'Dumping' the flaps

Discussion in 'Hangar Talk' started by woxof, Dec 6, 2018 at 9:53 PM.

  1. woxof

    woxof Pre-Flight

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    The term seems to mean to retract the flaps to the up position after touchdown resulting in more weight on wheels and therefore, more effective braking.

    I can see how that might work on an aircraft with manual flaps where they can be retracted in a very short time but with the electric flaps, it would seem to me that they retract too slowly and the pilot can expect less drag combined with an overall increase in lift versus drag as the flaps approach 10 degrees or so, which would take away any overall positive performance gain.

    But I read today of someone who selects the electric flaps up in the flare. Has anybody tried this technique.
     
  2. Walboy

    Walboy Line Up and Wait

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    Sounds like a good way to do a gear up landing someday.
     
  3. IK04

    IK04 Pre-Flight

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    No. Bad habit. Worry about other things, like a potential go-around. In a Cessna with 40 degrees of flaps, leave the flaps down and take advantage of all that drag. Improved braking efficiency will be achieved by SLOWING DOWN and keeping the wing STALLED...
    Low wing airplanes with less effective flaps are a different story. They don't provide nearly as much DRAG, and will transfer a tiny bit more weight to the wheels for braking.
    I don't trust those itty bitty airplane brakes to make me stop fast anyway.
     
  4. IK04

    IK04 Pre-Flight

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    EXACTLY! I was going to say that, but was hoping the OP would understand why I said it was a bad habit!
    Thanks!
     
  5. Stewartb

    Stewartb Final Approach

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    Used to do it all the time with my Hawk XP. Not a big deal. Still do it now but with manual flaps. In a taildragger dumping flaps drops the tail without the plane flying off. Important in the wind. And with the tail down I can use more brake.
     
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  6. James331

    James331 Touchdown! Greaser!

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    I do it a bit, mostly with manual flap planes, comes in handy in crosswinds, I’ve also once runway assured pulled the power back and used the flaps almost like a collective in the round out.

    Not sure I’d recommend this in all planes for for all pilots though.
     
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  7. GRG55

    GRG55 Final Approach

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    No, never have. Wouldn't actually understand the purpose behind selecting flaps up in the flare.
    Braking after touchdown in a tricycle gear is all about dumping lift and keeping as much weight off the nosewheel (e.g. on the mains) with the elevator as possible.

    Come fly with me in my twin and I'll show you a low wing that has PLENTY of drag from the flaps. ;)
     
  8. Stewartb

    Stewartb Final Approach

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    In a Cessna the wing will fly a lot slower with 40* flaps than when clean. That’s the point of retracting them. Flight over.

    BTW, with enough horsepower up front a Cessna will perform it’s shortest takeoffs using 40* flaps.
     
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  9. IK04

    IK04 Pre-Flight

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    Oh yeah! Those split flaps on Cessna 400 series will impress you the first time you use them!
     
  10. Bill Watson

    Bill Watson Pattern Altitude

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    Had a sailplane that practically required dumping flaps in the flare (PIK20-b). But learning to crank the flaps while pitching up produced some entertaining PIOs.


    Sent from my iPad using Tapatalk Pro
     
  11. GRG55

    GRG55 Final Approach

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    I notice in the Husky a full flap take-off gets the mains off the ground slightly before the tailwheel.
     
  12. asicer

    asicer En-Route

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    In a 172/182 I dump the flaps after touchdown because that's what the POH says to do and I'm a "by-the-book" type of person.
     
    Last edited: Dec 6, 2018 at 11:31 PM
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  13. Juliet Hotel

    Juliet Hotel Pre-takeoff checklist

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    I've always had a hard time believing getting the flaps up in order to get more weight on the wheels is really going to make all that much difference in a plane that weighs less than 3k soaking wet. Put another way, if that's what you have to do in order to land a particular runway safely, you're landing on the wrong runway IMO. Of course I also don't like landing on any runway that requires the brakes to actually work.
     
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  14. jordane93

    jordane93 Final Approach

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    I’ll do it for a short field landing. Normal and crosswind landing, I don’t touch anything until I’m clear of the hold short line.
     
  15. kath

    kath Line Up and Wait

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    The DPE on my CFI checkride challenged me on this question too, and I said no, I don't touch the flaps until later. "The 172's POH says you're supposed to do this," he said. But I pushed back, with an argument not unlike the one above. "There's already too much going on," I argued. "Sure, you see people doing this for the Valdez Short-Field competition... but I'd rather a student prioritize control and steering and proper crosswind correction and all these other things, on rollout." The DPE gave me a long look and said "OK. Plus, you might accidently hit the gear." I think he just wanted to see that I had given this issue some thought, and made a decision based on safety.
     
  16. Stewartb

    Stewartb Final Approach

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    Hmm. Is that how you fly when you’re PIC?
     
  17. kath

    kath Line Up and Wait

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    I don't do short-field competitions. So yes. I'm with JulietHotel.

