T/O performance.

[thing]

Went flying today but being as the weather was pants I decided to do take off experimenting. I took the 28 with fuel to tabs (133 litres) and me on board. I did a short field take off, two stages of flap, run up to full power on the brakes, rotate at 52 kts and I got off in 800 ft with 50 ft alt passing at about 1100 ft.

I then did a normal take off, no flap, no brakes, rotate at 55 kts and got off in exactly the same distance, however 50 ft came up at about 1300 ft which I was expecting. I was suprised that the no flap t/o run was the same. Runway is hard, wind was absolutely calm, temp around 0C. Anybody explain or is this normal? Incidentally the short field landing at 65 kts approach, full flap and fairly heavy braking was around 1000 ft from 50 ft.

[Pilot DAR]

It's great that you're experimenting with this, and observing the outcome. The results you describe sound right, as the PA 28, though an excellent wing, has a stabilator, which is a little less than ideal for optimum low speed pitch control. Several times, I have gotten PA-28's "hung" while experimenting with short/soft field techniques.

The stabiliator can be stalled, as it's only way of changing to create more lift (downward) is to change angle of incidence/attack. Whereas the traditional stabilizer & elevator changes camber, making it a bit more stall resistant, and the fixed (or nearly so for many C 180 series) stabilizer is more stall resistant. The early C 177's suffered from this, and that's why 177's have upside down slots in the stabilator.

I found the PA 28 to be extremely technique sensative in short/soft field takeoffs. Too much pitch up control, or at too low a speed, and the drag from the stabilator overcomes any advantage you might have had in getting airborne early. In one case, my buddy got his Arrow off the ground at too low a speed, and it would not climb, or accelerate, it was just hung in ground effect, and not going to clear the obsticles at the end of the 3000 foot runway. Once I knew we were committed, I raised the gear, and the drag reduction was enough to get us climbing a bit. In another case, I was taking off a frozen lake, and distance was not an issue, so I just let it keep going, 'till it climbed - it was a long way!

Takeoff performance data gathering takes dozens of takeoffs, with extreme precision, and the data averaged, and corrected for the local conditions. More than I'm sure you plan on. The determination of "50 feet" is more than just an altimeter - it's not precise enough.

Within the techniques in the POH, and your skills, keep experimenting. When you have a 1974 or later C 172, you'll see a much more pronounced difference resulting from the differing techniques, the PA 28 is just not the best plane to show the differences in contrast.

In any case, your challenging yourself, and making these observations will really increase your skills - excellent!

[Morrisman1]

Drag from the flaps perhaps? During short field takeoffs in the 181 I leave the flaps up till about 40kt, then click on two notches and start to bring the nose up and let it fly when it wants to, usually around 50-60kt depending on loading. Using this method I have had a pa28 airborne in just under 100m on grass, but it was empty, I don't remember the winds being significant either.

Good points you raise there PilotDAR, Ill keep that in mind in future.

[peterh337]

Too much pitch up control, or at too low a speed, and the drag from the stabilator overcomes any advantage you might have had in getting airborne early

That's why, IMLE, the total distance to get "off" the ground is not reduced by the soft field or short field takeoff methods, compared to a straight max performance takeoff where you rotate at the lowest flying speed and go straight up, with the stall warner just going.

[24Carrot]

the total distance to get "off" the ground is not reduced by the soft field or short field takeoff methods

Surely "soft-field" is a separate category?

On a "soft" surface with unusually high rolling friction, your top priority is getting some weight off the wheels to reduce that rolling friction (and maybe protect a nosewheel), and drag from the tail is a nicer problem to have, because at least you did get off the ground.

[BackPacker]

I was suprised that the no flap t/o run was the same.

The take-off run to rotation is a simple linear acceleration and there are only two factors important: Thrust produced by the engine, and aircraft mass. Both are not influenced at all by the flap setting.

Theoretically speaking deploying the flaps adds drag, but since the wing is hardly producing lift (as the AoA is close to zero), the induced drag from the flaps is virtually zero too.

Not entirely zero though, and that tiny little bit is the reason that people like Morrisman1 do the initial run at zero flaps, only selecting t/o flaps at the last moment. But that's just to squeeze that very last bit of performance out of the aircraft in tight situations. In your situation, I don't think you'd have the proper instruments/measurement tools to actually notice the difference. And this technique is not mentioned in the PA28 POH, so Mr. Piper apparently also doesn't think it makes a significant difference.

The reason for setting t/o flaps is that you can rotate at a slightly lower speed (but what's two-three knots between friends?), and get the best (angle of) climb performance straight after rotating. Setting flaps also increases the average angle of incidence across the wing, which in turn reduces the required fuselage AoA and allows you to see straight ahead over the nose better. And in certain aircraft you need t/o flaps to avoid a tailstrike upon rotation.

Where did you get the 52 and 55 numbers from by the way? I have a PA28-161 Cadet POH here and it specifies a rotation speed between 40 and 50 knots, depending on actual weight, both for normal and short-field t/o. Now the Cadet has a lower MTOW than a normal spec Warrior, so your -28 might have a graph extending all the way up to 55 knots or so. But you were not nearly flying at MTOW, so my gut feeling tells me that your rotation speed should have been around the 45-48 knots mark or so. Did you look at the POH for correct technique in both cases, including proper speeds? It takes book technique to achieve book performance, after all.

