Porpoising on take-off?!

[KandiFloss]

Hello all!

Ive just joined a new flying club and I am currently filling out paper work to assess my skills. One of the questions is 'How would you handle porpoising on take off'? I've heard of porpoising on landing, but not on take off.

I can only imagine that it would happen if you tried to pull back at too low an air speed? You could take off momentarily due to ground effect, but that's all. The action that I would take is to make sure that full power was applied, let speed build up and then pull back. HELP?!

Has this ever happened to any one?

Thanks!

[Lister Noble]

I've never heard the expression,but "googled" it, and basically it is trying to lift off before correct flying speed is reached ,promoting a series of mini stalls,dives and climbs along the runway.
Answer must be to lower nose a little and wait until correct speed is reached.
I'm sure you will have much more experienced pilot replies than mine.;)
Lister
Indoors because there is a 25 knot crosswind!
Now going gardening.

[Genghis the Engineer]

The term "Porpoising on take off" sounds to me as somebody is trying to describe a pitch circuit pilot induced oscillation (Pitch PIO). The best way to deal that is usually to stop chasing the pitch attitude on the stick/yoke.

So, the short answer is keep the pitch control still in about the right place, then make only small slow adjustments.

I use to have a little Canard single seater called a Goldwing which did this very readily; it's likely a consequence of either an aeroplane with a lightly damped longitudinal short period mode, or a pilot who is too ready to make rapid control inputs (or both together!)

I've never heard of a flying club that set a written exam for entry before? Whereabouts is this - an unusual way of doing things.

G

[BEagle]

1. Don't attempt to unstick (I hesitate to use the word 'rotate' for some light aircraft!) before the correct speed.

2. Select, hold and trim the correct attitude.

That said, I did once get into a nasty pitching oscillation in a PA28 after landing whilst attempting the same 'hover taxy' technique which worked OK in a Bulldog to expedite runway vacation. My own stupid fault, no damage but I Learned From That!!

[KandiFloss]

Thanks Lister, Genghis, Beagle for taking time to reply to my msg ... I really appreciate the advice that you've offered.

Genghis ... the question was from a sheet of questions that I have to answer before I can be 'signed off' by an instructor, just to check that i'm (or anyone else) not going to do anything silly/dangerous. There are other qu's that I would have expected to get asked as well, such as, how would I deal with an engine failure/ how would I deal with an engine fire on start-up, etc.

regards, PC

[KandiFloss]

That's interesting, as the aircraft that i'm choosing to fly is a PA-28. I think they're lovely to fly. I've never had a porpoise on take off at all ... in all my grand total of 67 hours:D! Mind you, i've never taken off/ landed on a grass strip, I think the nearest one to me is Barton. I bet its really bumpy to land on a grass strip :uhoh: ?

[TotalBeginner]

Can you really asses a pilot's skills by giving them written scenarios and ask them to fill them in at home?

What's wrong with instructional training, technical questions and then a check-ride?

[SNS3Guppy]

Porpoising can occur on takeoff or landing, and may be initially induced by the pilot or by the aircraft. The development of oscillations and their continued occurence, however, is nearly always a pilot issue. I've watched pilots damage airplanes on takeoff and on landing when porpoising.

One of the most common problems is not holding the nose off. Another problem, and I've seen airplanes damaged this way, is trying to force the airplane onto the runway. The airplane wants to fly, the pilot doesn't want it to fly, and the airplane becomes airborne repeatedly with the pilot forcing it back down...it bounces and the fight with the airplane only increases in magnitude until the pilot either gets wise and quits fighting the airplane, or something bad happens. Often it all starts with a bounce.

It's sometimes seen in a conventional gear airplane once the tailwheel is in the air. The pilot keeps pushing forward on the stick trying to hold the airplane on the runway...sooner or later it skips or bounces and the fight's on.

As others noted, it's a pilot-induced oscillation (PIO).