    Edit: maybe someday, when I'm a much better pilot, I'll start optimizing this kind of thing. But I know my own limits.
     
  18. Dan Thomas

    Dan Thomas En-Route

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    You cannot stall the wing in any normal attitude while on the ground. The geometry of the gear prevents it. Stall angle is notmally 17 or so degrees, but the wing on a typical taildragger in three-point attitude, or a trike with its tail on the ground, is only around 12 degrees. The "full stall landing" is a myth. Ask anyone who has touched down three-point or very tail-low in a gusty headwind and has had the airpalne leap off the ground again when a gust hit. It can't do that if it's stalled.

    Many taildraggers can actually take off mains-first, with the tailwheel still on the ground and the mains nearly a foot off the surface. Couldn't do that with a stalled wing, either.

    Keeping flaps all the way down and the nose high after touchdown simply creates a lot of induced drag, that's all. If you want traction for braking, the flaps need to be all the way up, and electric flaps are too slow for that. Cessna's flaps generate max lift at about 20°, and anything beyond that is mostly drag. The manual flaps are much handier that way, and I've done some really short landings in 180/185 and my old Auster by retracting the flaps quickly. Gets even shorter if you raise the tail during braking to reduce lift, too, modulating the brakes and elevator to keep it from nosing over. I haven't done that in a long time and wouldn't try it now without getting thoroughly reacquainted with the airplane.
     
  19. Stewartb

    Stewartb Final Approach

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    With all due respect, instructors instill their fears and misbeliefs in their students whether they're trying to or not. Retracting flaps on the ground is SOP. This topic started about initiated retraction before touchdown. I don't think you should teach new students that maneuver but I think it's your responsibility to teach the POH. The manual sets the bar pretty low as it is. Lowering it further isn't doing anyone any favors. Go fly crosswinds onto an iced and slippery Lake Hood Strip and you'll start retracting those flaps. Your students need to know how. But I'm not a CFI and you apparently passed the checkride.
     
    Last edited: Dec 7, 2018 at 11:45 AM
  20. Sac Arrow

    Sac Arrow Touchdown! Greaser!

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    I wouldn't have an issue doing that in the Arrow. I wouldn't even think about doing that in a Deb.

    172? Unless it's an RG, your gear shouldn't be retracting anyway. If it is, then you have other issues with your landing technique.
     
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  21. kath

    kath Line Up and Wait

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    Fair enough. And I agree that all of this is worth discussion with students, yes.
    But we're also supposed to be teaching students to set personal minimums, and instructors are allowed to have them to.
    One of mine is: "if the field is so short, that retracting the flaps is necessary to make it, that's too short for me."

    I wouldn't discourage a student from seeking out such experience, from an instructor who is better at it than me, from a real bush pilot. I am not one yet.
     
  22. Zeldman

    Zeldman Final Approach

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    I will raise the flaps as soon as I can when operating on gravel and other non paved landing areas to try to reduce damage to the flaps.

    To answer the OPs question, I have started raising flaps in the flair on really really short strips when I need the mains to touch down within a foot or so of a pre-determined spot. Dump the lift, hit the spot and hope the brakes won't fail.
     
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  23. SoCal RV Flyer

    SoCal RV Flyer Pattern Altitude

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    I land on asphalt runways, so I keep the flaps fully deployed until I'm down to jogging speed, for all the aero braking I can get.

    RVs have tiny little go-kart brakes. :D
     
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  24. CC268

    CC268 Final Approach

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    I gotta admit that I do this quit often (flaps up when I land), but maybe I ought to reconsider. When I recently read through the Complex Airplane section of the AFH I recall it stating that this could be a bad habit as you have the potential to select the gear switch instead of the flap switch (amongst other issues). Obviously this isn't an issue in a Cherokee ha. Recommends flaps up once clear of the runway. Can't say I've followed that advice myself, but maybe it's time to change my ways and repent!!!
     
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  25. IK04

    IK04 Pre-Flight

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    Not sure where you learned aerodynamics, but no, a wing can easily be fully stalled in a three point attitude during a landing.

    [​IMG]
    If you are moving through an airmass at 10-15 KIAS while on the ground, either taxiing or parked, because the speed (V) in the equation is SQUARED, the wing is stalled.
    As I said in my post, increase the braking efficiency by SLOWING DOWN (V squared). Aerodynamic braking works much better than those tiny brakes and tires above 20 mph or so...

    True, if you want TIRE traction, while braking, a wing producing no lift will help. If you want to stop on snow, ice, or without the excessive use of wheel brakes, that drag produced by 40 degrees of flaps is the only thing slowing you down, so leave the flaps alone.

    If you have Bushwheels and dual caliper Clevelands, by all means raise the flaps and nail those suckers until the tail comes up. That might work on a soft surface.
     
