Using pendulum to stabilize models - RC Groups
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Jan 08, 2009, 11:27 AM
Master Crasher
zultone's Avatar

Using pendulum to stabilize models

Could someone describe this to me?

Is it basically a weight on a string, connected to whatever control surface to control?

I googled it and couldn't find anything.

Anybody have pics of an un-covered model with one of these, or even just an example?

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Jan 08, 2009, 12:05 PM
Registered User
Gluehand's Avatar
This Sopwith Tabloid has a pendulum operated elevator.
Jan 08, 2009, 11:00 PM
B for Bruce
BMatthews's Avatar
Keith Laumer was a big fan of pendulums. But really they are just a novelty for the most part.

One spot where they can help is on a scale model that has very little or no dihedral. In THAT case a pendulum can sometimes maybe fill in for a lack of dihedral. The "sometimes maybe" part comes in because just like an old time mail plane pilot caught in some cloud it is all too easy for the antics of an airplane to fool the pilot or the pendulum into thinking the craft is flying just fine. For example a fully coordinated turn that develops into a spiral dive will not be corrected by a pendulum IF the coordination of the axes is perfect and there's no side G load. The model in such a condition will happily tighten the spiral turn into a spiral dive and plant itself like an Olympic javelin without the pendulum sensing any troubles at all.

Now having spouted all this doom and gloom a buddy did make a little CO2 powered scale model that used pendulum controlled ailerons with good success. So it CAN help.

As for pendulum elevator or rudder I'd say do not bother. In such cases the pendulums cannot make up for poor balance or force layout. Also with the proper wing to stabilizer angles and CG location you can turn the whole model into a very nice pendulum of it's own without using an actual pendulum arm and weight. A really high % of stability will be far more effective than a mechanical pendulum will ever be. And a pendulum elevator is all too easily fooled into driving the model into the ground instead of helping. And a pendulum rudder never made any sense to me at all. Again it's just something to mess up and it doesn't make up for poor trimming in the first place.
Jan 09, 2009, 02:12 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Bruce has expressed my thoughts on pendulums quite nicely. On elevator or rudder they make no sense whatsoever and even on aileron where there is at least some theoretical basis for their use I'd say there is no conclusive evidence that they consistently work. Most pendulum equipped models Iíve ever seen would fly just fine without the pendulum anyway.
The best argument for their use is probably Steve Glass' models. Steve goes by 'Stegla' and posts in this very forum quite often. He has built a number of wonderful scale models that use pendulum on ailerons. Whether these models fly 'because of' or 'despite of' the pendulum is still open to debate as all (arguably) had sufficient natural lateral stability built in via sweep back and/or dihedral.
His latest model should resolve the argument. It's a scale Hurricane with scale (almost zero) dihedral. If he gets that to fly consistently then even a cynic like me would have to admit that the pendulum must be helping... So far reported trimming flights appear to favour my side of the argument, but its early days and Steve is still working on it.

Steve B
Jan 09, 2009, 08:15 AM
Master Crasher
zultone's Avatar
I was wondering about that actually. If the forces on the pendulum act the same (or close to it) at all given times then it really wouldn't have any effect.

Howver, say the plane is in the air and a 10mph gust tries to blow it around, I could see the pendulum helping there, also.
Jan 09, 2009, 11:41 AM
Light and floaty does it
Work in Progress's Avatar
As others point out there are good empirical reasons to be pretty sceptical about the benefits of pendulum based stability augmentation, as every full-size instrument-rated pilot effectively learns when understanding why gyro platforms are needed to provide ground reference in flight. A skilled aerobatic pilot can loop and barrel-roll an aeroplane and keep a pendulum in the cockpit pointing straight down towards the cockpit floor, not straight down towards the ground. I've never seen a pendulum controlled model fly, but reports in Aeromodeller going back to the year dot always seemed to emphasise that the models flew best when the control movement was VERY restricted. I always suspected that in such cases things would fly best of all when the pendulum control was, in effect restricted to zero!
However, I must admit I've never tried it and really ought to just for fun. So until I do I won't say it can't work, only that I'd generally try conventional or gyro-based stability enhancement methods first.
Jan 09, 2009, 12:08 PM
Master Crasher
zultone's Avatar
With a gyro - Is it basically a gyroscope on gimbals with a similar hookup to a pendulum?
Jan 09, 2009, 12:32 PM
Registered User
TLyttle's Avatar
Wwweeellll... I built a Typhoon (PE Norman plan) with a pendulum elevator, and it worked exceptionally well (except when it counted, at a contest). Once that nose dropped below the horizon, the elevator hauled it back into a climb.

Hey, Bruce, you got scared off pendulums because of Bill's antics with his HP42....... ;-) (Private history lesson here.)
Jan 09, 2009, 12:43 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Originally Posted by TLyttle
Wwweeellll... I built a Typhoon (PE Norman plan) with a pendulum elevator, and it worked exceptionally well (except when it counted, at a contest). Once that nose dropped below the horizon, the elevator hauled it back into a climb.
Well here's a thing... i've never used a pendulum on elevator but my models do exactly the same thing... If they get into a dive they pull out by themselves and start to climb. All I have to do to achieve this is make sure the CG is far enough forward and that the tail is set to correct decalage.. not a pendulum in sight!

I think that you would have found that the Typhoon recovered from it's dive even with the pendulum locked or removed, just like any other properly trimmed free flight model does... Only without the pendulum it would have been lighter so flown better
Jan 09, 2009, 10:00 PM
Registered User
TLyttle's Avatar
True 'nuff, except for a couple of things: first, I likely didn't add 3/4oz to the model with the pendulum. Second, I tried it with the pendulum locked, and made VERY good use of the long grass I was flying over! The model would NOT fly without it, and I used every trimming trick in the book.

PE Norman's designs were amazing just in the fact that they flew. Small, very heavy, very tough, overpowered, they flew just the same. They were also nearly exact scale, and there was the rub: scale dihedral and tail surfaces. If you don't give yourself the latitude of changing the design, then stuff like pendulums (or radios) are the only alternative.

I built mine lighter than designed, but I met a guy who built PE's ME109 as designed. He flew it in South Africa, and his field had thermals that went to 20,000'. He launched into it, and watched it disappear vertically with too much fuel aboard, the pendulum doing its job. He heard it coming down long before he ever saw it, figures it was doing well over 100mph when it touched down nearby, unaffected by such a landing. He never flew it again...
Jan 10, 2009, 01:13 AM
B for Bruce
BMatthews's Avatar
Yeah, I'm with JPF on this for sure. Terry, I didn't have the pleasure of seeing Bill's HP fly but I'd still stand by the idea that a proper CG and elevator angle would have done the trick.

JPF, I'll stand with you on the Hurricane as well. If he can get that one to fly well then I'd have to agree that it'll be largely thanks to the pendulum. The side area and vertical tail area combined with the super low dihedral angle is just a spiral dive looking for a impact zone without some cheating being involved.
Jan 11, 2009, 06:40 PM

This thread will answer many of the questions you have regarding pendulum stability enhancement devices.
The addition of electronics may make pendulum assist practical.

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