On using a gyro for rudder

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If you've never flown a short coupled model equipped with a tail wheel, then it's quite likely you've never experienced the joy of taking off 90° to your initial heading. As pilots, we all know at some point take off includes a 'shoving the throttle forward' phase, and with the Pitts Special, a representation of the biplane popularized by designer Curtis Pitts, anything but perfect technique could mean a ground loop or a trip toward the weeds. Note; his 1943 design was considered revolutionary because of its small size, light weight, short wingspan, and rather extreme agility. The original hangs in the Smithsonian and mine (below) is in 1/3-scale. My model is also noted for agility, and in my case, a tendency to make an abrupt turn to the left as torque manifests itself as I add throttle if I don't smoothly add enough right rudder (once I have enough air speed to effect a countervailing force). In other words, I'll almost immediately screw up the take off if I'm not on top of my game. This brief read is about how technology can help.

Anyway, just like with the full scale aircraft, the model is small and the distance between the CG, main gear, and tail wheel is considered to be 'close' and the popular term for this is it's short-coupled. Note; unless you're an engineer, where the term couple immediately brings to mind two parallel forces equal in magnitude but opposite in sense and which do not share a line of action, or a pure moment, what this means for pilots (both full-scale and model) is the creation of rotation without translation. In English, translating means the model experiences an acceleration around the center of mass resulting in a tendency to ground loop (true in both the full scale world and with our models).

A Pitts Special is famous for this! And as if things couldn't be worse, for modelers there's the added reaction time of waiting to actually see the nose fail to maintain heading (an inherent processing delay due to the pilot's response to visual stimuli) before correcting. This, versus simply feeling it begin to happen in your butt when you're flying the full scale aircraft (because you're sitting in the cockpit). Anyway, whilst fun on grass, on pavement the tendency to jitter off into the weeds so to speak was bad enough that it got to the point that - while I loved flying the model - the stress of dealing with its rather poor ground handling and its especially nervous take-off characteristics led me to often look at it and decide to take something else to the field (and this doesn't even begin to address what this tricky rascal can be like on landing).

Anyway, take offs on our hard surfaced runway were sufficiently difficult to execute, that aborting happened as often as 1 in 3 attempts! While I'm an otherwise decent pilot, one out of three take offs were embarrassing. Moreover, while a ground loop was bad enough and just damaged my pride, proceeding to take off in a squirrelly manner at the aforementioned 90° to initial heading resulting in great mirth for the peanut galley (and those who dove under their car if it headed in the direction of the pit area).

Humor aside, it truly was sometimes dangerous to be around me when flying the Pitts off a hard surface. So I found myself leaving it at home more and more. Enter the gyro, the principal reason for relating all this. You see, we offer an inexpensive gyroscope for model helicopters, part number PDR0050. Its express purpose is to tame the tail rotor of a model helicopter like our Pantera.

In case you don't know it, flying helicopters is harder than flying a fixed wing model airplane because of the relationship of the main rotor and the tail rotor and the need to balance the torque between the two. Doing this successfully calls for a tail rotor gyro. Basically, it's a little box CNC-machined from a solid billet of 6061 T-6 aircraft aluminum, which houses a MEMs sensor (a tiny piezo crystal) and some electronics. Using it is easy as it connects betwen the servo and the receiver (just plug the tail rotor servo into it, and then plug it into the receiver) and Bob's your uncle because it's very sensitive to yaw. What it does is sense yaw and correct it for you before you ever get a chance to even see it.

This works beautifully in a helicopter because you can apply varying thrust from the tail rotor blades to add or subtract thrust. Works for fixed wing models too - but only if - there's sufficient airspeed over the control surface to provide a countervailing force. This means it works better at high speed than at low speed, or you need a good prop blast to create the force.

Note; if we're ever together at a fun fly, feed me a beer or two and I'll tell you an interesting story of the time I was goaded into flying my helicopter with the gyro turned off. Let's say i didn't crash and leave it at that. Anyway, I grabbed a gyro out of inventory (without telling Lynn, which ticked her off) and installed it (basically expecting magic things to happen). They didn't. Or at least not as much as I'd hoped.

