STOL approaches

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

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  1. Sep 14, 2019 #1

    PTAirco

    PTAirco

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    I came across this a while ago. It's been exactly year since I got my Maule M4 and for most of that time I was not getting the most out of its capabilities. (Landing capabilities - it only has an O-300 so take offs are less than spectacular.)

    I have played with the ideas put forward in this video before but never had such a good explanation why this is the bee's knees as far as short landing go. Most guys I know practice short field landings by approaching low and slow with lots of power and drag it in by the prop in a nose high attitude, cut the power when they reach their spot and flop it down. It works in some situations. This is totally different.

    I really like this alternative approach. In essence it's merely a steep approach using minimum practical airspeed and thuis forward speed over the ground. In case of the Maule in the video, airspeed is almost off the clock. Usually this requires a trickle of power to stop the descent rate from getting too high.In my Maule and another heavier version I flew recently, this sink rate is pretty high below 55mph. Bottom of my white arc is 53mh but I can fly an approach like that at 50 and it feels safe and controllable. I am still cautious about flying it with the ASI needle pegged at zero though!

    There is no need to be afraid of stalling since you're in a steep descent, not trying to hold that speed in level flight. Being on the back side of the drag curve, in a stable approach, it does not matter much if your engine were to quit; lower the nose and the airplane will go from high induced drag to its max glide speed and suddenly cover way more ground and sink less. Few pilots I know will raise the nose to avoid overshooting, most will aim for the threshold and pick up speed and still overshoot. Seen it many times. The "Stick and rudder" book touches on this subject too.

    The tricky part I found is that there is really not much left to flare with. A shot of power at the right moment helps, but then you're relying on that being available when you need it. However, if you're tying for a maximum effort short landing, not something you would have to routinely do, I guess the risk is quite small. Timing this shot of power is the tricky part. I got it wrong a few times, but the worst that happens is a "firm" landing, nothing terrible. Doing a really short landing you can't afford to grease it on anyway; you want to get it on the ground and you want it to stop flying at that instance, get the flaps up and stand on the brakes.


     
  2. Sep 14, 2019 #2

    BBerson

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    No need for a shot of power to flare. Instead just relax the back pressure for a second at 50 feet agl (more or less). The plane falls 40 feet and gets back the needed energy to flare at around 10 feet (more or less). It takes skill, isn't a "stabilized" approach but it works. Especially just after clearing the trees. Not that hard really, just be very gentle relaxing the back pressure at first practice. Relaxing the back pressure is almost always a good thing. (opposite of pulling back to stall stick position)
    For a tail dragger, then push the stick forward at contact to get full braking action and eliminate bounce if not stalled at contact.
     
  3. Sep 14, 2019 #3

    TFF

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    The takeaway is really it’s about precision flying. Yes STOL planes will fly slow, but that’s not really the secret. To get into a tight place, you must be precise and know your plane 100%. Most people, including me, are not precise. Precise is getting into what the standards is for professional pilots. Being a STOL pilot is not being a hot dog pilot. It’s about airplane control because you are going to remove any cushions a regular pilot cheats with.

    If the dream is to land in a high school football stadium, you are going to be dragging it in with power. That is double skill. I think most of the semi improved strips in the video could be visited with a C150. You just can’t be sloppy.

    The US Air Force did not believe the F86 Saber was controllable to the average USAF pilot. They thought it was too high performance and took above average skill. They were on the fence to buy it. What sold them was this. Bob Hoover flew the prototype from one end of the lake bed to the other, set up in landing configuration and at landing speed. Something like five miles, 10 ft above the ground at just above touchdown speed. Proved it could fly slow and in control. Wishy Washy does not cut it.
     
  4. Sep 14, 2019 #4

    bmcj

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    Great video!

