Rubber Motor lenght / Motor Stick length

calgoddard
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Re: Rubber Motor lenght / Motor Stick length

Post by calgoddard » February 20th, 2019, 9:46 am

Practice in your gym with one-third partial motors. By way of example, if your best results have been achieved flying with a 2.4 gram rubber motor that is X cm long, then make up rubber motors that are 0.8 grams and 1/3 X cm in length. Use a partial motor stick has a length that is 2/3 of the length between the prop hook and the rear hook and that weighs 1.6 grams. If you can get your model to climb to 25 feet in your high gym, it will climb to 75 feet with a full size-rubber motor at the State flying site when wound to the same torque as the partial motor.

Most flyers at your state competition will not optimize their models and rubber motors for flying in a very high ceiling venue. Their models will run out of turns while they are still quite high, at which time they will descend rather rapidly to the floor or fly straight into the wall and crab down the side of the wall.

Since you want to get your model to fly 75+ feet high, you will not be backing off turns when you wind, unless you need to do so to prevent the model from crashing at launch due to the effects of very high torque. To be on the safe side, launch your model while standing, and launch it high over your head.

Here is a link to a video of a flight by a Wright Stuff team that did an excellent job with their model, winding and launching, flying in a venue with a very high ceiling, probably over 90 feet:

https://www.youtube.com/watch?v=Wqt-FsekeqE

I am almost certain that this team lowered the pitch of the prop it used in a prior competition if the venue for that competition had a much lower ceiling height.

The model still lands "dead stick" in the video. If the rubber motor had been slightly longer so that it landed with unused turns remaining perhaps this team would have won the gold medal.
Last edited by calgoddard on February 20th, 2019, 2:20 pm, edited 3 times in total.

Polar
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Re: Rubber Motor lenght / Motor Stick length

Post by Polar » February 20th, 2019, 11:12 am

That is an amazing flight! Because their plane deadsticked at the end, could they have increased their pitch slightly to decrease the rate of usage of the motor? How do you know whether to increase the pitch or increase the rubber length in that case?

I will try to do the 1/3 motor testing in my gym.

calgoddard
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Re: Rubber Motor lenght / Motor Stick length

Post by calgoddard » February 20th, 2019, 12:03 pm

A slightly higher blade pitch on the prop of the model in the video would have used up the turns at a slightly lower rate with the same rubber motor.

However, the usual practice by experts is to adjust the length of the rubber motor, while maintaining the same rubber motor weight, in response to a model landing with too many unused turns on the rubber motor, or, in the other extreme, the model landing dead stick.

This is because it is much easier to change the length of the rubber motor than to accurately re-pitch both of the blades on the prop.

Moreover, an experienced flier wants to maintain a desired optimum P/D of the prop in order to achieve maximum flight times for a given air frame. Changing the pitch of the blades changes the P/D of the prop.

Experts measure the size of a rubber motor by its length and weight. This is because it is not possible to accurately and uniformly strip a rubber motor to a particular width over its entire length. Nor is it possible to accurately measure the width of a rubber motor over its entire length. In addition, both the thickness and density of the rubber can vary over the length of the rubber within the same batch (box) of rubber.

Changing the length of the rubber motor while maintaining the same weight is achieved by adjusting the width on the rubber stripper. If one does not have a rubber stripper, then one can change the pitch of the prop in order to fix the problem of too many unused turns or a dead stick landing.

I hope you find my comments to be helpful.

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Re: Rubber Motor lenght / Motor Stick length

Post by Polar » February 20th, 2019, 12:46 pm

Calgoddard,

You've made it very clear. Thank you for your explanations.

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