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Old Jan 15, 2013, 04:57 AM
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One thing you might consider is that in addition to (slightly) changing the aerodynamics and shifting the CG left/right, you have not only changed the roll and yaw moments of inertia, but also introduced cross-terms; I_xy, I_xz and I_yz. Given that the roll and yaw rates can be pretty high in a spin, perhaps those additional terms are affecting the dynamics. My inital thought was that this appeared to be an aerodynamic effect. It will be interesting to see to what extent that ends up being the case.
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Old Jan 18, 2013, 10:22 AM
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Some results re flat spins

Well, I've been having lots of fun with this, but haven't reached a definitive conclusion yet. I'm still seeing an increased tendency toward flat spins with the camera installed, versus with an equal amount of weight the same distance aft but on the aircraft centerline (near the tailwheel). But it doesn't happen 100% of the time, not as consistently as I thought it was happening that one day. Also, as of now I can't consistently replicate the effect with a lightweight balsa replica of the camera installed (with the tailwheel weight installed to keep the fore-and-aft position of the CG the same as with the camera on the stab). Neither can I consistently replicate the effect if I remove the camera, install the tailwheel weight, and install a weight on the wing that has the same lateral moment-arm as the camera on the horizontal stab did, so that the CG of the airplane should be at the same place as it was when the camera was installed.

Adding a heavier weight to the left wingtip CLEARLY causes spins to the right to go flat. Adding an equal weight to the right wing tip doesn't seem to change things much, in spins to the right. The spins still go flat, but not to any greater or lesser degree as far as I can see. In other words the weight or moment of inertia of that portion of the wing that lies between the centerline and the left wingtip, seems to be a key factor in whether or not a spin to the right will tend to go flat. The weight or moment of inertia of that portion of the wing that lies between the centerline and the right wingtip doesn't seem to matter. (Except that it is easier to enter a right spin in the first place, if the weight on the left wingtip is balanced by an equal weight on the right wingtip. Having weight on the left wingtip but none on the right makes it difficult to initiate a right spin.)

Adding a weight to the right wingtip, but none to the left wingtip, does not encourage spins to the right to go flat. It may actually tend to encourage a more nose-down spinning attitude.

The fore-and-aft CG position of the CG is also critical, of course. An aft CG tends to encourage spins to go flat.

I'm sure the camera's inertia, acting left of the centerline, did play at least a small role in encouraging right spins to go flat, compared to the spin behavior when a weight equal to the camera's weight was mounted on the aircraft centerline, near the tailwheel. But my more recent results suggest that that effect isn't very strong and doesn't really account for the consistent flat-spinning behavior I observed that first day of flying with the camera on the stab.

I'm not yet sure whether there was also a significant pro-spin aerodynamic effect from the camera.

I'm pretty sure the CG was not any further aft with the camera installed, than with the tailwheel weight installed.

I bought a more accurate scale and will do some more trials soon comparing the camera installed on the stab, versus a balsa replica of the camera installed on the stab plus some weight near the tailwheel to keep the fore-and-aft position of the CG exactly the same.

Meanwhile, I've gotten some GREAT video of tufts near the tail and on the tail during flat spins. Looking straight back at the tail, you can see the airflow is coming diagonally up and sideways at about a 45-degree angle to the vertical fin! The horizontal stab is blanketing the vertical fin and rudder, and a tuft on top of the vertical fin is lying limp, flopping slowly back and forth to drape over first the left side of the fin, then the right side. Pretty cool! Explains why full opposite rudder alone does not effect recovery! (Sometimes full opposite rudder plus full down elevator did effect recovery, other times some power was also needed. Presumably the propwash over the tail helped.)