What to do if a porpoise develops on takeoff? Let the airplane fly, or reject the takeoff. Usually the simplest thing to do is to freeze the controls and let the airplane stabilize and then handle it from there.

"Doctor, it hurts when I do this."

"Then don't do that any more."

Still good advice today.

[frontlefthamster]

Cabair developed the habit of asking this question and others in the 90s when they had a big fleet of Grumman Cheetahs. I say 'a big fleet', but not as big as it would have been head Grumman fitted as slightly more substantial nose landing gear assembly!

I suspect that the habit has been passed on...

Little point in asking it in what amounts to an 'open book exam' as people will only go around looking the answers up on google or posting the questions on PPRuNE...

Ah. Errrm...

[Pilot DAR]

Many of the comments, including Guppy's are right on. Proposing can also be casued or aggravated by operating on an uneven, or moving (water) surface, where the support or drag which the aircraft is experiencing from the surface is changing. This can occur on some types during the phase of takeoff when the nose wheel is off, and the mains are just coming off. The mains hit a bump, or catch a small snow drift, and pull the nose down. Then the fight can be on. "Checking" such movement is important, but not always as easy as it sounds.

I don't think that there really is one simple answer to the question as asked, other than to regonize that the conditions have occurred to make this possible, or it is already happening, and to continue to fly the plane. Sometimes PIO is not as easy to overcome as you'd think, particularly on a type which is new to you, while you're still figuring out control feel and optimum attitudes. This makes type training a good idea, even for pilots with lots of experience on other types.

[mikehammer]

(I hesitate to use the word 'rotate' for some light aircraft!)


........why?

[PompeyPaul]

I learnt on grass in a PA-28 which can be prone to it.
Hey G-EMMA, does that mean that you're qualified now ?

[Whopity]

.......why?The purpose of a rotation speed is to ensure that following the failure of the critical power unit, the aircraft will achieve the screen height by the first obstacle in the take off domain. Therefore it is not applicable to SE aircraft.

[Ivor_Novello]

I was having a dyslexic moment and thought the thread said "proposing on take off".

"Tortoising" would have been pretty cool as well ! Describes a VERY slow take off run :)

A helpless hopeless romantic !

[PompeyPaul]

Just waiting for the licence ;)
Congrats :ok:. That 3 week wait is agonising. Only decision now is to either be "captain sensible" and build confidence or whether you take everyone you know flying straight away. I opted for the latter.

[frontlefthamster]

Mike,

More practically: there is a world of difference between rotating a transport aircraft at VR and flying a light aircraft off the ground sympathetically at the right speed.

[RatherBeFlying]

Grumman Tigers and Cheetahs are lovely a/c, but they do get written off because of porpoising:{

Having had a number of porpoising incidents, one in a C-172 resolved by a go-around and a number in gliders, the key is to recognise quickly what you've got yourself into and then hold attitude.

Porpoising starts with a pitch excursion that is overcorrected and the [over]corrections to the [over]corrections tend to go divirgent; so you do not want to go past the first cycle.

Pick a reasonable pitch attitude for the circumstances and hold the stick there for a few seconds.

Prevention is of course preferable to cure; i.e. don't fiddle with spoilers in the last 50' if not current or in a new glider and think carefully about CG and trim before taking off in that new single-seater.

Grummans when light have a tendency to float past all available runway; so, it helps to reduce the landing speed if you have had a long flight.

[Genghis the Engineer]

I'm hopefully flying a Cheetah for the first time in a couple of hours - I shall look with interest!

G

[mikehammer]

The purpose of a rotation speed is to ensure that following the failure of the critical power unit, the aircraft will achieve the screen height by the first obstacle in the take off domain. Therefore it is not applicable to SE aircraft.


Yes yes yes, but effectively to unstick, you pull back on the control column and raise the nose, which is rotation about some axes or other. Glibness aside, I take your point.

More practically: there is a world of difference between rotating a transport aircraft at VR and flying a light aircraft off the ground sympathetically at the right speed.

