Rubber Motor lenght / Motor Stick length

banilmd
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Rubber Motor lenght / Motor Stick length

Post by banilmd » February 11th, 2019, 10:10 am

Hey all,
This is my first time competing in this event so forgive me if some of the questions I ask have obvious answers. Recently I competed at an invitational, and I saw some teams had a very long motor stick and long rubber motor. From my understanding, the reason for the long motor stick is so that you can decrease the angles of both the wind and stab and therefore reduce the overall drag. As for the longer rubber motor, I assume it is to allow for more turns as the length of the motor does not affect the torque. My question is one, am I understanding what is happening properly, and two how would I go about experimenting with longer motor sticks and rubber motors.
Thank you all very much!

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Re: Rubber Motor lenght / Motor Stick length

Post by bjt4888 » February 11th, 2019, 12:09 pm

Basil,

Yes, in general, you have one of the benefits of the long motor stick correct. If a longer stick allows a longer tail moment arm (essentially, the distance from the wing TE to the Stab LE), this moves the center of pressure more towards the stabilizer which allows, generally, the CG to be moved more towards the stabilizer which allows a shallower decalage angle (difference between the wing and stab incidence angles). Lower decalage angle will generally reduce drag. Whether this is optimal or not you would have to build and test.

If you want to test longer motor sticks, I would build and try. A difference of at least one inch would probably be significant.

To test longer motors, I would keep thickness actually density in grams per inch) constant and try longer and shorter in two inch increments (loop length) initially. The, of course, try different densities too. Lots of testing is the route to success.

Good luck,

Brian T

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Re: Rubber Motor lenght / Motor Stick length

Post by banilmd » February 11th, 2019, 2:50 pm

bjt4888 wrote:Basil,

Yes, in general, you have one of the benefits of the long motor stick correct. If a longer stick allows a longer tail moment arm (essentially, the distance from the wing TE to the Stab LE), this moves the center of pressure more towards the stabilizer which allows, generally, the CG to be moved more towards the stabilizer which allows a shallower decalage angle (difference between the wing and stab incidence angles). Lower decalage angle will generally reduce drag. Whether this is optimal or not you would have to build and test.

If you want to test longer motor sticks, I would build and try. A difference of at least one inch would probably be significant.

To test longer motors, I would keep thickness actually density in grams per inch) constant and try longer and shorter in two inch increments (loop length) initially. The, of course, try different densities too. Lots of testing is the route to success.

Good luck,

Brian T
Thank you very much Brian!
So basically i would just go inch by inch testing the longer motor stick. I’m assuming I would have to test with my rubber motor length as well. However by increasing the motor stick length would it throw off any proportion in relation to the tailboom? I am using an unmodified 2019 Freedom Flight kit for reference. Also there must be a point where the CG gets too far back. Would i be able to identify this by stalling, etc. Thank you!

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Re: Rubber Motor lenght / Motor Stick length

Post by bjt4888 » February 11th, 2019, 4:19 pm

Banil,

The stock FF kit design has a high static stability margin and can accommodate a CG that is further back towards the stab. So, extending the motor stick one or two inches is unlikely to cause stalling issues.

A long motor stick is more prone to bending due to motor tension though, and this is not a good thing. So, the stick needs to be light and very strong.

Motor length and stick length are not related. You’ll want to see the other threads that discuss this. Coach Chuck and I both have comments in these other threads that will be helpful for you to read.

Stick length may have only a small effect on duration, if any. Best to focus on propeller design, pitch and rubber density and length and winding procedures.

Brian T

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Re: Rubber Motor lenght / Motor Stick length

Post by banilmd » February 12th, 2019, 9:43 pm

bjt4888 wrote:Banil,

The stock FF kit design has a high static stability margin and can accommodate a CG that is further back towards the stab. So, extending the motor stick one or two inches is unlikely to cause stalling issues.

A long motor stick is more prone to bending due to motor tension though, and this is not a good thing. So, the stick needs to be light and very strong.

Motor length and stick length are not related. You’ll want to see the other threads that discuss this. Coach Chuck and I both have comments in these other threads that will be helpful for you to read.

Stick length may have only a small effect on duration, if any. Best to focus on propeller design, pitch and rubber density and length and winding procedures.

Brian T
Thank you very much for the information. After going through this years thread on ikara props and changing the pitch, i have decided that i want to venture into making my own balsa props seeing how too many pitch changes may break the ikara prop. In addition, balsa props usually weigh less. Is there any place where I can find information on how to make the varying styles of hubs, the design of the blades, how to actually make the different types of blades (flaring vs. non-flaring), and the materials that may be necessary? I know that it will come down to matching the prop design / pitch to the rubber, but I am rather confused as to where to start.

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Re: Rubber Motor lenght / Motor Stick length

Post by jander14indoor » February 13th, 2019, 5:53 am

INAV, Indoor News and Views, the LONG time newsletter of the hobby/sport.
https://indoornewsandviews.com/
Has all the old newsletters and articles. Many on building props, I'd focus on the ones for Penny Plane props as they are closest to weight and size for SO events.

A general search of the web for Penny Plane build info will also be productive.

Jeff Anderson
Livonia, MI

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Re: Rubber Motor lenght / Motor Stick length

Post by calgoddard » February 19th, 2019, 10:23 am

For an indoor duration rubber powered stick model, such as a Wright Stuff model, optimum motor stick length, optimum overall model length, and optimum rubber motor size are separate, but related concepts, in terms of achieving maximum flight times.

Rubber motor length by itself is not a useful parameter. One needs to optimize rubber motor length AND weight for a particular prop and air frame through extensive testing. The optimum size, P/D, and blade configuration for the prop must first be selected before one can begin to optimize the size (both length and weight) of the rubber motor.