Steve
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Old Jan 20, 2013, 09:33 PM
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Summary of results

Well, I've reached a conclusion but it's not very earth-shattering. At aft CG locations, the model does have a tendency toward a flat spin mode but it doesn't happen every time, often the spin mode is quite nose-down instead. Every spin is different. Flying from the ground rather than from inside a cockpit, it is impossible to consistently use the exact same entry technique (pitch attitude, airspeed, deceleration rate, etc.) So any exploration of spins with an RC aircraft is likely to produce very scattered data.

Nonetheless it sure does appear that the model has a significantly greater tendency to spin flat (in spins to the right) with the camera mounted well outboard (left) on the horizontal stab, as opposed to when an equivalent weight is mounted near the tailwheel (essentially the same distance aft of the CG, but on the aircraft centerline, and an inch or so lower.)

I can't replicate the effect when I have the equivalent weight mounted near the tailwheel and a lightweight balsa replica of the camera mounted outboard on the horizontal stab. Also, I can't replicate the effect when I mount a more compact weight, equal to the camera's weight but with a smaller aerodynamic "footprint", outboard on the horizontal stab, in place of the camera. I think the camera's aerodynamic effect alone is not easy to detect, and the camera's inertial effect alone is not easy to detect, but the aerodynamic and inertial effects combined are significant enough to make the spins go flat noticeably more often than when the camera is absent from the horizontal stab and an equal weight is mounted near the tailwheel.

That's not a very robust line of argument, but it is my current conclusion.

Flying with a chunkier, slightly larger balsa substitute for the camera on the stab-- as opposed to the more accurate balsa replica noted above--, and the tailwheel weight, I still couldn't make the spins go flat at a noticeably higher rate than with the same tailwheel weight but nothing on the stab. However, when I increased the yaw and roll rotational inertia of the aircraft by mounting a weight on the left wing tip, spins to the right definitely went flat at a much higher rate than without this weight. There's no doubt in my mind that mounting the camera outboard on the left horizontal stab did have some of the same effect--the yaw rotational inertia would have been the same as with the tailwheel weight, but the roll rotational inertia would have been higher-- even though I couldn't replicate it when I replaced the camera with a more compact shape of equal weight.

As noted earlier, when flying with a heavy weight on the left tip, spins to the right had a very strong tendency to go flat. I didn't find that counterbalancing the weight with an equal weight on the right tip made spins to the right go flat noticeably more or less often. It's the weight that lies to the left of the CG, that seems to have the greatest effect on the tendency of spins to the right to flatten out. And of course the fore-and-aft position of the CG.

When I say a "heavy weight on the left wing tip" I mean 3 quarters (coins), which together weigh about 0.6 ounces. The model (Hobby Zone Super Cub) weighs about 1.5 pounds. The camera weighs about 0.5 ounces.

Steve
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Old Jan 20, 2013, 09:44 PM
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Some practical notes on spinning and flat spins

In addition-- some practical notes on spinning and flat spins based on the spins I carried out over the last week or so in the RC SuperCub--

Every spin is different. Don't think that the next spin will necessarily progress like the last one did, even if you think you are keeping all the variables the same.

Fore-and-aft location of CG is critical, with far-aft CG locations inviting a flat spin mode.

Sometimes the anti-spin control inputs must be held for several revolutions before they break the spin.

Control throws are hugely important in influencing your ability to initiate a spin, and also your ability to recover from a flat spin. Use the trims to help. More up trim = more up-elevator, more down trim= more down-elevator, and so on with rudder.

(Note that in a real (full-scale) aircraft, especially a light plane where pilot has enough muscle force to over-power the trims, the trims have the opposite effect as described above. For example, dialing the trim to full "down" raises the tab at the rear of the elevator, which actually ADDS to the elevator's power when the pilot forces the stick or yoke to the full-aft position, and SUBTRACTS from the elevator's power when the pilot forces the stick or yoke to the full-forward position. So dialing in some UP trim, and some PRO-SPIN rudder trim, may help make the spin recovery be more effective, though it will also make your muscles work harder.)

