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Absolute minimum rotation speed

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Absolute minimum rotation speed

Old 16th Nov 2016, 09:30
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Absolute minimum rotation speed

Are there scenarios where the start of rotation could intentionally occur at or below stall speed, say for extreme STOL operation (e.g., bush pilots)? I'm especially curious if it would be important for a STOL aircraft to have the elevator authority to lift the nosewheel off the runway at (or even a little below) stall speed during TO?

I'm aware of, say Part 25 requirements for rotation speed VR (not be less than V1, 105% Vmc, speed that allows reaching V2 before height of 35 ft after takeoff, or VLOF @ 110% Vmu @ max rotation rate) -- but perhaps there are much more aggressive procedures for other types of aircraft/operations.

Under ground effect, an aircraft's wing produces more lift at a given speed but stalls at a lower AOA (note this effect contributed to the fatal Gulfstream 650 flight test accident). I'm guessing these competing effects (more lift, lower stall AOA) close to the runway are a wash and the actual stall speed under ground effect is the same as airborne -- so one could not exploit ground effect for added speed safety margin while accelerating past rotation speed.
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Old 16th Nov 2016, 10:36
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Unless something has changed in the background, civil certification doesn't really look at STOL .. ie the normal design standards still prevail.

STOL, as a fire-breathing animal, is a peculiarly military thing .. the military have protocols to manage the risks in a manner which would not be acceptable to the civil certification environment.

Now, as to just how Captain Bloggs might fly his STOL-advertised bird in the bush ... might be a different matter again ... just don't prang.
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Old 16th Nov 2016, 11:50
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On light aircraft the POH often says to get the nose off the ground as early as possible when performing a takeoff on soft ground. From the Piper PA-28 handbook:

Takeoff from a soft field with obstacle clearance requires the use of 25 degree flaps. Accelerate the airplane and lift the nose gear off as soon as possible and lift off at the lowest possible airspeed.
I think that this would result in some of the takeoff run being performed with the wing stalled.

At the other end of the spectrum, you've got the Concorde. If I remember correctly it was technically in an aerodynamic stall for both takeoff and landing.
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Old 16th Nov 2016, 12:15
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It's been a long time since I last did this. (22 years at least).

Muddy, pothold, unsealed and stone ridden runway in a C210.
Full flap, full back stick, full power.
Nose wheel off the ground at about 25 Kias aided obviously by the prop wash over the tail plane. Airborne at about 40-45 Kias, push the nose over to fly level with the airstrip in ground effect. Retract flaps as airspeed increases, once in normal takeoff configuration commence climb and retract gear.

Works great as long as you don't stuff up.

Last edited by Deejaypee; 16th Nov 2016 at 12:33.
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Old 16th Nov 2016, 12:40
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When one was young & not too bright, one would, occasionally, hold full back elevator on a BN2 prior to commencing the takeoff roll. Within one rotation of the main wheels, clearly visible out the left window, the aeroplane nose wheel would lift off. Judicious application of nose up elevator, would see the pitch attitude hold a steady takeoff attitude, and the aircraft lift off about 65 kts.
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Old 16th Nov 2016, 13:40
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I'm assuming the question relates to single engine only? There's a lot more to consider on multi engine take off performance and procedures.
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Old 16th Nov 2016, 20:38
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I'm not saying this is a practical case in the real world, but part 25 doesn't explicitly prohibit rotation starting before the in-air stall speed.

If we assume that the in-ground effect stall speed is Vmu (I know they are not) then in order to comply with part 25 I need to
(a) be able to get to V2 (min 1.13Vsr) before 35ft
(b) have a VLOF of not less than 110% Vmu

So I need a situation where I start rotating (well) before Vsr/Vmu BUT don't get airborne until 110% Vmu and don't get to 35ft before 1.13Vsr.

One thing that could do that is a mis-match between the ability of the aircraft to accelerate and the ability to pitch.

Suppose for some ungodly reason you decided to do a takeoff using only the stab, not the elevators. The stab rate is insanely low compared to the elevators, so you need to start trimming nose-up early - there's your early rotation. It's still going to take forever to get to VLOF, so we'll easily meet the 110% Vmu rule, and our "takeoff" will likely be at very low angles, so we'll take a while to get to 35ft, by which time we'll be well above min V2.