The precise length of the rubber motor loop relative to the length of the motor stick is not critical, but the ratio of these dimensions is important. A motor stick that is too short can result in undesirable rubber motor bunching. This can cause the prop to lock, the model to dive, and/or the model to stall. A motor stick that is too long can consume too much of the weight budget and/or result in undesirable motor stick bending. In my experience, the length of the rubber motor loop should not be more than about 1 ½ times the length of the motor stick.

Some indoor AMA classes, like A-6, have an effectively limit on the length of the motor stick. In order to use very long rubber motors on an A-6 model it is necessary to use sleeves that cover the front and rear of the wound rubber motor to prevent adverse rubber motor bunching.

Most indoor duration stick models utilize a very lightweight tapered tail boom made of stiff balsa wood that overlaps the rear of the motor stick and supports the horizontal stabilizer. The overall model length is typically the combined length of the motor stick and tail boom, plus the distance the prop assembly extends forward of the motor stick, and less the distance that the tail boom and motor stick overlap.

Unlike some indoor AMA classes, the 2019 Wright Stuff rules have no limit on the overall model length. To my knowledge, the Wright Stuff rules have never had a limit on the overall model length.

Depending on the existence and nature of ceiling obstructions at the competition venue, it can be advantageous to compete with a relatively long Wright Stuff model. This allows a competitor to maximize the distance between the wing and the horizontal stabilizer. The center of gravity can be moved farther rearward on a longer model, allowing stability in flight to be maintained while at the same time utilizing lower decalage and achieving lower drag. Such a model can sometimes be trimmed to fly in a slightly nose-up attitude, effectively utilizing the horizontal stabilizer as an additional lifting surface.

However, there is a practical limit on overall model length because one needs to build near the minimum allowed weight in order to be highly competitive. Also, a long floppy tail boom can lead to trimming difficulties. In addition, if the center of gravity of the model is too far rearward and/or the decalage is too low, the model will not recover quickly if it collides with a beam or a light fixture, resulting in a significant loss of altitude which is usually not fully regained during the flight.

Rarely the ceiling at the competition venue will be flat, without any beams, light fixtures, skylights, vents or other obstructions. In that case it can be advantageous to fly a shorter model with a CG located more forward so that the model can “skate” across the ceiling.

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Re: Rubber Motor lenght / Motor Stick length

Post by Polar » February 20th, 2019, 7:54 am

Thank you calgoddard for the information. Though a question about calgoddard's post..

Because my state's wright stuff will be held in a massive open area, I'm testing various modifications on the FFkit to best fit the competition ground. With regards to extending the motor stick, how much benefit does it provide? Is it worth extending the motor stick a few inches if it means going over the weight limit by ~0.1/0.2? I figure because it is unlikely for my plane to collide with ceiling obstructions, the disadvantage of the lack of stability upon collision of the longer motor stick can be disregarded.

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Re: Rubber Motor lenght / Motor Stick length

Post by calgoddard » February 20th, 2019, 8:24 am

Never fly an overweight model in competition, unless your No. 1 model has become stuck or got broken on your first official flight and your No. 2 model is overweight and that is all you have to fly for your second official flight. You do not have sufficient time to make and test repairs during your 8-minute Flight Period. Better yet, come to the competition with two models that are not overweight and that both fly well. One of these models will fly consistently better and it should be your No. 1 model flown for both official flights unless you encounter a problem.

As I recall, the late, great indoor flyer, Cezar Banks, once explained the concept to me this way. He said that you wouldn't start running the Boston Marathon with two bricks in your pockets.

Two tenths of a gram of extra weight can make a significant difference in flight times, enough to make you place second instead of first.

What do you mean by "massive open area"?

If your state Wright Stuff competition will be held in a flying site with a very high ceiling, you need to be concentrating on other things besides motor stick length. Start practicing with lower pitch propellers using partial rubber motors in your usual practice site if it is a typical size high school gym.

Also, it would appear that you are thinking of extending the overall length of your model by extending the length of the motor stick. If you want a longer model so you can move the CG further rearward, extend the length of the tail boom, and do not extend the length of the motor stick, unless you are encountering motor bunching issues.

Don't extend the length of your existing model in such a manner that it will then be one or two tenths of a gram overweight. If you want to test a relatively long Wright Stuff model, build one with a weight budget for each component to make sure that it comes in very near the 8 gram minimum weight.

A longer motor stick does not make a model unstable.

In terms of evaluating stability, you need to think of the concept of the static margin. Here is a quote from Wikipedia:

"In aircraft analysis, static margin is defined as the distance between the center of gravity and the neutral point of the aircraft, expressed as a percentage of the mean aerodynamic chord of the wing. The greater this distance and the narrower the wing, the more stable the aircraft."

The location of the neutral point depends on the stab area and the distance between the wing and the stab.

In Wright Stuff competition, you always want to maximize the size of the wing and stab. So this simplifies matters a bit.

For a given Wright Stuff air frame, the longitudinal placement of the wing along the motor stick will determine the location of the center of gravity which, in turn, will determine the static margin and thus the stability of the model in flight.
Last edited by calgoddard on February 20th, 2019, 8:57 am, edited 4 times in total.

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Re: Rubber Motor lenght / Motor Stick length

Post by Polar » February 20th, 2019, 8:50 am

By massive open area, I mean an indoor football field. The roof meets up in a center line like a triangle. The center of the triangle probably has a max height of 90 feet unobstructed, while the edges have a height of maybe 50-60 feet.

I will work on doing partial motor tests, but I am concerned that even with partial motor tests and scaling the max height, it would not accurately represent state's. My high school gym is ~30 feet.

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