Wednesday, September 17, 2014

decalage -- lots of interesting stuff

Decalage refers to the angle between a plane's main wings and the elevator, or the two pairs of wings on a biplane.  There's a great discussion in the RCG UMX Radian thread.  I'm capturing it here coz... well you know how searching on RCG is.  Post numbers are 3431, 3434, 3435, and 3442. Big Thanks to Phil Alvarez and turboparker for sleuthing these out of the thread.

My Summary:

  • Porpoising is caused by bad decalage.
  • Don't try to solve by changing CG.
  • Small foamies can incur twists due to their light construction.
Standard decalage test:

  • Perform a power-off vertical dive from high altitude (neutralize elevator).
  • A. Model continues straight down: No change needed. 
  • B. Model pulls to canopy: Increase stab incidence with respect to the wing.
  • C. Model pulls to belly: Reduce stab incidence with respect to the wing.

And the words of the masters:


no matter how many packs you try, if decalage is wrong and you don't correct it, there is no way to fix the problem. i had 1 with that and could do nothing until fixed it. it is not anything wrong with the design, it is your specific model. am talking based on personal experience and not on guessing.

Originally Posted by electrich
I agree with Phil. While the foam technology has come a long way, it is still soft and subject to "bad memory" due to how the plane is stored or even how the tail was taped in place - and it will effect your flights. After my dog ripped the stab off mine and I repaired it the porpoising was a problem. It wasn't an issue previously. I bent the fuse/tail joint slightly to give me some more "down" in the stab and then retaped it. Good as new.

Yep. Reduce decalage. Just cut tape and retape. So the stab produces less down force, so the plane flys more nose down. I had to do it on mine and now it flies great. I also had to cut the battery compartment to get a more rearward placement but I usually push the CG to max rearward on my gliders, just shy of unflyable instability. Floats like a dream


and how it started:
decalage and trim
after been able to fly my plane several times, i noticed that to fly and glide without stalling, i had to feed full trim down, and the elevator sits at about 3/32" down, so it was telling me that it had too much decalage (angular difference between wing and stab) , so i measured it again: i had it at -2 degrees (2mm), so i changed it to -1mm .
the cell had to sit full forward, otherwise it stalled wildly no matter what.
another thing i noticed was that if flies fast, and if i try to stall it at the moment of catching it, it does just a snap and falls, not a gentle stall as expected, so it confirms that something was not right.
whenever the weather allows me to try this, i will see and tell here.

just in case this is of any help to some1 that has similar problem (my plane came with zero wing-stab incidence (decalage).


OK, to clear up the confusion about what I did:

1) I removed 5mm of foam at the rear of the battery bay so the lipo can sit 5mm more to the rear
2) I removed the tape that holds the rear of the fuselage on
3) I inserted a 1mm shim in the gap made by cutting the tape on the top, i.e. a couple of cm in front of the fin
4) I retaped it with the bottom tape tight and the top tape normal.

Now the tailplane sits at a subtly different angle. I measured nothing, just took a guess based on the video posted recently of the full sizes Radian mods.

I lobbed it over the lawn to check it flew OK then went and soared it.

As a postscript, I've just cut another 2mm from the bay to test with both my standard battery and 190's. Report will follow.

Did you actually measure the CG? If not, do so. With UM/UMX planes, it is common to see variations between samples in where the battery ends up for a given CG. This is due to normal manufacturing variability - such as variations in foam density, and the amount of adhesive used during assembly.

Best glide performance is obtained with a neutral CG. The dive-test is the method of choice for determining where that point is on a glider. Take her up high & trim for best glide. Then put the plane into a steep dive (~45 degrees or steeper) & release the elevator. If the plane abruptly pulls out of the dive on its own, it's extremely nose-heavy. If it starts to tuck-under, it's tail-heavy. If the plane gently pulls out of the dive on its own, it's slightly nose-heavy. If it continues on the original path, it's neutral. For best glide, the plane must be neutrally-balanced. But it can be just slightly nose-heavy (pulls out of the dive very gently) without sacrificing much glide performance. Some sailplane pilots prefer the slight positive stability of a slightly forward CG, while others prefer neutral handling. On windy/turbulent days, move the CG further forward to improve penetration.

That said - Phil has a good point. A decalage problem will screw up the above test. My first UMX Yak had a major decalage problem that made the plane all but unflyable. I returned it to the LHS, inspected their remaining stock, and picked the one that was built the straightest.


Joel - I could use some advice regarding something that confuses me - I experimented with battery placement. When the battery was centered or as far back as possible the glider tended to porpoise power off with elevator at neutral position. To stop the porpoising it required some down elevator. When the battery was fully forward the porpoising stopped and it had a stable level glide at neutral elevator.
With the battery rearward the CG is 32, with it centered it is about 29, and forward it's about 25.
It flew nicely and as mentioned I was getting 15+ flight times using the throttle for 5 minutes mainly just to climb with the battery fully forward.

I have not tried the dive test yet.

Does any of this make sense to you?


Hi Midnite,

It makes sense. First off, you are supposed to retrim for best glide each time you move the CG. (You're always supposed to retrim the elevator after moving the CG, regardless of plane type.) If you retrim for best glide after adjusting the CG, the plane will not porpoise - unless it's nose-heavy and/or the decalage is off. Also - as you have discovered - CG is not a fixed number. It is standard practice for sailplane pilots adjust the CG to suit the conditions, and then retrim for best glide. Neutral or near-neutral handling for chasing light lift in calm conditions or when winds are light, and further forward to increase gliding speed and improve penetration in stronger breezes & turbulence. It is also common to ballast-up to suit the conditions. We use lightweight packs when lift is light & the air is calm, and then switch to heavier packs when it's windy.


Porpoising is a sign of a nose-heavy condition or a decalage problem. It is never caused by a tail-heavy condition. Try moving the battery aft. Be sure to remove the up-trim you added when you moved the battery forward. If the decalage is right, the plane should not need any visible up-elevator trim for a good glide when the CG is at the sweet-spot for best thermal performance (which is at or just slightly forward of the neutral-handling point as determined by the standard power-off dive-test). Also, there will be no tendency to balloon under power. If you can't get the plane to behave properly with minimum visible elevator trim, the decalage is most likely off enough to cause the behavior.


no problem. there is so much confusion on the subject that no dense here.
decalage is the angular difference between wing and horizontal fixed tail (stabilizer).
to reduce it, you increase the stab incidence or lower the wing incidence. usually it is easier to increase the stab. a small amount goes a long way. when a plane porpoises is believed it has too much decalage (some consider this subject to tests). it is independent of the elevator angle.
in my opinion is the 2nd most important factor (after the center of gravity) to have an airplane balanced and fit, and way unknown to most.
i hope this helps and does not open a can of worms.
Yup. If a sailplane exhibits the signs of a nose-heavy condition (porpoises during the glide, pulls out sharply on its own during the standard power-off dive-test) even after moving the CG decidedly aft & re-trimming for best glide - the decalage is off. Same goes for powered planes that exhibit strong throttle-pitch coupling even though the CG & thrust-line are correct.

Here's the standard decalage test. The vertical dive takes CG out of the picture, so that only the effects of incidence/decalage are present:

Perform a power-off vertical dive from high altitude (neutralize elevator).

A. Model continues straight down: No change needed. 

B. Model pulls to canopy: Increase stab incidence with respect to the wing.

C. Model pulls to belly: Reduce stab incidence with respect to the wing.

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