In addition to full anti-spin rudder and elevator, POWER may be needed to recover from a flat spin, whereas full opposite rudder alone may break a steep nose-down spin even if the stick is kept full aft. When adding power while holding full anti-spin rudder and elevator, expect the plane to "bunt" to a vertical dive or inverted flight as the full-down-elevator suddenly becomes effective and the spin is broken and the plane is flying again. So don't delay this step in the recovery until the ground is near! Using less-than-full power will help reduce this tendency to "bunt" to a vertical dive or inverted flight.

By the way I'm well aware that the standard spin recovery technique for light aircraft includes pulling the power to idle and leaving it there. I don't know what technique is recommended for a well-developed flat spin but after these experiments in the Super Cub, I surely would not hesitate to add power if full anti-spin rudder and elevator were not producing results!

Steve
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Old Jan 23, 2013, 11:17 PM
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notes on spin videos

Got an interesting video today, in a flat spin. Camera on the inboard wing, i.e. the retreating wing. Tufts located a few inches below the wing, approx. mid-span and mid-chord, streamed not too far off from parallel to the wing chord even during the flat spin. However a tuft several inches outboard of the wingtip streamed basically "straight-up" in the camera's reference frame, meaning basically perpendicular to the wing chord!

I suppose the indication of this outboard tuft is fairly close to true relative wind, i.e. the airflow that would exist if not for the physical disturbance of the aircraft, i.e. the relative wind caused by the combined effects of the aircraft's translational velocity and the wingtip's yaw and roll rotational velocities....

There was a camera on the outboard wing too and haven't looked at it yet, but would expect the tuft mounted outboard of the outboard tip to be raked considerably more aft of perpendicular to the wing chord , as this is the advancing wing tip.

In other words the relative wind is "twisted" by the aircraft's yaw rotation and roll rotation...

Will post links when I get the videos up, may take a few days...

Steve
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Old Jan 25, 2013, 11:57 AM
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I've been trying all sorts of airplanes to get some spin data, but all of them come down too quickly after the stall to let a spin stabilize. Mother Earth gets too close too soon!
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Old Feb 05, 2013, 03:06 PM
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Many people have been killed due to an inadvertent spin with an aft CG going flat and not recovering. Actually, one famous example is airshow great, Art Scholl. He was killed while filming the flat spin scene for the movie Top Gun.
One of my all time favorites! And I had no idea about this! Interesting! Thanks for the info :-)
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Old Apr 04, 2013, 01:16 PM
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I've been trying all sorts of airplanes to get some spin data, but all of them come down too quickly after the stall to let a spin stabilize. Mother Earth gets too close too soon!
You're going to love what I filmed yesterday-- since I've realized that my Hobby Zone Super Cup is tuned so that it stays in a flat spin indefinitely with the transmitter sticks at neutral, I've been climbing the plane up high, entering a flat spin, and then putting down the transmitter and filming from the ground!

With the current set-up, power is ALWAYS needed for recovery; full anti-spin rudder and elevator NEVER break the flat spin alone. However I've found that power will actually break the flat spin even with the rudder/elevator neutralized. This will help me do a better job of filming the recovery as I only need one hand on the transmitter (to open the throttle) and can leave the rudder/elevator stick alone and use the other hand for the camera.... Also this avoids the violent dive to inverted that normally happens when the full-down elevator suddenly "takes" as the throttle is opened and the tail feels the propwash. With the elevator full down, even gradually opening the throttle normally does produce this sudden dive to near vertical or beyond-- will be visible in the videos...

I was kind of surprised to be able to break the flat spin with throttle only, with the rudder centered and the elevator in the normal trim position for a slow glide...


Steve
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Old Apr 04, 2013, 05:14 PM
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Originally Posted by aeronaut999 View Post
You're going to love what I filmed yesterday-- since I've realized that my Hobby Zone Super Cup is tuned so that it stays in a flat spin indefinitely with the transmitter sticks at neutral, I've been climbing the plane up high, entering a flat spin, and then putting down the transmitter and filming from the ground!

