Why do models porpoise and how to prevent or stop it.

[Erfolg]

The heading says it all.

[Ron Gray]

Reduce the amount of elevator control and make sure CG is correct.

[Eric Robson]

They do it on porpoise

[J D 8]

  Usually they stop doing it if you let go of the stick.

One of the issues with rotary servos, [in early RC there were linear servos] there is more movement at the center than ends of travel] making things touchy at the center position. The right amount of expo can help here and some light fingers.

[Rich Griff]

May I ask, what about a free flight aeroplane ?

 

[steve too]

Insufficient pilot skill for the model and/or setup.

[Jon H]

More or less as the others. CG issue, Rates issue, definately expo can be a cause, and also pilot skill. Pilot induced oscillation is a big thing for full size pilots to deal with. 

 

However, porpoise can mean a number of things. Where does this occur? In normal flight, takeoff, landing, high speed, low speed?

 

My P39 becomes more and more unstable in pitch as speed increases. I need more and more forward elevator pressure but my elevator effectiveness increases with the speed. Beyond a certain point the model becomes very hard to control due to the high sensitivity of the elevator resulting in a PIO situation. My solution is simple, dont fly that fast! If i keep to a scale sort of speed its no bother, i just need to be careful on high speed passes as this wobbliness develops very quickly. 

 

In any case, the ultimate root cause of the problem comes down to when/how the problem develops and how it manifests. 

[Erfolg]

It occurs when opening the throttle.

 

When on the glide, there is no obvious porpoising on the glide or when diving (going faster). In short not obviously speed related. 

[Simon Chaddock]

erfolg

Its all to do with longitudinal stability and how close the plane is to having neutral longitudinal stability.

As you open the throttle there is more thrust so the plane speeds up. Its natural stability will cause the plane to climb to return it to its "natural" stable speed.

However if the thrust increase is significant and sudden the plane will climb faster and most likely at an angle greater than that required to achieve the stable speed. The result is the plane slows down too quickly ending in a nose drop which then causes the plane to speed up past the stable speed so it returns to climbing and so on.

Longitudinal stability is damped but not by much so any outside inputs can cause an oscillation which may or may not damp down by itself. Throw in some human elevator input and the system can get unstable very quickly.

Answer

1. Input any throttle changes gradually so the natural stability has time to return to the plane to its natural flying speed. It also gives the pilot time to retrim as required.

2. Add down thrust. Get the angle right and the nose down element of the thrust will match the tendency of the natural stability to make the plane climb, Note this will only be true once a "steady state" is reached. Until this state is reached the plane will either tend to dive or climb.

3. Fit a gyro system! That will keep the plane at a set attitude regardless of any external input. Human pilot no longer required. A gyro senses change faster than a human so it can even keep an unstable plane level.

 

That's my understanding of the issue.

 

 

  

   

[Erfolg]

Simon, I did think if fitting a gyro that was activated on opening the throttle, switching off when closing the throttle. Although not clear in my mind, if it was possible.

 

I had another thought that perhaps mixing in down elevator when opening the throttle, which I had mixed in some 30 years back (if my memory serves me well, with respect to time), I just cannot remember how I did it.

[GrumpyGnome]

I'd imagine that the vast majority of modern radios are easily capable of this simple mix.....

[Outrunner]

Erfolg. If you fit a gyro to the elevator there is no need to link it to the throttle. The gyro will damp out any unwanted pitching, throttle or no throttle. If you input elevator the gyro won't try to stop the effect.

[Brian Cooper]

It is the undercarriage position relative to the C.G.,

Wheels too far forward will do it on tail daggers.  

[Simon Chaddock]

Erfolg

Down elevator linked to throttle will work but like motor down thrust it is not "universal". The amount of down elevator will depend on the planes speed not just the throttle position. This means if the down elevator is set to exactly match the climb tendency at the planes final speed for a particular throttle setting then the down elevator will be too strong whilst the plane is speeding up.  