    The only time I ever wanted to carry power during an approach is to drag the plane low and slow for sightseeing. Other than that, I always prefer a power off high steep approach. I am a strong believer in nose high slips to landing... they give you better control, better visibility, no worries about ‘making the field’, and a little energy for your flare as the wing starts flying again as you kick it out straight for the touchdown. When I was in practice, it provides the ability to hit very close to the spot every time with a light no-squeak touchdown instead of the firm plunk you would expect. Also, with a nose high steep approach and full cross control, I found that the plane gently talks to you through an increase or decrease of the commanded angle of bank, letting you know if you need to pitch more or less to bring the opposing roll-yaw forces back into balance (without exhibiting any tendency to do a surprise stall break - possibly because of all of the crossflow on the wing).

    For those that say I’m crazy for tempting a stall-spin on final, go to altitude and try this maneuver up high so that you can see how the plane talks to you. Smoothly cross the controls and play with the pitch to balance out the roll against the yaw. I’m not sure if this works in all planes, but it serves me well in Cubs, Champs, Cessnas, Stardusters, etc.
     
    Last edited: Sep 15, 2019
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  5. Sep 14, 2019 #5

    PTAirco

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    Yes, I discovered that too. Once I'm past the obstacle, gently let the nose drop a little to give you just a bit more authority to flare.
     
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  6. Sep 14, 2019 #6

    delta

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    I don't know if the slips these guys use is applicable, but I'll bet they sometimes find it useful in the wild.

     
  7. Sep 14, 2019 #7

    Dan Thomas

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    Couple of thngs: You CAN stall in a steep descent. The load factor is still essentially 1.0, the angle of attack is high, and you might be in a spot where the propwash over the inboard wings is keeping the AoA low enough to discourage stall progression. If that engine quits, it can stall and recovery might not be possible in the remaining altitude.

    Remember that you have a high sink rate at very low airspeeds, and a sink rate can crash an airplane just as quickly as a stall. Ercoupes, stall-proof that they are, have crashed with high sink rates on final. Their short wings add to that sink, and your M4 has short wings, too. Beware. My Jodel, with its short wings, has a stall of 40 MPH but I can't approach even at 55 or the sink gets out of hand. As the sink increases so does the AoA, and you can stall the airplane in a level attitude or even a bit nose-down. It can bite the one who thinks AoA is the same as pitch angle.

    The lack of airspeed to flare is a sign that you're too slow. The prescribed method for STOL landings isn't much different than for regular landings. Approach at book value (should be around 1.3Vso), power off or very little power. It will be steep enough already. A lower speed, by raising the nose, will steepen it further, but don't let it get too low. Once the airplane is within about 30 feet of the ground, power should be off and the nose should be coming up already. Speed will decay and descent will slow, and if it's done right you arrive at the surface with NO float and insufficient speed to balloon or bounce or anything else. You can clamp the brakes on and stop pretty short. I've used that method for many years with many airplanes. I taught it as an instructor.

    Too many pilots are approaching too fast and carrying that speed into ground effect, flaring at five feet or something, and floating or bouncing or porpoising or ballooning far down the runway. So sad. They can't figure out what the problem is. And yes, they dive at the runway in an effort to fix a high approach. If they'd raise the nose they'd get down shorter and slower.

    Here's the prcoess. The initial nose-raising is called the round-out and it's the first stage of the flare. Note the altitude where it starts. NOT at the surface or anywhere near it. Ground effect has the effect of lowering the drag and lowering the stall speed as well.

    [​IMG]

    Those airspeeds are for a particular airplane. Certainly not a Maule.

    To get a touchdown at the threshold of a short runway, you need to aim at a point before the runway.
     
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  8. Sep 14, 2019 #8

    PTAirco

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    I have to politely disagree with some points: at an approach speed of 1.3 Vs you will never get a maximum performance short field landing. Not by any standards. And it won't be a steep approach to get you over trees either. Steep is relative. That diagram is a textbook "normal" landing following a standard approach, yielding standard results. Yes, you can fly like that and hit your chosen spot, sure. Provided the area in front of the threshold is clear of obstruction for long distance. But this is about hitting your spot at the minimum forward velocity after clearing obstacles. There is no round out. A round out in a short field landing means you were coming in too fast. The transition from approach to touchdown should be over the shortest possible distance or you're eating up runway (or sandbank or whatever).

