STOL take off technique

Discussion in 'Rules and Regulations / Flight Safety / Better Pil' started by PTAirco, Aug 14, 2019.

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  1. Aug 14, 2019 #1

    PTAirco

    PTAirco

    PTAirco

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    I have watched countless videos of various people doing STOL take offs in taildraggers, from the average Joe to the Valdez guys. The technique currently in vogue is:

    Brakes on
    Full power
    Lift the tail
    Accelerate with the tail up
    Rotate hard, at the same time deploy flaps.

    It always struck me as inefficient. Yes, these guys get off fast but is that really the best way to minimize ground roll (which is what this is all about)?

    I always felt that using about half flap (less than 20 degrees, say) and letting the airplane fly off when it's ready ought to be as efficient a method as any.

    Their arguments are: "it reduces drag at the beginning of the ground roll!". And : That's how it's done!" Can't see much scientific evidence beyond that. Maybe it is the best method, but can we analyze this a bit further?

    My points are :

    Drag at the beginning of the ground roll is downright negligible. Most of these guys keep the tail up for maybe a couple of seconds before slamming it right back down again.

    Pushing up the tail takes away energy that could be used to accelerate the airplane forward.

    In the three point attitude, with a little flap, most taildraggers are close to their maximum lift attitude which ought to be where you want it.

    The thrustline is pointed up slightly giving you a slight vertical component of lift. Maybe that's good or bad.

    I would like some opinions based on physics and aerodynamics, not current fashion. Let's hear it.




     
  2. Aug 14, 2019 #2

    narfi

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    Interestingly a coworker was just telling me this morning that if he pulled flaps too early on his Stinson that it took a very long time to get off the ground.
    It would be easier for you to test in an under powered plane than in a competition plane where the difference is probably pretty negligible.
     
  3. Aug 14, 2019 #3

    PTAirco

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    I agree that with the vast amounts of power these competition airplanes have it may be a moot point, but let's assume a "normal" airplane. My M4 Maule only has 145hp for example. Lands on a dime but take off is more sedate. I tried both techniques but not to the point of serious measuring and data collection.
     
  4. Aug 14, 2019 #4

    BJC

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    To takeoff in the shortest distance requires first accelerating the airplane to the minimum flying airspeed as quickly as possible, then quickly configuring it for minimum airspeed flight in ground effect while simultaneously rotating to the angle of attack for maximum lift. Then comes the challenge of accelerating in ground effect to the point of being able to climb out of ground effect. Lots of HP makes the acceleration, both on the ground and in the air, better, but excessive weight works against it.

    Tail up aligns the thrust for maximum acceleration. Note that a tricycle gear already has that taken care of.

    Flaps retracted allows maximum acceleration.

    There is an optimum flap extension angle for the minimum in ground effect flight. On many, that is full deployment.

    Once the minimum airspeed has been reached, any smooth rotation takes more ground roll than a quick, aggressive rotation.

    Rotating too soon either extends the ground roll or results in the airplane leaving the ground then settling back onto the ground. Timing is everything.

    The tests in the first video are not optimized.


    BJC
     
    Last edited: Aug 14, 2019
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  5. Aug 14, 2019 #5

    blane.c

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    Shortest takeoff I witnessed was as you described above, 21ft. However the airplane and pilot were as light as possible and on blacktop. I am certain at gross weight plus "it'll fly with another stack o' ribs on your lap" that the afore mentioned technique isn't the best. Plus doing it that way on a hard surface runway is ok on the propeller and paint but off a sandbar and the prop and paint get eroded and mud is another story too.
     
  6. Aug 14, 2019 #6

    Victor Bravo

    Victor Bravo

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    I have an opinion based on some amount of experience and a lot of research. Take it for whatever it is worth, I do indeed have ulterior motives coming out of my ears in this discussion :)

    The Piper Tri-Pacer official flight manual one year (mid 1950's) said that you could reduce the takeoff roll by up to 20% waiting to "pop" the flaps at the moment of rotation.

    The airframe drag at low speed may be negligible, but the flaps also deflect prop wash, which uses energy. Blocking or impeding the outflow behind the propeller makes some small amount of reduction in thrust. So you can add this tablespoon to the tablespoon of parasite and induced drag created by the flaps themselves.

    During the first part of the takeoff roll, the entire airplane is at some deck angle, which is more drag than a level airplane. So there's another tablespoon of drag. Most pilots have observed that raising the tail allows the airplane to accelerate faster, so the energy/drag that it takes to raise the tail is less than the reduction in drag from level (un-stalled( wings, a level fuselage, etc. So there is some net reduction in drag by raising the tail. Again, we're talking teaspoons and tablespoons, not parachutes.

    Tilting the thrust vector upwards by 10 or 13 degrees costs a little something.

    Having the flow horizontally into the tilted prop disc also results in a little tiny bit less thrust than clean straight inflow.

    So if you add up all these tablespoons and drops and medicine droppers worth of drag or reduced thrust, you get a cup full of lost performance.

    One more thing is that the timing of the flap deployment affects the drag of the control deflection. If you are rolling along, tail-up and level (no matter whether you pushed the tail up with the elevator or it came up on its own), and you pull the yoke back to lift off, your elevator has to push down hard enough to first overcome the nose-down pitch created by the flaps and then finally push the tail down and get the wings to the max AoA.

    If the flaps are retracted when you pull the yoke pack, the elevator can apply more force on the tail (able to "rotate" the airplane sooner) because none of that control force had to be spent on overcoming the nose-down pitch from the (extended) flaps.