And speaking of the POH, I just checked mine (as said, for the PA28-161 Cadet) and the t/o roll for 0 and 25 degrees flaps in similar conditions is virtually the same. If there's a situation where the difference is more than 100 feet, I didn't find it.

[peterh337]

Google (http://www.firstflight.com/lessons/flt07.htm) is useful as always.

[Pilot DAR]

the total distance to get "off" the ground is not reduced by the soft field or short field takeoff methods, compared to a straight max performance takeoff where you rotate at the lowest flying speed and go straight up

I'm not in agreement with this. Though the PA 28 is not the best type differences for showing the differences, failure to use the appropriate technique on other types will result in performance must less good that the POH presents.

The reselection of flaps during the takeoff roll, though done, and sometimes quite effective, is not a technique you're going to see in a POH, simply due the the risk vs benefit.

With a soft field technique, there is the presumption, that the ground run portion of the takeoff is the only distance you're trying to minimize. You're expecting to get airborne in ground effect, and stay there for a while.

It is my opinion that in general, the manufacturers simply don't want the liability of the risk that pilots will crash while experimenting with even shorter/softer field techniques. Even the amazing deHavilland Twin Otter is capable of much shorter ground rolls than the POH says, but the risks of getting it wrong are very high, so deHavilland won't tell you how. (one of their test pilots taught me though;)).

I am happy to think that other than takeoffs with high crosswinds, or granny and the aunt aboard, every pilot would purposefully practice either short or soft technique on most of their takeoffs, as Thing obviously has, It will make one a much more confident pilot. Many times I've had an air traffic controller (some have more time and are more bored in parts of Canada) ask me if an aircraft was STOL equipped based upon watching my takeoff or landing (no screaching of brakes or anything, just precise flying). "No", I'd reply, just practicing...

As for the application of full power on the brakes, yes most POH's say this, but I never do, unless it's a fliight test. Paticularly with fixed pitch props, the advantage is very small, and the risk of erroding the tips and leading edge much greater on some runway surfaces. If I really need that last bit of advantage, I'd rather plan well in the backtrack, and begin to apply the power as roll around to line up, never coming to a stoip, particularly on soft runways. A fixed pitch propeller blade, and most variables are pretty well stalled over most of the blade with full power and no forward motion. You're just making lots of noise and drag, with minimal thrust, until you're rolling.

[peterh337]

Sure; I was not suggesting that e.g. a soft field technique will be worse than a normal takeoff when departing from a muddy field.

What I meant is that I don't think doing a soft field technique from clean tarmac will be better than doing the normal technique from clean tarmac. The SF technique gets the aircraft off the ground sooner but at a big cost in drag.

[Pilot DAR]

Definiately, a soft field technique on pavement is going to result in a longer takeoff run than a short field technique, though both should be practiced often....

[thing]

Thanks for the replies. I got the 52 and 55 knot figures from the POH, it's a -161 Warrior II, 1978 I think. Bear in mind I don't fly to test pilot accuracy, which is precisely the reason I like doing that sort of thing, I know what the a/c will do when I'm flying it, not some steely eyed killer in a brand new a/c.

At the other extreme we have a 180hp 172. I took that flying in a 20 knot straight down the runway wind just before Christmas with around one third fuel and I was at rotation speed before I had full throttle on....I reckon the ground roll was 200 ft or less and climbing through the wind gradient had the VSI at 1700 fpm.

All good stuff.

Ah, forgot to mention, I did a ROC check as well on the 28. With two stages of flap and Vx of 63 kts it had 800fpm; with flaps up and Vy of 79 kts it had 1000 fpm at a weight of 1990 lb, 450 short of MAUW. The short field approach speed was supposed to be 57 kts for that weight, but I ballsed that up and approached 7 kts fast, so I suppose the short field landing distance from 50 ft would be more like 800 ft or so.

[Big Pistons Forever]

Went flying today but being as the weather was pants I decided to do take off experimenting. I took the 28 with fuel to tabs (133 litres) and me on board. I did a short field take off, two stages of flap, run up to full power on the brakes, rotate at 52 kts and I got off in 800 ft with 50 ft alt passing at about 1100 ft.

I then did a normal take off, no flap, no brakes, rotate at 55 kts and got off in exactly the same distance, however 50 ft came up at about 1300 ft which I was expecting. I was suprised that the no flap t/o run was the same. Runway is hard, wind was absolutely calm, temp around 0C. Anybody explain or is this normal? Incidentally the short field landing at 65 kts approach, full flap and fairly heavy braking was around 1000 ft from 50 ft.

This should not be surprising as the POH specifically mentions that for a short field takeoff, no obstacle: quote use of partial flap does not decrease the minimum ground roll, therefore, leave the flaps up or lower the flaps to 25 degrees as desired unquote

[thing]

Ah, missed that bit, good spot sir.