    What I like about the really slow approach in the video is that fact that your operating with a really high induced drag while aiming at your spot. Should you lose your engine, no problem, drop the nose to max L/D and carry on and land at a speed that will allow you just flare enough not too hit hard. To get the most out of it, you do need to keep some power in, but I don't consider that a huge detriment as long as you have an option if it does quit. I have flown it right down to the ground like that (uhm, semi-intentionally...) and by hauling back to the stick was able to arrest the descent to get a merely "firm" landing. A shot of power smooths things out nicely without adding forward speed .

    Of course, I agree, you can stall in that configuration a little easier but I never found it to be a problem. I only slow it down to these kind of speeds (50mph or less) on final, with no turns or anything. Some of that is due to my Maule's very benign stall characteristics. Nothing much happens. I keep it trimmed to about 60 and if I encounter a gust, all it takes is a momentary relaxation of the yoke and all's well. Same with an engine failure; just let go of the yoke a little and we're back to a "normal" approach speed. I would really like an AoA indicator but the airplane is very forgiving and finding the actual stall angle is a bit of a mystery anyway; it just mushes. Only way to get a break is power on and be really harsh on the yoke,

    I would not do this in some other airplanes, but Cubs etc respond well to this approach since they do not develop a high sink rate.

    To be super pedantic - if you're descending steeply, isn't your load factor below 1.0? Lift < weight. Maybe not much but it's a small margin.
     
    Last edited: Sep 14, 2019
  9. Sep 14, 2019 #9

    PTAirco

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    Here is another video on this approach to Stol Landings (fine pun..!) They basically teach the same technique, which would frighten most instructors at your local flight school to death.

     
  10. Sep 14, 2019 #10

    flyrite

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    PTAirco, Ain’t hard , Seat time will get ya there. When you get comfortable with it ,a whole new world will open for ya.Done all kinds of flying, Backcountry is the most rewarding!
     
  11. Sep 15, 2019 #11

    Pops

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    On my short wing 27' wing span Falconar F-12 , if light could get to down to about 78-80 mph before going into a high sink rate, if heavy its 85 mph, if GW its 87-88 mph. Since it set low to the ground, the TE of the wing is about 15" off the ground, it had a high ground effect so you had to keep the approach as low as possible. Stall speed out of ground effect about 68- 70 mph.

    With a low inertia, light weight aircraft the airspeed drops off very fast as the nose comes up with the huge increase in drag. Flying it on in behind the power curve can be a little more on the edge with the light weight aircraft because if the engine quits running, you have to be a lot faster getting the nose down to keep your airspeed above stall. A light weight, low inertia aircraft is my SSSC. EW of 485 lbs.
     
    Last edited: Sep 15, 2019
  12. Sep 15, 2019 #12

    Dana

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    Not at all. If you're in a steady uaccelerated descent the load factor is 1.0, lift equals weight. Otherwise your descent rate would be increasing, i.e. you'd be accelerating downwards.
     
  13. Sep 15, 2019 #13

    Dan Thomas

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    A steady, straight-line descent is at 1.0. A steady, straight-line climb is 1.0. Straight and level is 1.0. This is one of the things that crops up occasionally over on Pilots of America, with folks thinking they can do crazy stuff in a descent because the stall speed is somehow lower. The only time you'd have a lower load factor in a descent is if the descent rate is constantly increasing, which means an ever-steepening dive. Doesn't end well.

    In a very steep climb the propeller is taking some of the weight, so the wings are a little less loaded. If you had the thrust to climb straight up, the wings would be carrying nothing. In a steep descent, the drag is supporting some of the load, but it's small. With a bit of trig you could figure it, but at the shallow angles that even a steep descent involves, it's very small. I sure wouldn't rely on it to figure on a lower stall speed.

    If my trig is right: If we had a 10 degree glide angle, which is pretty steep considering that the standard approach angle is three degrees and the typical lightplane might approach at five degrees, you'd see a decrease in load factor to about .83, which would decrease the stall to maybe .96 of normal.