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  26. Salty

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    Not having enough lift to climb is not the same as being stalled
     
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  27. Dan Thomas

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    Exactly. An airplane tied down facing into a 15-knot wind is generating some lift. If the wind gets strong enough the airplane will try to fly away. That's why we tie them down, after all.

    I once saw daylight under the mains of my Jodel while tied down in a strong wind. I had hurried back to the strip when I saw a big storm approaching, and tied it down while the wind was trying to pick it up. Didn't get the ropes tight enough but didn't dare undo them to clamp it down tighter.

    There is a means of stalling the wings during tiedown to stop most of the lift from forming. It involves lift spoiler strips, and some wing covers have them. I've used 2x4s with some carpet on them and tied them on top of the wing. Here are wing covers with the lift spoiler strips built in:
    [​IMG]

    There's a common habit among some pilots: they think the flight is over once the wheels are on the runway and the airspeed is below stall speed. Those guys sometimes get nasty and expensive surprises when a wing picks up in the crosswind, or the braking skids the tires--maybe blowing one out--because the wing is still lifting and there's little traction. The lift decays gradually, not abruptly, if the attitude stays the same.
     
    Last edited: Dec 7, 2018 at 4:38 PM
  28. Dan Thomas

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    Nope. Not the same at all. Stall is dependent on angle of attack, not airspeed. Stall speed is based on the angle of attack at 1G that is required to maintain flight just as the AoA gets too high and the stall occurs. It goes back to the relationship between AoA and airspeed: low speed needs more AoA and high speed means less. When the runway is taking some of the weight, your speed means nothing with regard to stall. AoA still does.
     
    Last edited: Dec 7, 2018 at 5:38 PM
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  29. Ravioli

    Ravioli Final Approach

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    Wow. lots of options here. Anyone reading any POH's for the plane?

    I know in PA28 or C!7x I'm retracting the flaps during roll out. Always.

    In my RV6A, nope, leave them down.
     
  30. James331

    James331 Touchdown! Greaser!

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    I never got the grabbing the gear by mistake OWT.

    Maybe it’s the planes Ive flown, but I don’t see how you could confuse the two and grab the gear instead of the flaps, especially in manual flap planes.
     
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  31. Dan Thomas

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    I know of at least two instances, one in the flight school where I worked and another at a nearby flight school. Ours was a CPL undergoing flight proficiency assessment, and in the rollout he grabbed the gear handle and raised it. The examiner whacked it down real quick before the gear went up. The other flight school wasn't so lucky; in their twin the student raised the gear instead of the flaps, and got what was demanded. Expensive.

    It's not an OWT thing. It happens. You gotta be paying attention. LOTS of attention. With manual gear handles it's less likely, but some retracts have manual gear handles, too.
     
  32. Clip4

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    Probably more effective to open the doors full open than retracting the flaps.
     
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  33. James331

    James331 Touchdown! Greaser!

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    What aircraft?

    I just can’t picture how you’d screw that up.
     
  34. jordane93

    jordane93 Final Approach

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    Unless you’re regularly flying into a short field, I don’t see a reason to touch anything until you’ve cleared the runway. Focus on your roll out and exiting the runway.
     
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  35. Juliet Hotel

    Juliet Hotel Pre-takeoff checklist

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    I'd like to see a comparison in something like a 172. Pull the flaps in at touchdown, stop and measure the distance, then land again, leave the flaps alone stop and measure the distance. Do both 5x and average the numbers. If the difference is more than 50' I'll eat a bug.
     
  36. Clip4

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    The proximity of the gear and flap switches have nothing to do with it.
     
  37. CC268

    CC268 Final Approach

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    Yea seemed a little unlikely but hey I guess I’ll find out soon enough
     
  38. IK04

    IK04 Pre-Flight

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    Since we are discussing the use of flaps on landing, I was trying to avoid getting into the definition of a stall.
    I was simply giving examples of how exceeding the critical angle of attack is not dependent on aircraft attitude. If you are in a "three point attitude" of 12-15 degrees nose high, but you are descending at 1,000 fpm with power, you have exceeded the critical angle of attack and the wing is stalled. Sure, not all the wing is stalled, but you aren't making enough lift to stop the descent.
    At this point you need some Super STOL landing gear ;>)
     
  39. IK04

    IK04 Pre-Flight

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    I once pulled the mixture control back all the way in an Archer, confusing it for the carb heat because they were both levers and right next to each other. I was accustomed to push-pull knobs...
     
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  40. Dan Thomas

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    I know that. But we're not talking exceeding stall AoA at altitude; we're talking about AoA in the flare and touchdown, where the flight path is parallel to the runway. You can't get stall AoA in that scenario unless you're well above the surface, and have power on and are pulling the nose way up.

    I was a flight instructor some years ago. Taught all this stuff. Taught stalls and spins from various scenarios, not just the usual power-off gentle stall that really doesn't adequately cover the subject. And I taught Aircraft Systems, which addresses quite a bit about AoA and the various enhanced lift devices used on various airplanes.