DJA Chrysalis - Ruddervator Information
(Advice from the experts)

 

Don Stackhouse at DJ Aerotech took the time to explain the problems I was having in flying my Chrysalis. This may end up getting posted on their site as well but for now, here is the explanation. I suspect that other flyers who are used to super-stable ships like the Gentle Lady will find this to be useful.



Mike Whaley writes:

I have had exactly the opposite experience with ruddervator differential as what you are indicating on your website.

Well, maybe not EXACTLY! Let's look a little closer. I suspect you might be seeing a combination of several phenomena masquerading together as something else.

I started with equal throw on my Chrysalis, but have now helped squirrely flying by giving the ruddervators more up than down. Before, when I was turning, it would drop the nose dramatically. Pulling back helped a little, but I realized just to hold altitude in turns I was adding full up elevator and was chasing things around like crazy trying to keep up.

Mike, by some chance are you holding all or most the rudder deflection during the turn, then releasing it to roll out, the way you use a steering wheel on a car? If so, I think that may be most of your problem.

Forget the V-tail for a moment, let's look at the forces on the wing. When the model is in a turn, the centrifugal force from the turn combines with the weight of the model to create a resultant force that is perpendicular to the wing. Other than some quirky little side effects (which I'll discuss in a moment), the model does not know it is in a bank! For normal turns, you should need little or no rudder deflection after rolling to the desired bank angle, so whether or not you have ruddervator differential should be pretty much irrelevant during the turn. If no rudder input is needed once the turn is established, then obviously there is nothing you can do with differential settings that will have any influence on this problem.

If you are holding the same relatively large rudder deflection throughout the turn, then the model continues to roll to steeper and steeper bank angles. On an aileron model, it would continue to roll, even to the point of rolling inverted and then right side up again. In fact, this is the basic method for doing an aileron roll, just add a bunch of aileron deflection and wait till the model is right side up again. On a rudder-elevator model, the yaw caused by the rudder does tend to shove the nose down when you get to very steep bank angles, so the model tends to end up in a spiral dive like what you describe. To do a roll on a rudder-elevator model, you first need a bunch of extra speed, then a 15-20 degree pull-up at the entry of the roll so that the nose won't drop too low from this yaw effect during the first half of the roll.

This happened at the flying field, and then I took it to the beach and slope soared... a great way to test since you get long, continuous flights (have to be careful of elevator trims... you usually trim down a bit to hold it in place in the sky, so it thinks it's flying a lot faster than you would fly in dead air.) Based on that and upon thinking things through, I realized that in a turn, say to the right, the left elevator half was nearly vertical and yawing the fuselage like a rudder but the right one was acting like down elevator.

Don't feel bad, this is a common misconception, but for the reason I mentioned above, it doesn't quite work that way. If you hold a rudder deflection towards the inside of a turn while in a steep bank, there is a downward component to the yaw that results, but this will happen regardless of the type of tail. This will tend to force the nose down in a very steep bank, but this isn't the main culprit in most cases.

The fact that you are using some of the wing's lift to pull the model around the turn means that there isn't as much left to hold the model up. If you don't add some up elevator to generate some additional lift, the model will start to descend. Because you haven't changed the elevator setting, the model's natural stability in pitch will cause the nose to drop as the tail attempts to keep the angle of attack approximately constant. With the model now in a descent, the airspeed will increase, which then creates the extra lift required. If you're not mistakenly holding excessive amounts of rudder during the turn, then this is probably the main factor in the phenomenon you're observing.

Your inner ear cannot distinguish between centrifugal force and gravity force, which is why pilots flying inside of clouds can't keep the wings level without some sort of special instrumentation, usually based on gyroscopes. Likewise, in most cases your model can't distinguish between the two forces either, which is why it thinks the wings are level when in fact they're banked for a turn.

In a classic "coordinated" turn like this, you would use aileron or rudder to roll to the desired bank angle. You would then center the rudder and/or aileron and add a little up elevator to generate the extra lift coefficient required to keep the airspeed constant. Lastly you would neutralize the elevator and apply opposite aileron and/or rudder to roll out of the turn.

So far, what I've discussed is basically the same as what you would read in a good training manual for full-scale aircraft, and would apply as well to most typical R/C models. Now for the unusual quirks that complicate things:

It was very difficult to hold rudder and back elevator as well, with no differential set. So, I reset the servo arms to be one click farther towards the front of the aircraft, so that neutral was about 10-15 degrees towards the front of the aircraft so there was more travel pushing back (ie, up elevator) than down. I also set dual rates to have 75% throw on elevator and 85% on the rudder (still playing with that a bit.) But it seems that what you recommended would have made the "spiral death dive" effect worse, and though I'm still working things out it flies much more like a plane is supposed to now. I know you have good reasons for things you do, but I wonder why the difference in perceived effects and the associated solutions?