With the current set-up, power is ALWAYS needed for recovery; full anti-spin rudder and elevator NEVER break the flat spin alone. However I've found that power will actually break the flat spin even with the rudder/elevator neutralized. This will help me do a better job of filming the recovery as I only need one hand on the transmitter (to open the throttle) and can leave the rudder/elevator stick alone and use the other hand for the camera.... Also this avoids the violent dive to inverted that normally happens when the full-down elevator suddenly "takes" as the throttle is opened and the tail feels the propwash. With the elevator full down, even gradually opening the throttle normally does produce this sudden dive to near vertical or beyond-- will be visible in the videos...
I was kind of surprised to be able to break the flat spin with throttle only, with the rudder centered and the elevator in the normal trim position for a slow glide...


Steve
I have a friend with a full scale aerobatic setup- which also needs power blip to break a spin- - I advised him to sell it -after two turns it 'winds up" and normal spin recovery will not work
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Old Apr 05, 2013, 01:18 AM
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Flat-spinning the HobbyZone Super Cub-- videos

Flat-spinning the HobbyZone Super Cub-- videos

Video explaining set-up -- (Summary: I note that I've taped 8 quarters to the rear fuselage. Static stability is low but still enough to pull out of a vertical dive when trimmed for a slow glide. I've noticed that the right wing has very little washout, compared to a couple other spare wings I have on standby. The plane spins very well to the left.. About 30 to 50 percent of the spins were going flat, until I taped a quarter to the right wingtip, after which 100% of the spins to the left went flat. A few more details are given on the video.)

Flat-spinning the Hobby Zone Super Cub--Description of aircraft set-up (3 min 1 sec)


Video of flat spins and some recoveries-- This video is of a single flight. I did all the flying and filming. About 30 to 50 percent of the flight has been edited out. Sometimes I was filming while also operating the transmitter. Other times, such as during some of the climbs as well as during some of the spins, I set the transmitter down as I filmed. Spins were initiated at about half power, holding the nose up at a 30 to 45 degree angle and feeding in full left rudder and then applying full back stick and full left rudder as soon as the incipient spin started. Within a very few revolutions, the aircraft was well established in a flat spin, and I moved the throttle to idle and relaxed the pro-spin stick and rudder. The flat spin would continue indefinitely even with the rudder centered and the elevator at the trim position for a slow glide. For all recoveries, I gave full nose-down elevator and full anti-spin rudder and then I gradually fed in power. The aircraft never recovered from the spin without the addition of power. At about half power, the tail would apparently suddenly "feel" the propwash and the full-down elevator would "bite" and abruptly pitch the nose down to near or beyond vertical. This is quite visible in some of the recoveries on the video. Of course, as the plane bunted down into a steep dive, I had to put the camera down!

I have since discovered that with enough power, the aircraft will recover from the flat spin even with the rudder neutral and the elevator at the trim position for a slow glide. This gives a more gentle recovery without the abrupt "bunt" to a near-vertical or past-vertical dive. I'll film that next!

Flat-spinning the Hobby Zone Super Cub--Demo--1 (5 min 24 sec)
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Old Apr 18, 2013, 06:25 PM
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Flat-spinning the Hobby Zone Super Cub-- on-board camera

2 flat spins to the left. On the first recovery, note that full anti-spin (stick and rudder) controls have no effect until power is added. On the second recovery, note that the controls are centered and recovery is effected with power only.

Note the blanketing of the airflow over the fin and rudder-- the tuft on top of the fin is sometimes out of view but sometimes you can see it draped loosely over the top of the fin.

