End loopy takeoffs with a gyro

Aug 25, 2014 9 Comments by

Like many of you, I have a few different planes, and enjoy flying most of them. However, I do have one or two that are just plain nasty in their ground handling and take offs. You probably have seen these – some Piper Cubs, Beavers, and especially some of the narrow under carriage warbirds. Strangely, not all tail draggers do this. But with one of these “loopy” planes, they are lined up on the runway, power is slowly advanced, and the plane skitters to the left, it skitters to the right, and may loop in a complete circle. Even if it is kept straight for a while, as power is advanced and the tail lifts, it then skewers over to the left from torque. And if insufficient speed has been built up, for example it was heading for the fence and you horsed it up too soon, you are rewarded with a spectacular left wing cartwheel.

I know, when you are relating this story to your mates, you either get a “yup, that’s why I would never get a Beaver, Piper, warbird, whatever …”, or you get a “pilot skill problem, with more flying experience you will be able to handle it.” I understand that more skill may help, but honestly I go flying to have fun, not to work on creating an ulcer worrying whether the plane will get off the ground today. I sure wouldn’t mind some sort of electronic help. And after all, I don’t see people complaining that using exponential on our radios means we are cheating or aren’t skillful flyers.

Now our helicopter brethren long ago found out that even with super human reflexes, it was really difficult to keep the tail straight on their choppers – the least wind, change of engine speed, etc. would make it swing. They discovered gyros. Suddenly, their craft would stay straight, and hovering and flying became a whole lot easier.

The object of a rudder gyro then, is to provide some computer fast electronic help on keeping it straight down the runway on roll out, and fast correction of that torque roll to the left just as it just gets airborne. Once fast enough, the plane generally flies straight without any extra help for sure.

Originally, gyros consisted of a small electric motor spinning two heavy brass disks. These spinning disks resisted movement, just like bicycle wheels or a spinning top. Today, the gyros are solid state and utilize a piezo crystal to detect movement. And specifically, they detect movement only in one rotational direction. Thus a gyro used for rudder control will only detect rotational movement about the yaw axis. Electronics “read” this rotation, and output a correctional signal to the servo, in this case the rudder. Any other movement, i.e. acceleration forward, braking, or pitch and roll are ignored by the gyro.

 There are two kinds of gyros available for model use: rate control (also known as normal mode) and heading hold.

 In Rate Control mode, if anything other than you moving the rudder stick changes the plane’s horizontal direction (yaw), the gyro will give a brief but not sustained correction to help keep it straight.

 In contrast, the heading hold mode is best used just for the take off run. In this mode, when you first line up the plane, you ‘instruct’ the gyro that it is to stay on this exact heading. As you taxi, it will automatically, and lightning fast, make corrections to keep the plane on this heading. After, takeoff, however, unless you want the plane to stay on this course forever, best to switch to normal mode, so that you can turn and do a circuit, etc. Generally on these gyros, you utilize an additional switch on your transmitter to disable the heading hold mode right after takeoff, then defaulting to the rate control mode (normal mode) for the rest of the flight.

 What if we put one of their gyros in our ‘problem’ planes to help with takeoff rudder control? Clearly not all, or even many, of our planes need this, but when you have one of these “ground loopers,” maybe a rudder gyro could help. So, I chose my worst problem planes to try this. The first one is a kit-built electric conversion Mustang, 56” wingspan, about 8 lbs all up battery in. I know, kind of heavy. Never the less, this plane is a lot of fun to fly in the air, and with gear and flaps down, is not so bad to land either. But the take off always makes we wonder if this day is going to end with another trip to the repair bench with it.

 For my first test, I tried a Futaba GY401 heading hold gyro(~$135.00). It mounts next to the receiver with a supplied special double sided foam mounting tape. You plug the one gyro lead into the rudder channel of your receiver, and plug the rudder gyro lead into another lead on the gyro. The third gyro lead plugs into an unused receiver channel to select the gyro mode. I used the Aux 2 switch on my Spektrum DX7 transmitter to switch from ‘heading hold mode’ to ‘normal mode’. The manual describes a few adjustments and selection switch for Futaba digital servo or other analog servo.

After installation and testing, I was off to the flying field. Once I had the plane lined up straight on the runway, I rapidly flipped the Aux switch on and off 3 times, leaving it in the Heading Hold position. This instructs the gyro that this is now the desired course of the plane, and the rudder is in a neutral position. I then accelerated down the runway, and if the course was a bit off, I could still manually move the rudder to keep the plane headed down the middle of the runway. But, virtually all that nasty swing one side to the other was gone, and once some speed is built up, no more torque role to the left! As the plane lifted off, I switched off the heading hold mode, and continued with whatever my usual flight routine would be. Interestingly, in flight, and with the gyro in the Rate Control mode (Normal Mode), I could bank, climb, dive, etc. and the plane really didn’t feel much different to me. Landing was straight forward, perhaps even straighter than usual for me!

 For my next test, I tried the GWS PG-03 gyro ($38) in my electric Spitfire. This plane is 56” wing span, 6 lbs all up. Initially I flew with a 4S-5000 battery, and the plane was reasonable on take off, but still requiring a fair amount of finesse to keep straight and not torque over to the left. However, in the air the plane was a bit underpowered and somewhat anemic in normal flight patterns. Since the motor could h
andle it, I then switched to a 5S-5000 pack, and rebalanced the plane. Now takeoff was not so mild mannered at all, but once in the air a whole lot more power and fun. I installed the GWS gyro by merely taping it to the inside of the fuselage wall close to the receiver, using the supplied special foam mounting tape. The gyro plugs into the rudder channel of the receiver, and the rudder servo lead plugs into the gyro. This gyro is always in Rate Control (normal mode), so no other connections or receiver channels are needed. One simple adjustment, described in the gyro manual, is required to ‘center’ the servo output, and a second adjustment to set maximum sensitivity.