It still doesn't offend me to hear a light aircraft pilot talking of rotation, although I'll admit to being provocative if you like.

[SNS3Guppy]

"Rotate" is a perfectly acceptable word, found in many light airplane handbooks and training handbooks. It's not out of line, and doesn't describe anything to do with screen heights, minimum control speeds, or multi engine airplanes. It's the act of rotating the airplane from it's ground attitude to it's takeoff attitude or pulling the airplane off the ground if you will. Or simply putting it in a position to "sympathetically" lift off, as one poster noted.

In some advanced performance airplanes, various terms such as V1, Vr, V2, Vmc, etc, apply. And yes, in those contexts, Vr, or V speed Rotate, is a number which is selected to be above the minimum ground control speed in a multi-engine airplane...but it has nothing to do with screen height, either. That would be V2, and really isn't a part of the discussion in this thread.

Feel free to use the word "rotate." It's in common use in the hangar, probably from your instructor, and it's found more than likely in your pilot operating handbook or aircraft flight manual, and most likely in your text book(s) that are used for your basic flying learning and understanding.

[Mad Girl]

Quote:
Just waiting for the licence ;)
Congrats :ok:. That 3 week wait is agonising. Only decision now is to either be "captain sensible" and build confidence or whether you take everyone you know flying straight away. I opted for the latter.

Or.... Go and do the aerobatics you wanted to do.... and a tail wheel conversion... play with all of the experienced pilots who may have helped you through.......... with a bit of "chill" time for yourself.

.......... Just because you can........ now..... :D

[BackPacker]

G-EMMA, congrats too.

As far as the "rotate" debate, how about "levitation speed"? (Is the term Vl still available?)

[BEagle]

Try 'rotating' something like a Chipmunk....

In any case, you do not 'pull back on the control column', you use the control column to select the correct attitude for the initial climb. In some light aircraft that's only fractionally different to the runway attitude during the take off roll.

[KandiFloss]

Frontlefthamster - don't 'diss' my original question. The replies have been very helpful actually, with real accounts of the situation happening. Wind your neck in! :=

[Lister Noble]

BEagle is spot on.
In simple terms,the aircraft takes off when it wants to if you have the stick /column just very,very slightly back,almost imperceptible pressure exerted.
Well it does in the L4 with all of it's 65 hp;),and did so like this yesterday.

Another fault,anyway it was with me to start with,is to keep the aircraft forced onto the runway because you think the speed is too low,when it actually wants to fly away.

Lastly, there is a tremondous amount of knowledge on this forum,you just have to filter out the odd facetious remark.

Lister

[SNS3Guppy]

One of the airplanes I fly is the Air Tractor AT-802, which is a single engine conventional gear (tailwheel) airplane. The act of getting it into the air can occur from a three point attitude, or with the tail in the air; the physical change in the pitch attitude is to "rotate" downward, or nose down, to get the tail in the air, in order to accelerate faster and get off the ground.

Even though after that the aircraft flies off in nearly a level attitude, we still generally refer to this as rotation...not when the nose pitches down, and even though the nose doesn't pitch up after that, it's still generally referred to as rotation.

Regardless, so long as everyone knows what you're talking about, lift-off, unstick, takeoff, whatever...works. A lot of light airplanes start begging to go fly as they get light on the gear, skipping and hopping and ready to go, unless they're allowed to come off or coaxed off with a little back stick pressure. Sometimes it's a fine line between a fast taxi, and a takeoff. Sometimes the airplane comes off in a three point tailwheel attitude, sometimes a three point nosewheel attitude, and sometimes a two point attitude, either way.

The terminology really isn't that important. Generally speaking, it's still "rotation," no matter what you have to do to get it there. Call it what floats your boat, so long as you do it before you reach the far end where the little red lights are.

[Final 3 Greens]

Has this ever happened to any one?

