Porpoising

[NigelS]

Does any one have any good tips to alleviate porpoising?
Thanks in advance

Nigel

[Yogi-Bear]

Resign from the Mile High Club! :D
http://.stopstart.fsnet.co.uk/smilie/jump.gif

[Skylark4]

Don`t chase it. Make small stick movements and wait for the change to take effect. Lock your forearm to your knee to prevent inadvertent movement.

Porpoising in a glider is where you have a street of clouds on your track, you fly along under the lifting areas of the clouds and when you are in rising air you zoom up, fly slowly in the rising air and stick the nose down and cruise at high speed through the sinking air. Very high cross country speeds can be attained in this way.

Mike W.

[Chilli Monster]

1) Don't try and touch down too fast. Porpoising (especially in Cessnas) is caused by being too fast and panicking to try andget it down.

2) LOOK UP. As you look up at the far end of the runway you will automatically pull back into the flare. As you sink just pull back gently and the aircraft will settle by itself.

It really is that easy :)

CM

[GoneWest]

Go Around?

[Tricky Woo]

NigelS,

I think that the term 'porpoising' is used for two separate problems:

Firstly, there's the out-of-trim state that causes an aircraft that's supposed to be flying straight-and-level to gain and lose say fifty feet in a sort of porpoising rhythm. The only answer I have for the one is 'learn to trim, young man, learn to trim'.

Secondly, there's a rather nastier nose-wheel-bending phenomenon, that can occur after a botched landing, that's also known as porpoising. The only answer there is careful speed control and a proper flare.

Not sure which one you mean, but that's my two bobs worth.

TW

[Fujitsu]

TW - what do you mean by an out of trim state? Surely the aircraft will be trimmed to an airspeed, although perhaps not the one you want. I understood that you you can trim for say 100kts and when settled down the aircraft will fly at that speed. Add power, and again after settling down the aircraft will climb at that speed, reduce power and it will descend at that speed etc. If you trim for 100kts and try and fly at 90 or 110 I imagine you'd spend a fair amount of time climbing and descending,is this what you mean by out of trim or have I got the wrong end of the stick?

[New Bloke]

Still Learning, sort of right.

You are correct that you trim for a speed but the power setting must be appropriate for that speed.

In other words, if you trim for 90 kts and have, say, 2200 RPM all will be well and you will fly at a constant altitude. When you put carb heat in, the revs will drop and if nothing is done (ie pull back slightly on the stick) you will descend. If you plan the approach part of a cross country correctly you can trim for S&L and just reduce power to descend to circuit height without re-trimming.

[Genghis the Engineer]

There are three modes of flight which could be interpreted by the rather untechnical term Porpoising.

The first and most common is the Phugoid, which is a height-speed oscillation with a period of around 30-60 seconds in most light aircraft. It is due to the aeroelasticity of the aicraft, and can be dealt with by careful slow movement of the pitch control. It isnt strictly dangerous in itself unless you're trying to land, but is sickmaking and makes altitude control a tad fiddly.

The second is the Short Period Oscillation, or SPO, which is pitch nodding with a period of a couple of seconds. This is a natural mode of the aircraft which you rely upon to make attitude changes, but if poorly damped can be a nuisance. The solution is generally to clamp the stick in the middle and let it sort itself out, if that doesn't work, letting go of the stick usually will. However if it is a constant intrusion into your flying, there's probably a fault in the elevator circuit.

The third is the "Pilot Induced Oscillation", or PIO, which happens when the pilot's corrective inputs are at the same frequency but out of sync with the SPO. It can get you in serious trouble if you're not careful (I've a little single seat Canard which does it a treat if I'm not careful). The solution is to stop trying to chase it and clamp the stick in a central position until it stops.

G

[FlyingForFun]

Hmm - I always understood "Porpoising" to be pulling back too hard in the flare, resulting in the aircraft gaining altitude and loosing too much speed....

Nigel, maybe you could describe exactly what you mean, to avoid people giving you advice to fix a different problem to the one you're experiencing?

(Sorry, no advice from me, except to keep practicing, and it will come together - there are lots of people around here who are far more qualified than me to advise on flying.)

FFF
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[TheSilverFox]

Still Learning,

Whoever told you that you "Trim for Airspeed" hasn't given you the whole story.

We have all heard instructors say "trim for 90 knots" etc. which is in fact a little misleading.

Far better to get into the habit of thinking of trimming to maintain a particular attitude!

It may be useful to remember this:-

For a specific power setting combined with a specific attitude (eg straight & level) you will achieve a specific performance!

POWER + ATTITUDE = PERFORMANCE (AIRSPEED).

Make sure that you do things in the correct sequence:-

1) set the power

2) select the attitude

3) Trim to maintain the desired attitude.

PAT Power,Attitude,Trim!

There is an exception to this rule which is when you level of from a climb, but I,m sure that you are already aware of this.