Flat-spinning the Hobby Zone Super Cub-- On-board camera take 1 (1 min 45 sec)
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Old May 07, 2013, 11:43 AM
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some new insights

* I've been flying the plane at 2 cg locations, the more forward involving 6 quarters taped near the tailwheel and the more aft involving 9 quarters taped near the tailwheel. One quarter is also taped the trailing edge of the right wing near the tip, to facilitate left flat spins as noted in the previous posts. No change in the elevator linkage is needed-- the more forward cg trims to a nice engine-off glide with elevator trim neutral (and I've set the linkage so the elevator is pretty much flat with the tail in this configuration), and the more aft cg trims just a bit slower than ideal for a nice glide with full nose-down trim, engine off.

* Obviously, both configurations have the CG well aft of stock configuration.

* Surprisingly, it is only slightly harder to get consistent flat spins in the more forward configuration than with the more aft configuration.

* The entry technique is important to getting a consistent flat spin. Here's what is working for me: power at 3/4 to full, power trim full down (so engine will be full off when throttle is full retarded), hold airplane wings-level, slowly add back stick to raise nose to about 40 to 45 degrees high and hold it there as airspeed bleeds away. When a wing starts to drop or the nose starts to drop or the stick reaches full aft, give full up elevator and full left rudder. As soon as a marked yaw rotation rate is visible, cut power to idle. Hold full pro-spin controls until the spin goes good and flat. At this point the controls can be neutralized and the flat spin will continue.

*Cutting the power to idle as soon as the yaw rotation has started seems to be the key to getting consistent flat spins in this plane. Holding some power for too long tends to produce a more nose-down spin, even at the more aft cg location. The plane will automatically recover from a nose-down spin when the flight controls are neutralized, even at the more aft cg location. Considering reaction time and so on, I'm guessing that I'm chopping the power to idle at about the same time as the aircraft has completed the first rotation of the developing spin.

* The above observation seems a bit contradictory to standard full-scale airplane spin recovery technique...

* My favorite technique for a smooth recovery is to slowly open the throttle while leaving the flight controls neutralized. At some point before the throttle is full open, the plane will recover from the spin and pitch down into a gentle dive. Holding full anti-spin controls while adding power ends up pitching the plane down into a vertical or past-vertical dive when the plane recovers from the spin. Full anti-spin controls with no power does not break the flat spin and has no obvious effect on the pitch attitude or rotation rate.

* I've set the linkages in the positions that give the maximum throws, but the throws are still very modest in this trainer-type airplane. This undoubtely has a significant influence on my observations.

* The more forward cg position has good static and dynamic pitch stability-- phugoids are well-damped, and there is good static pitch stability, at least as long as the airspeed is not allowed to build to too high a value. It is a pleasure to fly and the final approach to landing in smooth air involves a nice stable hands-off glide to the flare point.

* The more aft cg position is noticeably more squirelly. In the power-off case, there is good static pitch stability at least at lower airspeeds--when I trim for a slow glide and pitch down to vertical or beyond and release the controls before the plane has time to gain too much speed, the plane does pitch up toward a gliding trajectory. But phugoids are not well-damped with power off (trimmed full nose-down for slow glide), and are somewhat unpredictable. Sometimes even a large phugoid will damp out, and sometimes a slight disturbance from a slow glide will develop into an extreme phugoid. Extreme phugoids often involve the plane pitching up to 80 degrees nose-high, running out of airspeed, dropping the nose abruptly to about 100 to 110 degrees nose-down (10 to 20 degrees beyond a vertical dive), and smoothly accelerating with smoothly rising nose and then continuing on to the extreme nose-high attitude again. Sometimes the plane will stabilize into a long chain of these phugoid maneuvers with little to no visible variation between each one. Another variation I sometimes see at the low-airspeed, nose-high point in the phugoid, is for the nose to flip "over the top" in a tight, low-airspeed loop followed by a vertical or near-vertical dive or past-vertical dive.

* With power full on, trimmed for slow flight (climbing), phugoids are well damped with either cg position. Pitching the nose down and then releasing the controls never results in a sustained phugoid with power on.