To anyone who ever flew a T-tail Lance :}

[SNS3Guppy]

Gemma,

Sometimes when one sees a runway that's 12,000 feet long, it seems excessive...especially if one is flying an airplane which requires 1,200 feet. Some large airplanes however, need it (reference the video).

When I flew large air tankers, we used fields more suited to small airplanes, sometimes, because those fields had been selected as central to the areas in which we fought fire. It wasn't uncommon at all to roll full length, or occasionally right off the end as we became airborne. I've stood at the end of the runway at Hemet (California, USA) and watched other tankers rotate, sucking clods of earth out of the ground from wing tip vortices with a very aggressive rotation, followed by the gear clearing the fence at the end by a few feet. I departed Reno-Stead in a C-130, a few years ago where the Reno Air Races are held each year. Reno sits in a bowl, with rising terrain around it....not very high terrain in most cases, but rising terrain. We got off the ground, but couldn't clear the terrain and had to fly the air race course around the pylons as we flew around the valley scratching for altitude. I did the same thing last year in single engine tanker, much of it for the first lap below 15', because due to the temperature and downdrafts from high winds, the airplane just wouldn't climb. I used all the runway.

Presently (airline freight) we depart at 833,000 lbs, and just as the ground school instructor said on day one...we see the red lights on every takeoff. We calculate everything into the takeoff; runway slope, wind, wet runways, weight, climb performance after takeoff, accelerate stop, accelerate go, refusal distance, speed, runway remaining, time to dump fuel to return in an emergency, commit speeds with an engine failure, brake settings, power settings, etc...it all gets figured out for each takeoff. We have an exact speed and pitch attitude to achieve for rotation. An early or late rotation throws all the calculations out the window and increases the distance. No matter how much we might want to rotate as we see those red lights coming up, we can't.

The russian airplane in the video went a bit farther than we might have done. We do have minimum altitudes alluded to earlier in this thread, but they're very small, called a "screen height" that are part of performance calculations. Generally speaking, the takeoff distance is calculated to be the longer of either the balanced field length, or that necessary to achieve a 35 foot screen height (15' in wet weather). Balanced field is when the distance to accelerate to decision speed, lose an engine and take off, and to do the same and stop...when those distances are the same, ...or the distance to take off with engines and achieve a height of 35'...that's our minimum runway. Whichever is longer. The ruskies appear to be working without those margins in the video, which is the same way we worked in tankers.

It's much simpler,and a whole lot more fun, in a light airplane, when one gets to feel the airplane into the air. In a large airplane, it's all numbers, which takes some of the fun out of it.

[jxk]

A word of caution about 'lifting' off too early - you may not have full aileron control and may not be able to stop a roll. Most POHs quote an 'unstick' speed for a reason.

[Maoraigh1]

Lift off too early, and you might be only able to fly while in the ground effect, with too much drag to build up speed without dropping the nose and touching down again - assuming you have enough runway left.
(It isn't easy to do this, but it has lead to accidents)

[Pilot DAR]

It is easy to do that on Piper PA28's, I was a passenger to a pilot who did it in an Arrow, The only way we made it out is that I retracted the gear, and the reduced drag let us climb away. It was a risk, but the runway was gone anyway. I later experimented in a PA28-140 on the long frozen lake. even at light weight, it is surprizingly easy to get the nose so high that you could not climb out of ground effect without landing back. I did it many times.

I do not support the dependance on numbers in light aircraft, as much as I expect that pilots should be able to "feel" the plane. If you are watching the instruments, you are not feeling the subtle cues that the aircraft is offering, and you're not learning as much. Hopefully your night training is including zero instrument lighting (for simulated electrical failures) so you have only the feel of the plane to go by. That's where you'll be able to feel when flaps should be selected up or down. If the operator asks to have their procedures followed, then you are bound by respect to do that, but regognize it as a safety procedure, which may be more restrictive than the aircraft manufacturer intended.

Pilot DAR