Now, that's not a terribly practical aircraft, but it would (at least in regards to Vr) be compliant to the letter of 25.107.

Could there be some kind of high T/W limited pitch authority aircraft design - maybe one of these supersonic ideas floating around??
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Old 16th Nov 2016, 22:37
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I sense a Bernoulli vs Newton conversation coming!

Note that many Alaska bush pilot STOL lift off very quickly. Just about as much flap as wing...

May 19, 2016 - Twenty-two-year-old Bobby Breeden, EAA 1131136, of Sterling, Alaska, won the short takeoff/landing (STOL) competition in the Alternate Bush Class at the 13th annual Valdez Fly-In and Air Show held May 6-8.

Breeden, who has been competing in STOL competitions since he was 17, made a 40-foot takeoff and 55-foot landing to claim his second consecutive victory in as many years.

The STOL contest, which featured aircraft specially modified to take off and land in short distances, included competitions in five different aircraft classes. Breeden’s takeoff was the shortest overall.

Other winning pilots included:

Ben Brown of Kasilof, Alaska, Light Touring Class, - 244 feet total, 161-foot takeoff and 83-foot landing, in a Cessna 172
Matt Conklin of Boise, Idaho, Heavy Touring Class – 258 feet total, 90-foot takeoff and 168-foot landing, in a Cessna 180
Jacob Williams of Anchorage, Alaska, Bush Class – 165 feet total, 54-foot takeoff and 111-foot landing, in a PA-18
Robert Pedersen of Oriental, North Carolina, Light Sport Class – 103 feet total, 54-foot takeoff and 49-foot landing, in a Just Aircraft
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Old 17th Nov 2016, 00:24
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You want short? This is short.

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Old 17th Nov 2016, 00:42
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One aspect to consider is that the actual stall speed will be a function of power due to the propwash over the wing. Stall speed with full (take-off) power is rarely measured in flight and if it is then it will be reduced even more than will result from propwash because a steep nose up attitude will occur which will result in a normal acceleration which is less than 1g. Therefore, in a STOL aircraft take-off the unstick speed may well be below a quoted 1g stall speed for the same configuration because that will inevitably be measured with idle power.

Stall certification for Part 23 aircraft requires stalls to be flown with idle power and with 75% MCP set. I have flown an aircraft in a wings level 75% MCP stall and have been about 45 deg nose up with close to full aileron and full rudder and the ASI pegged solidly on the bottom stop but the wing had not stalled aerodynamically!
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Old 18th Nov 2016, 00:08
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I haven't done the math admittedly, but I believe early rotation will only lengthen (not shorten) the required takeoff distance for a given obstacle clearance.

So it may be good for soft field but not for short field.

On airliners, early rotation is also a frequent cause of tail strikes.
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Old 18th Nov 2016, 09:14
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To answer the OP's question, yes there is at least one legitimate time when airborne at absolute minimum speed is done.

I used to fly a transport category aircraft on skis and the takeoff procedure was full back stick until the nose lifted, check the attitude holding it off the snow, then when the mains were in the air, push forwards, holding it in ground effect until v2 then climb out.

The stall warning is blaring the whole time until speed is gained in ground effect. The nose rotation speed depended on CoG and snow conditions, but was definitely below Vs.

Of course you are airborne below vmca, so "V1" was V2 but there is no temptation to keep going as you're not about to run out of tarmac!
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Old 18th Nov 2016, 14:09
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peekay4 - that matches the PA-28 POH. The short-field procedures call for acceleration to 52kts and then immediately lifting off, whereas the soft-field procedures call for lifting the nose as early as possible, and then lifting off as early as possible.

Presumably on very soft ground the drag from having the wings stalled (especially at low airspeed) is less significant than the drag from having all the weight on the wheels, so you could end up with a shorter takeoff run like that. For a "nice" runway (where drag from the wheels is likely to be minimal) you'd definitely get better acceleration and therefore a shorter takeoff roll by keeping the nose down until reaching liftoff speed.
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