* Landings are a bit more challenging in the aft-cg configuration. One is always making some inputs against the phugoid tendency, even in smooth air. It is not posible to make a hands-off final glide to the flare point.

* I'm having almost as much fun with this little plane, as I have with my molded 6-servo slope glider!

* Next project-- with my improved spin entry techniques, try the other two wings I own for this plane and see whether or not I can get a flat spin, at either cg position. As noted in previous posts, the wing that flat-spins so well to the left appears to be warped to have less washout than normal, especially on the right side. It's hard to tell for sure though. Hmmm, now that I think about it, shouldn't a lack of washout on the right wing promote spins to the right, not the left?

* No radio problems at my current flying spots even with the little DX-4 transmitter (which I see is now advertised as having only 2500' range); I'm sure there were some sources of interference at the other places where I had the problems. Still, I always check that the plane is set up so that neutral trims neutral stick does not yield an nosedive. This receiver goes to servos-neutral upon loss of signal.

That's all for now!

Steve
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Old May 19, 2013, 10:18 AM
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flat spin update

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Originally Posted by aeronaut999 View Post
* Next project-- with my improved spin entry techniques, try the other two wings I own for this plane and see whether or not I can get a flat spin, at either cg position. As noted in previous posts, the wing that flat-spins so well to the left appears to be warped to have less washout than normal, especially on the right side. It's hard to tell for sure though. Hmmm, now that I think about it, shouldn't a lack of washout on the right wing promote spins to the right, not the left?
For various reasons, including planning ahead for an aileron project, I own 4 wings for the Hobby Zone Super Cub. Yesterday I found that I could indeed make the plane enter a flat spin with any of these 4 wings installed. I still find that the wing featured in the above tests and videos does flat-spin a little easier than the other 3.

In fact, I had these interesting results even while flying with a CG position (5 quarters taped just ahead of the tailwheel) slightly forward of either of the 2 CG positions described in my previous post. At this more forward CG position, with no weights on either wingtip, the plane is reluctant to spin at all. Bear in the mind that the control throws of this trainer-type airplane are rather limited. To the right, I can't get a spin at all with any of the wings. To the left, I can get a spin with the nose about 45 degrees down or so, but the rotation rate is fairly low and the spin ends as soon as I cut the power, even with full pro-spin controls. So it's not really a fully-developed spin. Unlike the more developed spins with the nose pointing nearly straight down that I got with a further aft CG position, and no weight on either wingtip.

Yet, despite the plane being fairly reluctant to spin at this CG location, if I added a single quarter near the right wingtip I could get the plane into a fully-developed left flat spin with any of the 4 wings installed. Some of the wings yielded this result more consistently than others, but the maneuver was possible with any of the four.

I definitely have gained a better understanding of the entry technique that yields a consistent flat spin in this plane. On a full battery charge, the best technique is to start with less-than-full power-- with full power, the nose ends up too high before the maneuver is completed, inviting a tailslide or backflip and also just generally unloading the wing too much, so that the airspeed doesn't bleed away during the spin entry. Pitch the nose smoothly up to about 40 degrees nose-high or so and hold this nose-high attitude with the wings level as the airspeed bleeds away. As soon as there is evidence of a wing drop (either wing), a drop of the nose, or the control stick reaches full aft, apply full aft stick and full left rudder and hold these inputs. As the rotation starts to develop, cut the power. The power should be cut by the time the plane has completed several rotations. Continue to hold full pro-spin controls (full aft stick and full left rudder) for a few more turns, or however long is needed until the spin really flattens out, with a near-level pitch attitude and a high yaw rotation rate. At this point the controls can be neutralized and the flat spin will continue indefinitely. The smoothest recovery technique is to smoothly open the throttle while leaving the controls neutralized. Holding full nose-down stick during the recovery yields an abrupt pitch-down beyond vertical to an inverted dive. Some of the wings would sometimes recover upon the application of full nose-down stick and full right rudder, without power, but this is the exception to the general rule.

