Airborne_Again wrote:
Of course, but is that caused by elevator drag or by drag from the wing at the high AoA?
Any lift creates drag, so nitpicking it would be both (at least when centre of gravity forward of centre of lift), but I would tend to think the lift created by the wing greatly outperforms the one of the elevator very quick
Yes, elevator up creates drag, but not so much when beginning to accelerate and not enough to make liftoff and acceleration impossible on it’s own. While it’s all dependent on the size and power and model of the aircraft anyway, with a single engine in a soft field takeoff most people (1) set flaps, and (2) pull up, to save the nosegear, (3) right from the beginning. Now if you pull too hard and too much, what happens is the air flow from the prop under full power pitches the plane so far up that the wings, flaps, frame and rudder in their combination create so much drag that the plane does not accelerate fast enough for the wings to create enough lift to take off. In a soft field, with weels sunk into the ground, and you are heavy and / or in hot conditions, your engine may not produce enough power to pull you “beyond the curve” in time before the end of the airfield.
So, Noe, yes, in a ‘normal’ soft field takeoff you have drag, yes, from both the flaps, elevator, weels in soft field, etc., but as you accelerate, the wings quickly create enough lift. But if you pull too hard, the wings themselves are in an angle of attack that doesn’t even allow enough acceleration in the first place. You’re then stuck in a situation where the engine does not create enough power to pull the aircraft, against all resistances from soft field, flaps, elevator, heat, weight, beyond the speed necessary to lift off. If the pilot then does not abort, but tries to forcefully lift off by i.e. even more pitch, desaster will happen.
Plenty of opinions on technique here, illustrating the eclectic nature of GA – from airline pilot instructors who don’t teach STOL ops because there’s no off-airport flying in Europe (except for the Alps, the Pyrenees, the British Isles and Scandinavia) – to European bush and mountain pilots who watch their flight instruments on take-off as assiduously as any duck or ptarmigan.
I’d never risk taking off from a short field, let alone a soft one, but an aspect which hasn’t been discussed is how to prepare the machine. Newton’s second and third laws suggest leaving non-essential mass/fuel on the ground and, for a soft surface, setting minimum tyre pressure ( = ground pressure).
Walking any makeshift runway, and/or feeling it with the main wheels before landing might be wise.
What else could we do to stack the odds before starting the engine?
It’s really no rocket science, IMHO. I’d say the ground roll gets a little longer with full back pressure, but once the plane is a little faster you can release the back pressure just enough to keep the weight off the nose landing gear.
I made this simple video.
To a large degree, the takeoff technique which will become most to your liking, will be the one which you have learned works best for the conditions most common to you. So, trying variations is wise, as long as you’re attuned to the outcome. Thinking back, I would estimate the number of “other than hard surface” runway takeoffs I have done to be in the range of 10,000 to 11,000, with 7000 of those at my home runway. Sometimes firm, sometimes so soft that taxiing difficult. That has afforded me ample opportunity to find what works well, and not so much. I either learned, or did not go flying.
The least likable combination was the Tomahawk in a muddy 1600 foot runway. I learned that Tomahawk takeoffs are really all about the same, with little difference in outcome for differing techniques, other than the spread between regular and scary! Cessnas and high wing Pipers, on the other hand, particularly those with STOL modifications, have remarkable capability on soft ground if you work at your technique. The worst of those for me was the 182 amphibian at 3350 gross weight, on 6.00 – 6 mainwheels, on my runway so soft that there was sod stuck in the axle ends when I arrived at the next (hard) runway. I’d left 6" deep ruts, but I got out. The stall warning was screaming for 500 feet of the runway run. Then I spent a day repairing the ruts I should not have made!
One in ten landings I make will be on a paved runway. Otherwise, grass, sand/gravel/ water, ice or snow. Often, when landing on an unknown surface (beach sand, or snow, for example), I’ll drag one or both main wheels/skis to assess the surface. This amounts to a soft field touch and go. There certainly have been times when the touch indicated that an immediate “go” was necessary. An example is a snow landing on a lake, when after touchdown, you realize that the snow is only on top of useless slush, rather than firm ice. I have never landed a plane onto a surface form which I could not take off when I desired, though there has been some shoveling a few times!
Jacko wrote:
I’d never risk taking off from a short field
Funny statement. 
Define short. Also why would you never accept the risk? What is the risk exactly (in POH terms)?
Airborne_Again wrote:
Of course, but is that caused by elevator drag or by drag from the wing at the high AoA?
Flyer59 wrote:
It’s really no rocket science, IMHO. I’d say the ground roll gets a little longer with full back pressure, but once the plane is a little faster you can release the back pressure just enough to keep the weight off the nose landing gear.
This is exactly the technique that I’m talking about that is appropriate for soft fields. But again, consult your AFM/POH or operator’s manuals for the type of aircraft you fly.
Jacko wrote:
I’d never risk taking off from a short field
Funny statement. Define short. Also why would you never accept the risk? What is the risk exactly (in POH terms)?
Ignoring the POH (which may contain no performance data at all, or none of any direct relevance) a short field is one where the departure cannot be aborted without injury as soon as the pilot determines that obstacle clearance is not assured. The risk, unlike most landing incidents, is of more than mere material damage.
A soft field is much easier to define empirically: a field where the bearing capacity of the soil is lower than the tyre pressure. For most light aircraft running about 18 psi in the mains (for grass runways), this is roughly where the boot heel of a traditionally-built EuroGA contributor leaves an impression in the soil.
N.B. 18 psi is typically not sufficient to prevent hydroplaning at more than 40 knots on a wet runway, for which reason the POH may recommend a higher pressure.
The POH shoukd contain data for both the takeoff roll and the takeoff over a 50 ft obstacle. But if you want to make sure you can always come to a stop, or land again, from any phase of the takeoff … you can only takeoff from very long runways.
What plane? And what are your limits regarding runway length?
Jacko wrote:
I’d never risk taking off from a short field, let alone a soft one
Sorry, but you lost me on that one. Why not ??
For Maule aircraft produced from 1957 to the present day there is no performance data whatever in the POH. The same may be true of other aircraft certified under CAR 3.
In a modified MX-7-180 at ISA, and at “play” weight, I tend to pay attention to any flat field with less than 400 ft ground roll or 800 ft to a tree line. These are not absolute limits, for instance 200 ft ground roll may be enough if we can roll onto water, and/ or if there is no obstacle. Likewise if the obstacles are birch or spruce trees, the tops of which are quite soft. On the other hand, ski lifts and power lines demand a decent margin.
Having established obstacle clearance at minimum weight, we can land and add fuel/pax/cargo as appropriate.
The decision to abort or continue is taken as soon as the tail lifts, typically after three seconds.
A steep (say >10%) down slope shortens the ground roll appreciably but may shift the abort point to the top of the the slope.
In general, if landing is comfortable (which is a straightforward energy calculation based on actual stall ground-speed and runway slope), the Maule will take off OK.