    Your airspeed indicates lower than stall because the pitot isn't headed straight into the wind at high AoA. I could fly my old Auster in slow flight and see the airspeed reading a bit negative!
     
    Last edited: Sep 15, 2019
  14. Sep 15, 2019 #14

    pictsidhe

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    This thread is making me wonder what spots my Hurricane would be able to get out of. With a sub-cub stall speed, getting in should go OK. Bigger tyres and more power seem a good idea. I think I'll leave some extra room in the wheels wells...
     
  15. Sep 15, 2019 #15

    PTAirco

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    Ok, I stand corrected on that one.

    What model Auster did you have? My very first basket case ever was an Auster 6, 1946.
     
  16. Sep 15, 2019 #16

    PTAirco

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    That video shows exactly what the steep, slow approach, back of the drag curve approach is not. Nothing wrong with that, but this is approaching the touchdown point from low level, at a shallow angle with lots of power.If the engine stops over the water, you get wet.
     
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  17. Sep 15, 2019 #17

    Dana

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    That was Dan with the Auster. My first plane was a '41 T Craft, though, which has a lot in common with the Auster.
     
  18. Sep 15, 2019 #18

    Aesquire

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    With my glider background, I dislike the power on behind the drag curve approach.

    I see the utility of the gliding below max L/D speed for a STOL landing, but have seldom used it, preferring to fly above max L/D until quite low, then flaring to landing attitude & bleeding the speed off, idle power all the way.

    But that's not perfect for a minimum runway landing. It does however reduce the risk of unexpected stall when running into wind gradient.

    Remember, on a strip surrounded by trees, brush, or hills, the drag of the taller, uneven surface means a very high likely gradient. You might have 20 knots on the nose at 500 feet, then suddenly only 10 at 30 feet. The plane in a speed stable condition, has to recover that loss, by losing altitude, or adding power. The sudden sinking feeling and apparent acceleration across the ground, can trigger a pitch up by the pilot to compensate, if you haven't anticipated it. That is not an uncommon cause of bad landings, stalling early & higher than expected.

    The 3 important speeds in this corner of the envelope are, in descending order, Max L/D, Minimum sink, & stall.

    Flying at Max L/D in approach is wrong. Wrong. Wrong. Speeding up OR slowing down, lands you short of your aim point. Either faster or slower is necessary to allow for flare or any possibility to stretch the glide of you come up short for any reason. ( wind shift, pilot misjudgment, gremlins )

    Flying below Minimum Sink has you jammed into the corner... Any slower, and you sink faster & steeper. Even if you don't stall, without an increase in speed, or blast of propwash, the landing will be "firm" to "impact".

    This is where the timing, and skill from practice mentioned above applies.

    Flying above Minimum Sink, but below Max L/D, gives you more energy to lose, but that also is a cushion against gradients and error.

    That's my long winded 2 cents of obvious ;)
     
  19. Sep 15, 2019 #19

    PTAirco

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    I would agree - for a normal landing. Flying at max L/D, your judgement would have to be perfect to hit your spot, should you lose power., you have nothing left to play with. In my hypothetical case of an approach over obstacles into a very short field and a 50 mph approach, I am flying much slower than max L/D ( about 70 for mine) so if I misjudged and was under shooting, I could pitch down for best glide and make up for it. You could do the same by flying faster, and pitching up to best glide, yes. Given enough room, that might be a better option, but not if you have limited space.

    Like I said, I would not do this in a lot of other aircraft. Flying on the edge of a stall with some airplanes makes me nervous because of the sharpness of the stall, dropping off on wing etc. Maybe my Maule has spoiled me a little but with Cubs and similar aircraft this is not a big deal. The stall is so non-eventful in this approach configuration that instant recovery is extremely easy. And yes, I did practice these kinds of descents endlessly at 4000 ft first.
     
  20. Sep 15, 2019 #20

    pictsidhe

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    If you come in steep as the first video suggests, then relax the stick a little to gain some flare speed, won't you drop in even steeper? That seems like it could be useful.
     

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