What I learned is this was due to my initially using a somewhat slow servo for the rudder. Our sport-class servo DS360DLHV, which is about twice as powerful as the model needs turned out to be too slow for the gyro to reliably 'catch' the swing about the CG (which you'll recall is slightly aft of the main wheels), e.g. rotation, at which point it was katy-bar-the-door we're coming around into a fully developed ground loop (or foolishly taking off 90° to the original heading). This last, especially, either scares your fellow pilots, or leads to snide comments from the peanut gallery, which I could live without.

So as it turns out, a servo that's perfectly adequate torque-wise for the size of the control surface and anticipated loads for this splendid sport model 'also' needs to be up to snuff speed-wise when it's gyro-coupled and thus, responding faster than human fingers. Anyway, on the first few attempts before I switched to a faster servo (and note, the original was rated at 0.17sec/60° which is just fine for mere mortals), I was having to bail about one out of three take off attempts. This, despite my otherwise decent piloting technique. So what I ended up using was a servo rated at 0.10sec/60°, instead. Add in 50 bcks for a gro and cost delta for the servo of another 50 bucks and I added a bill to the cost of the build. I can live with that. maybe you will decide to also. It's why I'm sharing!

Speaking of technique (and now I'm addressing anybody of unknown skill level who may be reading this), I learned I can't just shove the throttle forward all the way and count 100% on the gyro to make up for my poor rudder skills. So instead, and despite now using a high performance servo along with the gyro, I roll throttle in a little slowly at first (just to get the model and whilst exercising my left thumb with the rudder joy stick). Then, and modestly quickly, I go to about 3/4-throttle to perform the maneuver.

This, for me constitutes decent technique and along with good components (gyro and a sufficiently fast servo) mean about a 90% shot at a good take off with this model. Adding the gyro offers me the best opportunity to control the model in yaw at take off. And I'm not shamed of using this 'crutch'. Note, I theorize this works because shoving the throttle forward abruptly from the get-go results in propeller torque swinging the nose of the model before there's enough airspeed for the rudder to generate enough force to compensate - true whether there's a gyro to help, or not. The gyro is simply much faster at beginning to compensate for the yaw than a mere human. At least this human!

This brings up the natural question; could good technique totally compensate for not using a gyro? Yes, definitely - but - in my case, I found I liked having a gyro to back up my 'usually' good technique. Especially on days when I wasn't fully on top of my game. In other words, I wouldn't remove the gyro and servo and switch back to an ordinary servo to save the 100 bucks.

Anyway, these two things helped me tame the beast (and if you've flown a Pitts Special you know how miserable the ground handling can be due to its being very short coupled. Especially on unforgiving asphalt or concrete. Grass, a little less so but you'd still better be on your game. Anyway, after installing a gyro and switching to a servo that was quick enough, added to a little bit of a refined rudder technique, instead of heading toward the weeds 1-in-3 I'm maybe 90% certain of accomplishing a takeoff without scaring anybody when operating off a hard surface. And to reiterate, flying off grass is considerably easier, even without a gyro. Finally, once in the air, it's nice having a gyro because, as I'm sure I don't have to tell you, the gyro compensates for a lot of gusts and such.

Finally, as we all know, the Pitts is a very short coupled model in comparison to the Bf-109, where the tail wheel is considerably further aft (even more so than a Mustang which due to tail wheel placement is also rather short coupled). And while I'm not sure how sensitive every taildragger rudder/tailwheel is in comparison to the Mustang, I bet every warbird benefits from using a gyro on the rudder. Especially since (and I mean no disparagement of anybody) but it's also my experience scale pilots are generally less capable perhaps due to spending an inordinate amount of time in the workshop adding details (instead of out polishing their skills by flying a lot). So if the little bit of added technology (a gyro on rudder) helps to make for a major upgrade in the user experience for the pilot (me included) then I'm all for it.

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