At this CG position, If the controls are released before the spin really flattens out, the plane will recover by itself.

I find it interesting that at this CG position, where it is difficult to get a well-developed spin of any kind with no weights on either wingtip, and the plane tends to recover automatically when the controls are neutralized or even simply when the power is reduced, I can add a single quarter to the right wingtip and consistently demonstrate successful entries to fully-developed left flat spins, which will continue indefinitely when the controls are neutralized (and often even after the application of full anti-spin controls.)

I didn't test it in the last round of tests, but previous results suggest that adding the same amount of weight to left wing tip (so that both wings were equally weighted) wouldn't make a significant change in the plane's tendency to flat-spin to the left. It's the amount of weight on the outboard tip that matters. That would be a good thing to test again, now that I've moved the CG forward and perhaps as a result have gained more resolution to detect a slight increase or decrease in the plane's tendency to flat-spin.

Steve S
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Old May 20, 2013, 03:24 PM
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more insights

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Originally Posted by aeronaut999 View Post
I didn't test it in the last round of tests, but previous results suggest that adding the same amount of weight to left wing tip (so that both wings were equally weighted) wouldn't make a significant change in the plane's tendency to flat-spin to the left. It's the amount of weight on the outboard tip that matters. That would be a good thing to test again, now that I've moved the CG forward and perhaps as a result have gained more resolution to detect a slight increase or decrease in the plane's tendency to flat-spin.
I've done some more tests and have now concluded that--

Adding weight X to one wingtip promotes a very-nose down spin mode, if the direction of spin is toward the weighted wingtip. In this case and in general, when the aircraft is spinning in a steep-nose down attitude, the aircraft will typically recover from the spin as soon as the pro-spin control input is ended by neutralizing the control stick.

Adding weight X to each wingtip (for a total weight increase of 2x) only creates a small increase in the plane's tendency to flat-spin, compared to the case with no weight on either wingtip. In both of these cases, the spin will sometimes go rather flat, but usually recovery can be accomplished by neutralizing the stick or giving an anti-spin control input, even without adding power.

Adding weight X to one wingtip creates a much larger increase in the plane's tendency to flat-spin, if the direction of spin is toward the unweighted tip. Typically, the spin can not be broken without adding power. In some cases, the spin can be broken with full anti-spin controls, especially with the addition of full anti-spin trims, but recovery typically takes many turns. Adding power is an effective recovery method, even if the controls are left neutralized.

The observation that weighting the inboard tip makes the plane harder to spin in general as well as harder to flat-spin in particular, both in the case where the inboard tip is weighted compared to the case where no tip is weighted, and also in the case where both tips are weighted compared to the case where only the outboard tip is weighted, contradicts posts 14, 17, and 18. In those posts I suggested that weighting the inboard tip had little effect; I now find that weighting the inboard tip strongly discourages spins in general and flat spins in particular. It's possible that the earlier results have some validity for very far aft cg locations, but I tend to doubt it. For one thing, it makes little sense that the axis of yaw rotation could be near the inboard wingtip as I suggested in some of the earlier posts; as the plane falls nearly straight down in a very flat pitch attitude, I think the axis of yaw rotation must be nearer the aircraft centerline. At any rate, I think the latest results are more robust and generalizable. Some possible problems with the earlier tests include--the tests were limited to just one wing-fuselage combination rather than demonstrated with several wings. The most effective spin entry technique was not yet known, so variations in entry technique may have caused variations in results. Extreme aft cg may have so strongly promoted flat spins, that the results of changing the tip weights were partly masked.

The secret to getting a consistent flat spin without having to move the CG too extremely far aft, is to add ample weight (up to 3 quarters in these tests) to the outboard wingtip.