A full 6 axis gyro on the other hand is universal and will keep the plane level regardless of the throttle setting, how rapidly it is applied or the final speed the plane achieves.

I am not recommending such a gyro but a simpler 3 axis 'rate' set up is rather different. It reacts to limit the effect of any change but does not restore the plane to its original attitude. Such a system thus slows down the effect of any disturbance which is more in keeping with my now rather old human reflexes. It also does not limit the ultimate effectiveness of the control surfaces so aerobatics are still possible.   

[steve too]

15 hours ago, Erfolg said:

It occurs when opening the throttle.

 

From this and your other topic it sound like the thrust angle is incorrect and you don't have the skill to manually control the throttle induced pitch change. Sort out the thrust angle before doing anything else.

[Nigel R]

What airframe are we talking about here?

 

 

[Erfolg]

We are talking about two old 2m Sagitta's. Both originally gliders.

 

One was modified to electric foe some past electric power contest. This one was set up with some down elevator trim on throttle up. It worked well for a 30sec vertical climb. That was years back, since then I now use a different transmitter, plus I stopped flying gliders for at least 10 years. Now I do not have a clue how the original set up was done.

 

The second model is also some 10-20 years old (not as old as the first), In essence I just controlled the climb out using the elevator. It was lively, but did not bother me.

 

However at the club I now fly at, there are gasps of horror from other members when climbing vertically, heard one suggesting that they feared the model was going to crash. It seems the difference between a power club and and a flat field competition glider club, where wing bending bungee, tow or winch launches, start with vertical climbs often with a ping or even aggressive manoeuvres to maintain line tension (bungee, tow line). In short I decided to start scale glider type climb outs (to avoid upsetting others), then the short comings of the power set up have become apparent. I think you have guessed I have only recently taken them out of retirement (recommissioned, in modern jargon).

 

For club flying the models are OK, the wings are overspared for todays flying styles. Plus we are in the days of Carbon/GF wings and bodies, excellent wing sections. Electric motors allow me stay up consistently far longer than I ever could achieve in my youth. Not quite the challenge of yore.  It seems that they and me are all dinosaurs.

 

 

[Basil]

13 hours ago, Brian Cooper said:

It is the undercarriage position relative to the C.G.,

Wheels too far forward will do it on tail daggers.  

Brian, do you mean like a pendulum .

Bas

[Simon Chaddock]

Erfolg

"It worked well for a 30sec vertical climb."

I think you have hit the nail on the head. As you found out in the past it is perfectly possible to set up down elevator to control a high power near vertical climb but the same setting would not be exact at other power levels. Not really an issue for power on/off  high performance gliding.

It does take a bit of trial and error to get the throttle/elevator mix right but like many things if it works for you go for it. If you are not sure how to programme the mix on your transmitter I am sure there would be help from somebody on this site. 

As you point out many power flyers would call such a climb "aerobatic" and not a safe thing to do for many planes.

 

My home design Big Dragon has a "pylon" glider configuration but as a pusher it has a high thrust line. A configuration made worse by its light airframe and a heavy battery low down in the nose. 

With a thrust to weight close to 1:1 applying full power in level flight causes a significant nose dive but it does allow a perfectly controlled near vertical slow climb. Very impressive when started from a low slow pass. Exactly the sort of thing you should not do with a club trainer!

[PatMc]

Erfolg, looks like you're model is experiencing "phugoids", there's a good description & explanation in Wiki of the probable cause. 

 

Re your clubmates' reaction to the vertical climbing Saggitas : you could point out that since the wings are not contributing any lift during a vertical climb it's actually less stressful on them than a "gentle" scale like climb. Also that the speed of a vertical climb isn't actually as fast as it might appear to be, a typical 30 sec burst that takes a model to 200mtrs is only 15mph. 

 

 

[Trevor]

Gliders porpoising with no control input is a sign of too much uptrim and/or a forward cg. People often try to fix this by piling more lead in the nose so I’d suggest you take some out and re-trim the elevator accordingly 

 

good luck.

 

Trevor