Circular

torqueburner
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Re: Circular

Post by torqueburner » January 18th, 2015, 12:53 pm

jander14indoor wrote: . . .Now the rest of the trim is just to keep the turn a constant size over the full speed range these planes fly at.
The prop is actually set with its access down and left to offset the excess lift at high torque (max winds) and off set the excess left wing lift (which increases left turn radius, sometimes to point of right turn) at higher speeds with left thrust.
The tilt of the stab to the right causes a left turn because of force vectors. To all intents and purposes the lift from a flat wing or stab is perpendicular to the surface left to right. If the surface is tilted right that force vector points up AND right. That right component pushes the rear of the plane right, pivoting the nose to the left. Another left turn. This force becomes dominant at slower speeds.
The vertical stab is typically set for a left turn too. But set properly it doesn't so much control turn, but just minimize drag in a plane that's turning anyway.

. . .Hope that makes sense, if not, feel free to keep asking questions.
When we originally read this, it matched up perfectly with our experience, and we had no trouble with our planes flying well, in a circle of approximately constant radius. But during this morning's flying session we had an experience that has us scratching our heads.

The plane in question has twin stabilizer end plates at an angle of about 15 degrees from vertical. The stab is angled and the tailboom is set at an angle consistent with a left turn. The thrust line is angled to the left. Originally, we had the tailboom angled too much, and the plane flew in a circle with a diameter of only about 15 feet. Too tight, but it flew well enough to do 2:45 in a venue with about 25 feet of flyable height. This plane was also flying the bonus wing with a 7 cm chord. Not too bad, but we wanted to enlarge the circle, so we decreased the tailboom angle.

The first few flights were made with a prop pitched to P/D = 1.5. This went pretty well, with a nice circle of about 30-35 feet, though we did notice that it took a little while for the plane to start circling properly. This was easy to compensate for by shifting the launch position to a different part of the gym.

Next, we wanted to try a prop with a higher pitch - P/D = 1.7. Now the plane really didn't want to turn; it headed for the side wall, not quite a straight line, but pretty close. We retrieved it from its landing spot and relaunched, and then it circled to the left, , about diameter about 30-35 feet.

So it appears that the plane doesn't want to turn when the torque is at its greatest, and the plane is flying fastest. Do I remember hearing that the rudder is more effective at higher speeds? If so, do the above observations suggest that we increase the tailboom angle a bit? We are planning to try this, of course, but we have the usual problem that gym access is severely limited, as we are not basketball players, or the like:( So if we can get an idea of what else to try, it might help us to save some time.

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Re: Circular

Post by bjt4888 » January 18th, 2015, 5:14 pm

torqueburner,
One possibility would be that the motor you are using cannot operate the higher pitch propeller well enough to create the airspeed necessary for the tailboom offset or the stabilizer tilt to create adequate turn. We had this exact result as propeller pitch was increased or when substituting a "softer" flaring propeller or when substituting a broader bladed propeller. One possible solution would be to try a slightly thicker rubber motor. Our experience doing exactly this was the thicker motor (we substituted an 0.94" motor for a 0.90" motor) tightened the turn up nicely and flew the airplane through an excellent climb/cruise/letdown pattern. That being said, my students are getting their best duration with a slightly narrower bladed propeller with slightly less pitch (about 13.5" pitch) that has good flaring capability and with 0.087" motors. Best duration using this combination has been 3:16 in a gym with 27 ft. flyable height. Best duration with the broader bladed balsa "Bucket" propeller (also about 13.5" pitch and weighing 1.2 grams) is 3:10 at a flying height of 21 ft (different site). This propeller shows a lot of promise, but so far the students have not had the time to continue testing it. Unfortunately, the students need to set their airplanes to fly in about a 20 - 25 ft circle as the regional site they are preparing to fly in is the size of a single basketball court with goals projecting into the court on three sides. This smallish flying circle probably affects duration a bit.

Good luck and good flying,

Brian T.

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Re: Circular

Post by jander14indoor » January 19th, 2015, 10:25 am

First, whenever you notices sudden changes in a plane that has otherwise been flying OK, check for something broken. Can be very subtle sometimes and not always the cause, but check.

Turn radius with a higher pitch, surprises me that your turn opens up. Normally more torque rolls the left wing down decreasing turn radius. Perhaps the pitch was so high the prop stalled and you were not rolling as much as before? Are you sure the prop axis is angled left? Again, higher pitch prop, more thrust, generally tightens turn.

Reading carefully I'd really check the prop axis. Consider more tail boom angle and stab tilt, one at a time.

Jeff Anderson
Livonia, MI

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Re: Circular

Post by torqueburner » January 19th, 2015, 12:30 pm

bjt4888 wrote: One possibility would be that the motor you are using cannot operate the higher pitch propeller well enough to create the airspeed necessary for the tailboom offset or the stabilizer tilt to create adequate turn.

jander14indoor wrote:First, whenever you notices sudden changes in a plane that has otherwise been flying OK, check for something broken. Can be very subtle sometimes and not always the cause, but check.

Turn radius with a higher pitch, surprises me that your turn opens up. Normally more torque rolls the left wing down decreasing turn radius. Perhaps the pitch was so high the prop stalled and you were not rolling as much as before? Are you sure the prop axis is angled left? Again, higher pitch prop, more thrust, generally tightens turn.

Reading carefully I'd really check the prop axis. Consider more tail boom angle and stab tilt, one at a time.

Jeff Anderson
Livonia, MI
Thanks, Brian and Jeff. Since this happened right after decreasing the tail boom angle, that is probably the first thing we'll try, after double checking the thrust axis. It was angled to the left, but perhaps it has loosened up and shifted.

Dave D.

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Re: Circular

Post by torqueburner » January 21st, 2015, 3:18 pm

jander14indoor wrote:First, whenever you notice sudden changes in a plane that has otherwise been flying OK, check for something broken. . . . Reading carefully I'd really check the prop axis. Consider more tail boom angle and stab tilt, one at a time.

Jeff Anderson
Livonia, MI
Well, some bad weather forced an early dismissal, but we were able to sneak down to the gym for a few flights. We discovered that the thrust bearing had indeed loosened. We fixed that. Back in business! So as Jeff said, the lesson here is to check the plane for something broken if anything changes suddenly.

Dave D.

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Re: Circular

Post by Toms_42 » January 22nd, 2015, 8:30 pm

torqueburner wrote:
jander14indoor wrote:First, whenever you notice sudden changes in a plane that has otherwise been flying OK, check for something broken. . . . Reading carefully I'd really check the prop axis. Consider more tail boom angle and stab tilt, one at a time.

Jeff Anderson
Livonia, MI
Well, some bad weather forced an early dismissal, but we were able to sneak down to the gym for a few flights. We discovered that the thrust bearing had indeed loosened. We fixed that. Back in business! So as Jeff said, the lesson here is to check the plane for something broken if anything changes suddenly.

Dave D.
Yeah that happened to us at competition. We neglected to do a test flight for fears of getting the plane stuck in the rafters, and the first flight never climbed at all and was under a minute. We thought we miscounted winds or something, so we flew the next flight with more winds hoping it would climb. Nope. It immediately fell to the ground after shaking violently on release. Turns out the prop shaft had bent, and the whole prop was bouncing around wildly. Somehow still got 9th though...
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