This is true even when the rudder trim lever is kept in the centered position throughout the tests, rather than moved toward the unweighted wingtip to trim the plane for wings-level flight. (Note that the plane has no ailerons.)

Small differences in control position are very important. Adding weight to the right wingtip, then re-trimming the rudder to the left to yield a straight-line glide, will magnify the effect of the wingtip weight (compared to if the rudder trim lever were kept centered), in terms of the spin's tendency to go flat when full left rudder stick is applied during the entry, and in terms of the spin tending to stay flat when the stick is neutralized, and in terms of the plane's tendency to not recover when the stick is moved full forward and right without adding power. Similarly, full anti-spin stick plus full anti-spin trim can sometimes effect recovery without power, even when full anti-spin stick with neutral trim does not effect recovery without power. (Note that trim tabs on the rudder or elevator would have the opposite effect-- for example, in a full-scale light plane with trim tabs, full up trim actually gives the pilot less authority to make a nose-up input and more authority to make a nose-down input, so long as his arm muscles are stronger than the pitch forces.)

It seems most useful to explore the results of adding weights to the wingtip while leaving the rudder trim centered. If we re-trim the rudder for a straight-line glide, we may overestimate the effect of the tip weight. Nonetheless, even with the trim lever kept centered, adding weight to the outboard tip definitely increases the spin's tendency to go flat, while adding weight to the inboard tip has the opposite effect.

I also did some tests where I shifted the fuselage 15 to 17 mm to the side, relative to the wing. Not surprisingly, this had an effect much like adding weight to a wingtip. With the fuselage shifted to the left, a left spin entry yielded a semi-stalled diving spiral that tended to automatically recover even as I continued to hold full pro-spin stick and rudder inputs. With the fuselage shifted to the right, a left spin entry yielded a good spin with the nose about 45 degrees down. Neutralizing the stick at this point would cause the plane to recover from the spin, while continuing to hold full pro-spin rudder and elevator would cause the spin to flatten into a good flat spin. At this point full anti-spin controls would generally not end the spin, even with the addition of full anti-spin trim. Power was generally needed for recovery. Except as noted, the shifted-fuselage spin tests were carried out with the rudder trim centered.

Note: shifting the fuselage to the side entails removing the wing struts-- high-g maneuvers should be avoided! Generally speaking, adding weight to a wingtip is a better way to obtain a good flat-spin capability in this plane!

Summary of airplane set-up for consistent flat spins to left-- stock except for linkages set for maximum possible throw, 5 quarters taped on near tailwheel, 1 to 3 quarters taped on to right wingtip, linkages adjusted for a good fast power-off glide with elevator trim neutral (elevator is approx flush with horizontal tail), spin entries conducted via the specific technique described in post #28. Full nose-up trim is good for a full-power climb in this configuration, and I typically leave the elevator trim in this position throught the spin entry and recovery, unless I'm specifically exploring the effects of other trim settings.

A note on CG position-- with no battery, wing, or rubber bands, here's where the fuselage balances: when the battery cover latch is moved aside, this exposes a short groove in the battery cover, running fore-and-aft. The fuselage appears to balance with the wing platform exactly horizontal, on a pointed object placed exactly in the middle of this groove. To this bare fuselage, add the stock battery, wing, wing struts, and rubber bands.

Note: check your plane's spin tendencies before weighting either wingtip. If it spins best to the right, then add the weight to the left wingtip to optimize for flat spins to the right. I've owned several fuselages and wings and most combinations preferred to spin to the left, but note the reference to right spins in the first post of this thread!

In the latest tests, I added the wingtip weights on the "dot" (a product of the molding process) that is about 1 inch aft of the quarter-sized circle visible on the upper surface of the wing, near each wingtip. This should be fairly near the aircraft cg, in the fore-and-aft sense, when the extra weights are installed near the tailwheel as described above.

Steve
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Last edited by aeronaut999; May 21, 2013 at 11:28 PM.
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