Hope that this is of some use to you.

Good luck with the flying!

[FlyingForFun]

SilverFox,

Sorry to be pedantic, but I believe you trim for an Angle of Attack, not for attitude. The two are only the same if the relative air is coming from directly in front of you (which it won't be during a climb or descent).

The best illustration is to trim for straight and level at roughly cruising speed. Then, add power. The attitude will change (nose-up), but the angle of attack won't (the relative air will be coming from exactly the same place relative to the aircraft - not easy to see, though, unless you attach a streamer, or some other kind of wind-sensitive device, to the aircraft).

Not quite sure what this has to do with the original question, though.

I wonder if this thread should be moved to the Instructors forum? I don't know anyone on here except myself well enough to comment on their level of knowledge, but I suspect there are a lot of people in here who will unknowingly give bad advice. And yes, I most definitely include myself amonst them!

FFF
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[Squawk 8888]

FFF, you're referring to "ballooning". In the flare porpoising can happen if you overcorrect for the ballooning, dive back down, overcorrect an balloon again.

[FlyingForFun]

You're quite right, Squawk.

Just goes to prove what I was saying in my previous post about lots of people in this forum who don't know what they're talking about, doesn't it!

I'll shut up now....

FFF
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[TheSilverFox]

FlyingForFun,

I too am sorry to be pedantic, but if you give incorrect information you leave me no choice!

YOU DO NOT TRIM FOR ANGLE OF ATTACK!!!!

THE TRIMMER IS NOT A FLYING CONTROL!!!

The Elevator/Stabilator is the flying control!

The trimmer is used simply to relieve control column pressure.

Therefore the power has been set, the pitch attitude has been set with the elevator
(Angle of attack has now been established)now is the time to trim to relieve control column pressure AND MAINTAIN ATTITUDE!

Too many qualified pilots fly on the trimmer (I see it regularly during prof checks).

I spend a long time during FIC groundschool convincing future FI's that the trimmer is not a flying control in the hope that the message will be passed on to their future students!

Regards,

TheSilverFox

[FlyingForFun]

SilverFox,

Apologies if I've given incorrect information - as I said, I'm probably not qualified to be giving information. So can I please turn this into a question? Clearly my understanding is not the same as yours, and if you're an instructor and I'm a low-time PPL, that probably makes me wrong, and waiting to be educated - so, here goes.

When I'm levelling off at the top of a climb, I think we agree I set the new angle-of-attack/attitude using the elevator. (I'd probably also make a power adjustment - although that's not relevant to my example.) Then, once everything's settled down, I trim off the pressure using the trim wheel. I am now flying at a constant angle of attack, and a constant attitude, with no control inputs necessary.

Some time later, I decide to add power and enter a climb. I do not touch any controls other than the power. (Ok, maybe a bit of rudder to counteract the slipstream effect, but certainly no elevator - for the purposes of this example.) Doing this, unless I'm wrong, will change the attitude - that is, it will change the angle that the nose of my aircraft makes with respect to the horizon. Therefore, my attitude has changed, but the position of my trim-wheel hasn't. Is that not the case? Because if it is, I don't think it's fair to say that the you trim to maintain a particular attitude. If you disagree, care to explain why, for my education?

Also, I was in no way implying (and I apologise if my mail was misinterprested this way) that you should fly on the trim wheel. You set the attitude/angle-of-attack that you want using the elevator, then you trim the pressure off with the trim wheel. (I'm pretty sure we agree on this point.)

And, once you've done that, the angle of attack which the aircraft has (compared to the relative airflow) will remain constant, even if you change the power. (But it will obviously change if you move the elevator. It will also change if you adjust the trim-wheel, although it would be bad technique to do so.) However, I'm not certain whether you agree on this part - if you disagree, could you please explain why I'm wrong.

Always ready to learn...

FFF
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[TheSilverFox]

FlyingForFun,

Stop putting yourself down because your "Only a PPL"!!

Yes I am an Instructor & Examiner but I certainly don't claim to have all the answers!! I've yet to come accross the person that does (With the exception of Scroggs-but he only thinks that he knows it all!)

To respond to your given scenario:-

Yes, If you are in unaccelerated S&L flight and you increase the power the nose will pitch up (in the type of A/C that you are used to flying) by design. This is realy a by-product of the A/C being designed to pitch down if you lose power!

Yes the Pitch attitude has now changed and NO you have not touched the trim wheel.

The Aircraft was trimmed to maintain the previous S&L config at the previous power setting.

You have now altered the balance of the forces acting upon your A/C!

So start again.

Check the power is at the setting that you require.

Check & Adjust the attitude as required.

Re-Trim to maintain new attitude.

Hope this answers your question!

PS. Where do you fly?

Regards

TheSilverFox.

[NigelS]

Wow!! I just got back and line and I've attracted loads of response. Excellent! And thanks.

