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Author Topic: WS plane circles nicely as it climbs and cruises, but then flies straight  (Read 666 times)
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Little-Acorn
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« on: January 27, 2018, 07:13:59 PM »

At my son's middle school we have six Wright Stuff teams, two students each. All six teams have built a copy of a plane that one of the coaches designed, from plans that I drew up from the prototype. Pretty standard layout, wing up on pylons 2-1/2 in. long, tail on skinny CF booms some 6" behind the motor stick and level with it, fin mounted centrally on top of a flat stabilizer.

After a lot of adjusting, the planes are starting to fly very well. One student has turned in a 2:04 flight, the rest have all broken the 1-minute mark. They're all trimmed to climb while making a 30-foot circle to the left, and they do.

But while they climb and cruise for the first half of the flight making a nice left circle, for the second half they mysteriously straighten out and start flying straight ahead, not circling any more. They usually proceed into a wall or something.

It's not just one oddly trimmed plane, all six planes do this including the prototype. We've gone thru several theories of why it might be:
1.) Motor torque twists the fuselage, leaning the rear wing pylon to the left more than the front pylon and inducing a slight warp in the wing, which goes away as the motor runs down. But since the prop rotates clockwise as seen from behind, I think that would actually cause the plane to circle RIGHT under high power, while going straight or circling more left as power ran down - the opposite of what we're seeing. So much for that theory.
2.) The tail is glued to two round 1mm carbon fiber spars side by side, that go forward on each side of the motor stick and are held to it by orthodontic rubber bands. The booms are around 7" long. Held this way, it is very easy for the booms and the entire tail to roll from side to side, and it's not uncommon to see a plane in someone's hand with the wing and fuselage level for the tail canted over to a 45-degree angle, which the student straightens out before launch. But a strong slipstream from a fully-wound motor might be rolling the tail to the right, and then letting come back closer to level as power runs down. Depending on whether the tail is lifting upward or downward, this can cause the plane to turn one way during full-power climb, and less as power runs down. Haven't checked this theory much yet, except that the student with the 2:04 flight has his booms glued to the fuselage and the tail unable to roll at all, yet that one still loses its circling ability in the later parts of the flight. It bounced off a wall twice during the record flight, and managed to keep going anyway.

Those are the theories so far, all on shaky ground.

Can anyone suggest any other possible causes, for a plane that circles properly left for the first part of a flight, but flies straight for the later part?

P.S. We first started out with glide tests, and adjusted each plane to glide in the gentle 30-foot circle with no power. But when we wound it up and launched, it started circling very tightly (15 foot or less diameter) and not climbing at all, flying fast. We took out a lot of the left-fin setting to get it to circle properly at higher power, but now found it won't curve at all in a glide, hence the present situation. All planed went thru pretty much the same process.
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ceandra
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« Reply #1 on: January 27, 2018, 09:24:13 PM »

There are a number of factors that affect the left turn. The big ones are motor torque, fin offset (angle), and tail tilt. All three affect the plane during different parts of the flight. There are other factors as well, such as warps in the wing, which can be speed-dependent.

I believe that generally the fin offset has more impact under higher power, and the tail tilt at lower power. So if the teams are straightening the tail before a flight, you may be removing the low-speed turn, but you are certainly changing the trim of the plane each and every flight. The required decalage for best flight (time) changes with circle size, so changes in the tail position will change the trim point. If you read some of Bill's posts on trimming, you will find he almost ALWAYS indicates that after making any change to the plane, but especially to tail setup and circle size, you need to re-test and set the decalage.

Chuck
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OZPAF
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« Reply #2 on: January 28, 2018, 01:36:06 AM »

A little more info and questions following Chuck's advice.

What adjustments did you make to achieve the turn on your glide tests? Rudder or tail tilt or wing tip weight? Have all the model been trimmed the same way?

After you achieve your glide trim I would suggest you attach the tail boom permanently to the motor stick.

Have you wound the motor to launch torque, held the prop and visually checked for motor stick twist? The motor stick if it is flexible enough would twist the rear wing mtg post further to the left(when looking from behind) then the front post - that is the opposite direction to that of the prop. This will induce a small amount of wash in on the inside wing which would tend to open the turn if it twists enough. The twist would also tilt the tail right side high(also looking from the rear) but under power unless trimmed close to the stall, this would have little effect on the turn compared to rudder or wing twist.

As the power side of your flight seems to be ok - then it is hard to say what the problem is unless the glide trim adjustments are known.



Good luck with it.

