will easy enough to solve it then. No need to sign-up for something that may not be needed.

    Thanks,
    Bob Wilson

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    you could always try well planned descents...

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    steveinindy's Avatar
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    If you want to know what it is like to come down at 4000'+ per minute, go buy a right seat ride at the local skydiving center.
    Yeah, the idea of doing a spoiler deployed, power off descent on the way home from a $100 hamburger run. There's a reason why the military cargo guys joke you can tell whether a tactical approach was left hand or right hand based on which side of the compartment the vomit is most prevalent on.

    The better speed brake is a constant speed prop and full RPM. Another plus on the list of reasons to use a constant speed prop.
    Assuming you don't mind pissing off the airport neighbors and listening to the NIMBY jerks.....much better to just deploy the "rubber spoilers" landing gear) and increase your rate of descent and slow down that way.

    And if your speed brakes are not a manual linkage (heavier) you generally do not want to deploy them to slow down as you cross the numbers. They generally do not deploy exactly synchronized so you will have the airplane wiggle a little just as you are flaring. Tried all that.
    We're working on a system for our primary design that deploys when the weight goes on the wheels based on the squat switch activating.

    you could always try well planned descents...
    This is true. My intent is for the spoilers to only be used in flight if we need to make an exceptionally rapid descent (for example, loss of cabin pressure, etc). They also come in handy when ATC completely changes what you had in mind (Example: "If you can get down quick, we can squeeze you in with a direct vector for final".....being nice to the ATCers and getting to know them has its advantages).
    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.



  9. #9

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    Good Thinking

    Bob: I like the out of the box thinking; combining functions is always a good thing. But, the problem with cowl flap speed brakes is that when you most want them open (takeoff/climb) you don't want the added drag, and when you want the drag (coming down) you want the engine cowled to keep it warm. I also like Tony's comment: Plan ahead.

    Nobody has mentioned slipping the airplane either. Oh, that's right, we don't teach using your feet anymore (tic).

  10. #10
    Eric Witherspoon's Avatar
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    Quote Originally Posted by bwilson4web View Post
    I was remembering a belly-board, speed brake seen on a Velocity.
    As Wes mentions, on a tractor prop, there's disturbed flow to worry about. That's probably a good reason these are more commonly seen on pusher configs. Also, canard-configured aircraft (generally) don't have flaps, so the need for something to increase drag may drive more designers/builders/operators of those types to want to add the additional hardware. Bob - I'm not sure what you are working on, I'm just saying that there's reasons you saw a control surface on a canard-configured airplane that don't translate to "more traditional" configurations.
    Murphy's 13th: Every solution breeds new problems...

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