    So I do agree with the concept that putting the tail up early, then rolling along until minimum flying speed is reached, then pulling the tail down at the earilest possible time (to get the wings to bite), then FINALLY snatching the flaps to increase lift to the maximum .... will deliver the shortest takeoff roll.

    The f*****g idiot in the AOPA video left out several techniques and ideas from his "scientific test" and did not bother to reply to me when I tried desperately to get him to consider my ideas and re-run the test. Again, yes, I had a pretty significant ulterior motive, but that motive is not relevant here on HBA.

    All those tablespoons, plus the greater available control power at the tail, adds up to a measurable reduction in max performance takeoff roll.

    At Chino, it will not make any difference whatsoever. If you would like to fly in to WHP one Saturday, I will be happy to let you test out the techniques I am suggesting, with and without the use of the product I manufacture. If you can rig up some sensors or other data recording stuff, or measurements, I will certainly help with the testing. Addicted2Climbing will likely loan us one of his wing strut camera mounts and we can get actual video and time stamps of the wheels leaving the ground, etc.

    Anyway, sorry for the rant. I've had to argue these points with countless pilots of nosewheel and tailwheel airplanes, Pipers, Maules, Cessnas, Cherokees, you name it.

    Holy crap, this post is almost as long as one of Armilite's... where's that text color button again?
     
    Last edited: Aug 14, 2019
  7. Aug 14, 2019 #7

    blane.c

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    You raise the tail, you lower the prop into sand or brush or what have you. Sometimes like on mud or ice with a little overflow on it you never let off the brakes because it'll increase drag to roll the tire than skid it on the "grease". There are many variables in real world flying.
     
  8. Aug 14, 2019 #8

    Victor Bravo

    Victor Bravo

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    PTAirco's inquiry was about short takeoff, not rough field, mud, etc. Of course you'd have to use different techniques for different surfaces.
     
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  9. Aug 14, 2019 #9

    blane.c

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    I was thinking "useful". There isn't much use taking off an empty airplane were it's easier to travel by car. Now go somewhere and toss a couple Salmon in the back and come home, that's useful.
     
  10. Aug 14, 2019 #10

    TFF

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    I have noticed watching that you don’t lift the tail to level; you lift above level. the clock is ticking so anything before flying speed is using up time; use wisely. Tail high, held at about half takeoff speed, then they start to rotate. All that time spent now accelerating is with pitch up on elevator not holding tail up. If timed right the plane hits flying speed right at perfect pitch. I have not looked, but I bet a notch of flaps would is added right at rotation. Plane around level, give or take, has less induced drag and one less wheel on the ground. Flaps right at end means they are not adding drag until wanted.
     
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  11. Aug 14, 2019 #11

    Pops

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    One year at Lockhaven, PA at the Cub fly-in I talked to a man that won Valdez 6 straight years in his Cub. Cub highly modified. He had a piece of string with a lead weight and a pitch scale on the side of the cockpit. He said he raised the tail up so the string was on a mark on the scale. Also said being just a little off made a big difference in the takeoff length. Then he let go of the brakes and counted 1-2-3 fast and rotated. Best takeoff was 17' and best landing was 28' of I remember correctly. Has a sea-plane prop on the GREAT running Cont- 0-200 and a huge amount of down thrust. So much down thrust when he went to full power with the brakes on the landing gear unloaded a huge amount as the air from the down thrust going over the top of the inter part of the wings and fuselage top developed lift.
    He was so concerned about weight even the tiedown rings were removed. For traveling cross country he had the cruise prop on the engine and the climb prop tied to the left wall of the cockpit.
    I remember the wings were squared off at the end of the round wing tips, ailerons moved out to the end of the squared off wings and the flaps make longer.
     
  12. Aug 14, 2019 #12

    Victor Bravo

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    Sounds like Jerry Burr's "White Bear", which I believe was a PA-11?????
     
  13. Aug 14, 2019 #13

    Pops

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    I believe you are right. Been several years ago.
    Wasn't he from Washington state ?
     
  14. Aug 14, 2019 #14

    blane.c

    blane.c

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    Was the tail wheel tire turned down on a lathe so it was tinier? And diminutive front tires as well compared to bush planes? Also I thought it was a Continental O-240? Then it was the one I saw take off in 21ft.
     
  15. Aug 14, 2019 #15

    spaschke

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    putting 2 notches of flaps at 45mph, but not before, in a cherokee 140 will get you off the ground and into ground affect sooner than a normal take-off. It minimizes the drag while accelerating to 45.
    I hate electric flaps, they are slow to deploy and take away some of the fine control.
     
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  16. Aug 14, 2019 #16

    BBerson

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    I had a climb prop on my Cherokee 140. It would lift off in about 300 feet with full flaps solo.
     
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  17. Aug 14, 2019 #17

    Pops

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    I had a Cherokee 140 with a climb prop for 5 years . Wife and I did a lot of traveling in it for those years. Never measured the take-off but it did a good job and still cruised a little better than a C-172. 172 would beat it in short field performance.
    C-140 is a good little airplane for 2 people with lot of baggage, same for the C-172.
     
  18. Aug 14, 2019 #18

    Hephaestus

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    My Cherokee 140 I did as above, held off flaps for acceleration. But it was pretty tired needed all the help it could get.
     
  19. Aug 14, 2019 #19

    flyrite

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    No salmon in the back, but there is a bunch of coolers and about 20 pounds of catfish
     
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  20. Aug 14, 2019 #20

    blane.c

    blane.c

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    I like catfish.
     

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