John
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mkirda
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« Reply #3 on: January 30, 2018, 03:48:22 PM »

At my son's middle school we have six Wright Stuff teams, two students each. All six teams have built a copy of a plane that one of the coaches designed, from plans that I drew up from the prototype. Pretty standard layout, wing up on pylons 2-1/2 in. long, tail on skinny CF booms some 6" behind the motor stick and level with it, fin mounted centrally on top of a flat stabilizer.

After a lot of adjusting, the planes are starting to fly very well. One student has turned in a 2:04 flight, the rest have all broken the 1-minute mark. They're all trimmed to climb while making a 30-foot circle to the left, and they do.

But while they climb and cruise for the first half of the flight making a nice left circle, for the second half they mysteriously straighten out and start flying straight ahead, not circling any more. They usually proceed into a wall or something.

It's not just one oddly trimmed plane, all six planes do this including the prototype. We've gone thru several theories of why it might be:
1.) Motor torque twists the fuselage, leaning the rear wing pylon to the left more than the front pylon and inducing a slight warp in the wing, which goes away as the motor runs down. But since the prop rotates clockwise as seen from behind, I think that would actually cause the plane to circle RIGHT under high power, while going straight or circling more left as power ran down - the opposite of what we're seeing. So much for that theory.
2.) The tail is glued to two round 1mm carbon fiber spars side by side, that go forward on each side of the motor stick and are held to it by orthodontic rubber bands. The booms are around 7" long. Held this way, it is very easy for the booms and the entire tail to roll from side to side, and it's not uncommon to see a plane in someone's hand with the wing and fuselage level for the tail canted over to a 45-degree angle, which the student straightens out before launch. But a strong slipstream from a fully-wound motor might be rolling the tail to the right, and then letting come back closer to level as power runs down. Depending on whether the tail is lifting upward or downward, this can cause the plane to turn one way during full-power climb, and less as power runs down. Haven't checked this theory much yet, except that the student with the 2:04 flight has his booms glued to the fuselage and the tail unable to roll at all, yet that one still loses its circling ability in the later parts of the flight. It bounced off a wall twice during the record flight, and managed to keep going anyway.

Those are the theories so far, all on shaky ground.

Can anyone suggest any other possible causes, for a plane that circles properly left for the first part of a flight, but flies straight for the later part?

P.S. We first started out with glide tests, and adjusted each plane to glide in the gentle 30-foot circle with no power. But when we wound it up and launched, it started circling very tightly (15 foot or less diameter) and not climbing at all, flying fast. We took out a lot of the left-fin setting to get it to circle properly at higher power, but now found it won't curve at all in a glide, hence the present situation. All planed went thru pretty much the same process.

Any video of the flight(s)?

How stiff is the tailboom?

Regards.
Mike Kirda
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Hepcat
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« Reply #4 on: January 30, 2018, 11:31:37 PM »

Little Acorn
This won’t be the answer you are looking for but I think it needs saying.
Since the earliest days of flying, aeroplanes have used propellers to drive them along, model and full sized and quite often, particularly in models, the propeller moves clockwise when seen from the pilot’s seat. By the basic laws of physics, if the propeller is rotating clockwise then what is rotating the propeller, petrol engine or rubber motor, will apply a torque force trying to rotate the rest of the aeroplane in the opposite direction, i.e. to cause it to bank to the left. If an aeroplane banks it sideslips which initiates a turn in the direction it is banked.  Now if an aeroplane is turning the outside wing of the turn is moving faster than the inside wing so it generates more lift.  So if an aeroplane is left to its own devices the torque force and the increased lift of the outside wing will quickly spiral dive the aeroplane into the ground.  Very soon pilots and out door model flyers realized this and from choice would turn to the right thus balancing the asymmetrical lift of the wings against the torque.   
Indoor flying apparently became popular in the 1920’s and I know little about the origins. If someone has old magazines I hope they can correct my guesses.  I guess that with the opportunity to use these large buildings with no wind to worry about that big light aeroplanes were the way to go. So that is what they built and took them along to test and trim and found that they turned quite well to the left (because of torque) and they were happy to leave them that way.  After a while naturally flyers started using more rubber to get longer flight and the left bank became too steep and one of the first ‘fixes’ was to make the left wing longer which did give more lift but was a little heavier and had a little more drag. Since those early days improvements have been never ending and sometimes breath-taking.  The most intricate bracing, no bracing, new materials, and what might be call a flexible airframe to use the torque of the motor to good purpose. I acknowledge that if there ever was a problem with top class indoor models flying in left hand circles it has been solved.
However I do think that SO models should not ape international class indoor models.  Fly your model to the right.  Use some right side thrust.  When the torque is high trying to turn the model left then the right thrust will also be high, counteracting the torque.  Then a little right rudder offset will probably give just the circle you require.
John
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John Barker UK - Will be missed by all that knew him.
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