 

Again at the flying field, I now lined up on the runway and slowly accelerated. But this time I still needed to input some torque correcting right rudder transmitter stick movement, but it was much easier. There was no nasty or sudden swinging left to right, and a very smooth takeoff. Once in the air, again, I was not really aware of any impact on my usual flying routine form the gyro. Landing was uneventful, and even easier to keep straight.

 Overall, the heading hold gyro is superior, but more expensive and requires an unused function on your transmitter to turn the modes on and off. The rate control gyro is considerably cheaper, easier to install, and does not require any other transmitter channels. Thus, if I have any more “ground looper” planes, I probably would try the simpler and cheaper rate control (normal mode) gyro first.

 If you try this, and it works nicely as it has for me, then it will be up to you whether you tell any of your fellow pilots about the gyro, or whether you just bask in their complements about how skillful your takeoffs have suddenly become.

By John Falconer

  

Fixed-Wing Flight School, How-tos

About the author

Come fly with us! The original since 1929, we ignite our readers' passion for radio-control flight.

9 Responses to “End loopy takeoffs with a gyro”

  1. Bob Moore says:

    Thanks for a great article with the full explanations..

    This is what I needed in two of my models.. I am going to try an E Flite.. about $65.00 ea.

  2. Nelson Johnson says:

    Interesting read but just to be correct the tendency to yaw on take-off and climb-out is caused mostly by the spiraling slipstream of the airflow produced by the propeller. The propeller blows air backwards but also causes this column or fast moving air to rotate in the same direction the propeller is turning. This rotating column of air then strikes the vertical stabilizer with more force on one side than the other, causing the craft to want to yaw. It is not torque doing this. Also, as the tail then rises gyroscope procession kicks it to cause a yaw force as well. But, procession only happens as the tail is in the actual act of lifting up, as the fuselage is pitching forward when the tail is rising. Again, it is not torque that is causing all of this left turning tendency in single engine aircraft with clockwise rotating propellers as seen from the cockpit. In fact actual torque and P-factor are the least of the 4 forces acting on an airplane during the takeoff roll and subsequent liftoff and climb-out. Spiraling slipstream of the propeller “prop-wash” is by far the strongest off all of them. If the vertical stabilizer was pointed downward and not upward the airplane would yaw in the opposite direction if you did not change any other factors.
    Nelson

  3. Pat says:

    Excellent article. Yes, P-factor is the cause, but the results are what counts. I have used a 3-axis gyro from HK and it is a miracle worker. I first tried it in Tucson on cross-windy days with an Apprentice. it was though it was on rails.

    As an Aircraft Instrument Mechanic in the mid-sixties, I built, repaired, calibrated and flew with autopilots which had spinning wheel gyros…rate gyros as well as three axis gyros. The solid state technology that allows all of the same functions that were in the 80# packages back then, in a tablespoon package today, is just amazing. I love it.

  4. Steve Stresman says:

    I have a Cub that is challenging on take-offs and since I have a spare gyro right now, I know where I am going to be installing it. Thanks for the informative article.

  5. clive durham says:

    Toe out fixes the problem as well

  6. Nelson Johnson says:

    Pat, you did not understand. I said P-Factor is NOT the cause. There are 4 forces or “things” that contribute to causing a clockwise rotating (as seen from cockpit) single engine, propeller driven aircraft to tend to yaw to the left during the takeoff roll and or high power settings and low airspeed. 1) Spiraling slipstream from the propeller 2) Torque reaction 3) Gyroscopic precession 4) P-Factor. With P-Factor being the least! Not the most, the least!! Spiraling slipstream or propwash is NOT P-factor. The propeller does not just blow a column or tube of air straight back it gives it a small twist while blowing it backwards causing it to slightly curl around the fuselage and hit on the left side of the vertical fin more than on the right side pushing the tail right causing the nose to yaw left. Gyroscopic precession is the next strongest but it only kicks in when the tail is in the act of raising up off the runway. Once the tail is straight out precession is no longer a factor, it stops instantly. P-Factor is often mentioned but in reality is almost nonexistent. It just has a cool sounding name so everyone mentions it. The long and short of it is to learn to fly your aircraft and stop coming up with all these “fixes”. You will be a much, MUCH better pilot in the end if you just practice more. Practice taxing your plane all day long if you have to. Taxi a few gallons through it, high speed taxi, taxi, taxi, taxi! That’s what makes a good pilot. And while you are at it, fly a slop glider for a while. Flying gliders will make a mediocre pilot GREAT!!!!! Trust me on this one. Flying slope will make you an amazing pilot.
    Nelson

  7. Thomas Swift says:

    Unless I’m misreading the picture, it looks like you mounted your second gyro on the wrong plane to control yaw axis movement. Shouldn’t it be mounted on a horizontal plane (like your first gyro) in the airplane?

  8. GC says:

    Each to their own. Use it if you want. Doesn’t mean you are any less of a pilot. Its not a competition its a hobby. Its about haaving fun any way you want it

  9. Nelson Johnson says:

    Here is the super short version: Apply an little right rudder BEFORE you add power for your takeoff roll and you will be fine.

Copyright © 2014 Air Age Media. All rights reserved.