I have obviously mislead though. I am still with low hours and my trouble relates merely to straight and level. I have no trouble (at the moment)with landings. My instuctor makes it all look so easy and does it efforstlessly. He just points the nose, gets the power and airspeed right, trims quickly and then just leaves it alone. When I do try the aircraft oscillates through about 50 feet. It bugs me and it bugs the hell out of my instructor...

Sound familiar?

Anyway, thanks for all the advice. I'll keep practicing.

Kind regards

Nigel

[Fujitsu]

Thanks for that Silver Fox - funny how terminology can lead to misunderstanding etc. etc. Thanks again
SL

[BEagle]

This all goes to show why 'point and power' is a much easier technique to use when learning how to land!

Roll out of final turn, select land flap, trim to approach speed. Then just aim ac at touchdown point, keeping that point nailed in the windscreen. Make continuous very small power adjustments scanning touch down point, airspeed, touchdown point, airspeed. At the moment critique, change aim to far end of runway, close throttle fully and hold ac off until it can't be held off any more.

[FlyingForFun]

SilverFox,

I wasn't claiming you know all the answers, just that, since you have more experience and training than me, if we disagree, there's a good chance you're right.

But I think we agree on this one, it's just the terminology which is a bit confusing. Now I understand what you're saying - I think it's fair to say that you're refering to how you use the controls during flight, rather than a technical description of what happens when you use them.

Having re-read and re-thought about what I said, I can see why you disagree with the statement that trim setting control angle of attack - the angle of attack can be changed (and should be changed) without using the trim wheel, but with the elevator instead. I'd still argue that trim doesn't control attitude, for exactly the same reason, but I can see what you're saying and it really is only semantics which we're disagreeing on.

I tried to come up with some better descriptions of exactly what the controls do (as opposed to how you use them, which is what you've described), and the best I could come up with was this:

Elevator is used to control angle of attack
Trim is used to control the amount of pressure which you need to apply to the elevator to hold it in a specific position (i.e. a specific angle of attack).
There is no direct "attitude control" - this is combination of angle of attack, and direction of relative airflow with respect to the aircraft - although, in practice, you set the attitude by using the elevator to set an angle of attack which produces the correct attitude, then using the trim wheel to hold that attitude.


I think that's correct, but it's probably far too theoretical to be useful in flight.

Anyway, I'm getting bored of this now. Since we both agree you set the attitude and then trim, let's move on to some more lighthearted stuff :D (or at least try to answer the original question, instead of carrying on this tangent...)

Oh, I fly out of White Waltham, by the way...

FFF
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[kabz]

Closes eyes, and jumps ...

As far as I know from flying (PPL student), the trim control allows you to trim the control pressures off to maintain a given **airspeed** ...

Here's an example ... downwind ... reduce power and pull back on the stick to maintain altidude and approx 80 kts ... trim off the pressure ... the aircraft is now **trimmed** for 80 kts ...

Pass abeam the numbers, and reduce power to initiate descent ... aircraft is now at 80 kts, but descending ... reduce power and flare and land ...

Full power ... and aircraft takes off ... climbs automatically at the **trimmed** 80 kts ...


As for thinking about this, doesn't the trim control manipulate the speed at which the elevator and wing balance each other ??? i.e. more power will push the speed up and climb will occcur, less power and the aircraft will maintain speed, but in a descent...

[FlyingForFun]

kabz,

Speed is very closely linked to angle-of-attack. I remember reading somewhere an extremely good explaination of why this is, but I can't remember where I read it. It may have been Langeweische's (sp?) Stick And Rudder. I'll try to find it, and post the reference.

So, for all intents and purposes, you do trim for a specific speed. (I know this is the way I was taught to fly, anyway!) But an aircraft designer would argue that this is not actually true. I just tried typing up the reason, but got stuck half way through and realised I don't know enough to give any more details.

(As I said in my last post, my definitions of what the controls do were probably too theoretical to be of practical use during flight. I really regret posting on this thread now. I have understood what SilverFox was getting at, but probably confused a load of people in the process. Oh well.)

FFF
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[FlyingForFun]

kabz,

Have a read of the first 11 pages of Chapter 7 (What The Airplane Wants To Do) of Langewiesche.

This describes the role of the elevator/trim first of all in terms of airspeed, pretty much as you did. It then explains that, while this is true for straight flight, it does not apply during turns. If you trim for a specific speed, then put the aircraft into a turn without altering the elevator/trim positions, the aircraft will speed up. However, the angle of attack will not change, and Langewiesche explains the whole thing over again using angle of attack instead of speed. Of course, he explains it far better than I can!

(If you don't have a copy of Langewiesche, get one - I've never heard anyone who has a copy and regretted buying it. "Stick And Rudder" by Wolfgang Langewiesche, published by McGraw-Hill, ISBN 0-07-036240-8. Now I've said that, watch someone come along and say they didn't like the book....)

FFF
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