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Apr 01, 2012, 08:47 AM
Registered User
stemo99's Avatar
Thread OP

when a plane porpoises is it tail or nose heavy?

I maidened my Stevens Aero Micro Stick this morning and the plane was very twitchy and exibited pretty bad porpoiseing. Is this a nose heavy or tail heavy condition. I thought nose heavy but when I moved the battery back a bit, upon launch my prop saver o ring snapped and I didn't have another with me. Your help is always appreciated.
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Apr 01, 2012, 09:18 AM
Registered User
Did it glide at all? If it tried to dive with no power, nose heavy. BUT, I bet, being "twitchy" and porpoising .. it is probably tail heavy.
Apr 01, 2012, 09:23 AM
Registered User
Ken Myers's Avatar
Tail heavy and possibly too much elevator throw. Too much elevator throw really exacerbates a tail heavy problem.
Apr 01, 2012, 09:41 AM
characters welcome!
Mark Wood's Avatar
My question is, did you carefully balance the plane per the instructions before committing aviation?
It actually took me a few years to discover how important this really is. Maiden flights got MUCH easier after this revelation.

Apr 01, 2012, 10:14 AM
The 6 P principle works for me
elecfryer's Avatar
+1 to KM and MW plus, I have come across more than one kit / arf that had an incorrect CG listed! There are now many web sites that will assist you with proper CG location. One example below:

to paraphrase a commercial, "an informed pilot is our most successfull flyer"

that being said, I still plant em every now and then!

Michael (if your not fryin, your not tryin!)
Apr 01, 2012, 10:45 AM
Registered User
elfwreck's Avatar
Hey now,
It's likely a tail heavy situation. However. It could also be a case of incorrect incedence or decolauge. Um, the angle of the wing in relation to the ceneter line, or the relation between the wing and horizontal stabilizer orceven thrust angle of the motor. Any or all of these could cause the problem.
Apr 01, 2012, 11:30 AM
Registered User
E-Challenged's Avatar
With no elevator input and 3/4 throttle, does it climb or dive? If it tries to climb then stalls and goes nose-down it is tail heavy. If it needs up-elevator input to keep from diving, it is nose heavy. All airplanes will try to climb at full throttle, lift increases with airspeed. A tail-heavy model may even loop after launch with full throttle. With a properly balanced model, use elevator trim to fly level at 1/2 throttle to full throttle speeds.
Apr 01, 2012, 11:51 AM
Registered User
stemo99's Avatar
Thread OP
Thanks for the replies. I balanced it per the instructions, 2in back from the leading edge. As far as wing and stab incidance, the wing sits square on the fuse per plans as well as the stab. I purposely balanced it a bit nose heavy for the maiden, but I need to fly it again to experiment with the CG. I will be sure to have spare O rings next time out (I hate prop savers).
Apr 01, 2012, 01:51 PM
Electric Coolhunter
Thomas B's Avatar
If the model is indeed properly balanced, you need to rate down your elevator throw and add expo, if your radio is capable of adding those things.
Apr 01, 2012, 03:02 PM
An itch?. Scratch build.
eflightray's Avatar
Assuming the model is in reasonable trim with the correct CG, almost any model with a reasonably high lifting wing section, (under-cambered as against a fully symmetrical section), has a very limited flying speed for level flight without some elevator trim needed.

There are some models that get a little nearer to neutral trim with speed if the down thrust is set right.

Even on full sized aircraft you have to adjust the elevator trim with different throttle settings to fly level.

You cannot increase the throttle and expect level flight to stay the same with many 'simple' models, (the 'simple' isn't meant in a derogatory way).

The 'porpoise flight' is a classic of flying faster than the models trim setting.
The wing lifts more with the increase in speed, the nose goes up, the drag increases, the model slows, often a shallow, (or vicious) stall occurs, the nose drops and speed builds up again, more lift is generated, and off it goes again.

Balancing changes in speed and maintaining level flight needs some elevator trim changes as well as throttle.
Apr 01, 2012, 04:01 PM
payne9999's Avatar
+1 on Eflightray's comments. I flew for years using the elevator incorrectly. When it hit me that I was doing this I changed techniques and my flying skill almost immediately went to the next level. In the past I used the elevator too much and basically ignored the throttle. It would be good to add some expo to the elevator if it seems twitchy. I am not saying you are using the elevator incorrectly but just saying this might be something to consider.

I would, if your not already doing this, use the throttle more. Get more active on the throttle to manage ascent and descent. I tend to use the throttle a lot to manage altitude and I only use elevator for minor corrections, or to maintain level flight in a banking turn or if I have to do an extreme maneuver to abort a landing (some kind of urgent situation).

On approach I use almost no elevator but constantly change the throttle to get the descent rate and to target the spot on the end of the runway where I want the touchdown to occur. On departure I just pulse the elevator a little to rotate into flight when I know the airspeed is correct.

I learned this by flying twins and multi-motor planes and the reason is they tend to fly in a very narrow band of throttle settings. You don't idle in with them but just decrease throttle 15 -20% (roughly) and the descent starts to occur. These planes really only fly properly on the throttles. However, single motor planes are better controlled this way too. I see lots of folks throttle back to an idle setting and point the plane in for landing. Without the perfect approach setup and no variation in wind will this ever work out.

Practicing precision landings really helped me learn this. I used to put a small towl out on the grass and try to manage the airplane by throttle and make it set down as near to that spot as possible.

Apr 01, 2012, 04:36 PM
Long-Time Member
James Frolik's Avatar
Originally Posted by Mark Wood
It actually took me a few years to discover how important [balancing a plane] really is.
Hearing that from a long-time-E-Zone-guy with over TWENTY-THOUSAND posts, I now wonder whether to ever trust you again. I learned about weights-&-balance with my first paper plane, when my daddy said, "Son, you need weight in the nose or it will stall and crash." I think I was 5 years old. (I used brackets in MW's quote to define the referent word "this," which is used in the original post for "balancing a plane.")
Apr 01, 2012, 04:39 PM
Retired US Navy
Evan D's Avatar
Usually caused by too much up trim put in to correct a nose heavy condition....
Apr 01, 2012, 04:59 PM
Tarot 550/600 & OMPHobby UK
Smoggie's Avatar
Hard to say exactly what the cause was without further information but contrary to what was stated a few posts back (and somewhat counter-intuitively) a nose heavy plane tends to climb under power. If you found the model pitching nose up only when you added power and flying level or diving when you throttle back then that's probably a sign of nose heavy, and/or you may need some downthrust.
I'd tend to go with Evan's suggestion that you are nose heavy and carrying too much up trim to compensate.
Apr 01, 2012, 05:05 PM
Tarot 550/600 & OMPHobby UK
Smoggie's Avatar
Originally Posted by elecfryer
+1 to KM and MW plus, I have come across more than one kit / arf that had an incorrect CG listed! There are now many web sites that will assist you with proper CG location. One example below:
That calculator only works out the MAC of the wing, it doesn't calculate where the actual CG needs to be. For that you need to factor in the tail area and tail moment arm (among other things). The better calculators out there do take the tail into consideration:

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