Quirky Effect no. 1 :
If the aircraft is in a tight turn, the inside wingtip is moving slower than the outside wingtip. Years ago, before the widespread availability of gyro instruments, an inventive full scale pilot used this phenomenon to develop a simple but reasonably effective way to keep his wings level while inside a cloud. He went down to his local five-and-dime store and bought a box of "wolf whistles", the kind that make a higher note when you blow harder. He held them each out the window of his car until he found two that made the same note at the same speed. He then mounted them on the wing tips of his Curtiss Jenny, and ran some tubing from each back to the cockpit. The tubes were connected to each side of a stethoscope. When his left ear heard a lower note than his right, it meant he was turning left, and vice versa. When both whistles blew the same note, his wings were approximately level. Worked ok, but the constant dissonant bleating of the whistles in both ears could drive the pilot bonkers after a few minutes! Soon after, Sperry invented his gyroscope, and the whistles and stethoscopes quickly went in the trash!

This difference in airspeed between the wingtips will require a corresponding increase in angle of attack on the inside wingtip to keep the lift generated at both wingtips the same, or else the aircraft will try to steepen up the bank angle by itself. On aileron models we can do this with a SMALL amount of "top" aileron, which has the added benefit of increasing camber at the inside wingtip. On lightweight sailplanes, the difference in airspeed can sometimes be by a factor of 2:1 or more, which means that the difference in lift coefficient between the tips must be 4:1 or more! This is one of the major causes of tip stalls. On a polyhedral rudder-elevator model we don't have ailerons, so all we can do is yaw the model a little to the outside of the turn with a SMALL amount of "top" rudder, causing an angle of attack increase on the inside wing and a decrease on the outside wing.

Quirky Effect no. 2 :
In a very lightweight model with a long tail moment (like your Chrysalis) some other factors come into play. In a turn, the airflow past the model is curved. This effect exists for all aircraft in a turn, but for most models and full scale aircraft the radius of the turn is so great in comparison to the dimensions of the aircraft that this curvature of the airflow can be ignored. In a lightweight model such as a good R/C HLG, the turning radius in tight thermal turns is so small that it is in the same league as the length and span of the model itself. When this happens, the relative wind in the pitch and yaw sense at the tail will be inward and upward in comparison to the wing's local relative wind. The difference can in some cases be more than 10 degrees in both control axes! When this happens, the model will naturally want to yaw to the outside of the turn, which may be enough to deal all by itself with the effects of "Quirky Effect no. 1". It could also be too much, or not enough, in which case you will need to hold a certain amount of rudder one direction or the other to keep the bank angle constant. I've seen it go both ways, depending on the design of the aircraft. For example, to do a coordinated turn in a full-scale Aeronca "Champion" I used to fly, I needed to apply a SMALL amount of "bottom" rudder (rudder in the same direction as the turn, because the curvature of the flow at the tail wants to yaw the aircraft too much to the outside of the turn) plus some "top" aileron (aileron opposite the direction of the turn, because the lower airspeed at the inside wingtip causes the aircraft to want to roll to a steeper bank angle). I saw the same situation happen in a full-scale Schweitzer 2-22 sailplane. In R/C trainers it's probably best to size the tail and its moment arm such that some rudder is required to keep the model turning, so that it will tend to naturally roll itself back to level flight if the controls are released.

The upward airflow at the tail also tends to push the nose down and increases the amount of up elevator required to hold a tight turn. The net result of all of this is that to do really tight turns, you have to learn to use the controls properly while rolling in, during and while rolling out of the turn.

I think the reasons above explain the effects you're observing. BTW, all of this is equally true for conventional, "T" or "V" tails, and has ABSOLUTELY NOTHING to do with the reasons for ruddervator differential on V-tails.

The reason for ruddervator differential is to balance the forces between the two sides during rudder deflections. Because the up-moving ruddervator gets an end plate effect from the other tail panel, and the down-moving ruddervator doesn't, for equal deflections the up-moving ruddervator develops more force than the down-moving ruddervator. If we kept the deflections equal, this would result in a small amount of un-commanded pitch up whenever we gave a rudder command. By making the down-moving side move a little farther than the other one, we balance out this effect so that the forces on both sides of the tail are equal. When tail differential is perfectly balanced, a rudder command will result in ONLY yaw and roll, with no change in pitch, just like a conventional tail. Get the model on a flight path directly away from you, then give it a series of left-right, left-right rudder inputs. You should see the tail yaw back and forth in response, but it should not move up and down in pitch.

Some folks (like my partner Joe) like to intentionally drop the nose a little when entering a turn, so they add a little extra differential to cause this effect. Personally I prefer mine to be neutral, with no pitch response either way. You should set yours at whatever makes your model fly the way that is most comfortable for you. Just be sure you know what your model is doing and why, so you don't get all confused fighting the wrong problem!

Don Stackhouse @ DJ Aerotech
djarotec@bright.net
http://www.bright.net/~djwerks/




I would like to hear from anyone who finds this information helpful. Please email me at merlin@ov-10bronco.net or catch me at the next IRKS meeting. Or of course you might see me at the flying field or the beach!! May all your launches be high, your thermals be big, and your landings on the spot!



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