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Author Topic: Should the distance from the prop hook to the back hook, equal the relaxed lengt  (Read 345 times)
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Little-Acorn
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« on: December 03, 2017, 11:53:56 PM »

Hi, all.

I noticed a Wright Stuff plane a few weeks back which, when the motor was totally unwound, the motor sagged quite a bit, with the center like 4"-6" lower than the propeller and back hook. The student was using 1/16" rubber, which in my experience has a relaxed length of about 19". But the distance from the prop hook to the back hook on his plane, was maybe 15" to 16", hence the sagging rubber.

In all his flights (approx. 1 min 30 sec), the motor never fully unwound. I don't recall seeing it sag during any flights, even at the end. Doubtless because there were still a fair number of winds left.

I have always built planes with the distance from prop hook to hack hook, equal to the motor's relaxed length. So the motor never sagged (well, not really) even when fully unwound. But now that I think about it, I can't really give a reason why I should build them that way.

Is there some cosmic reason saying the motor should never sag this way? I could save some fuselage length (and weight) if I made it 15 or 16 inches long (ahead of the tail boom), while the motor's relaxed length was 19". Would this do any harm, either to motor life or torque or etc.?

Is there any reason not to make the motor stick 10" long? (Going to an extreme here)

Thanx all!
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lincoln
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« Reply #1 on: December 04, 2017, 02:12:46 AM »

I don't know the rules for the event, so I can't advise you about motor stick length. I'm assuming they have a stick for the fuselage and that the rubber is just below the stick,.

It's fine to use rubber that's longer than the motor stick. However, the longer you make it, the bigger the knots may be. So you might have to increase the distance between the prop shaft and the stick. Ditto for the rear hook. If you're flying indoors, it's best if you have a few turns left when the model lands. If a model runs out of turns in the air, it descends quickly.  In general, for endurance models,  the motor should weigh a large fraction of what the rest of the model does. For instance, Chuck Slusarczyk, who is famous among people who fly indoors, won open pennyplane at the 1989 USIC championships. According to the drawing in Winning Indoor Designs, the model weighed 3.3 grams without the rubber. Calculating by a specific gravity of 1.5 and the given dimensions, the rubber weighed about 3 grams! BTW, that model weight is for a biplane with 18 inch wings and about about 153 square inches for the pair of them!

I've noticed a trend in EZB, which has no minimum weight, for people to use shorter fuselages when building for low ceilings. (Low compared to a blimp hangar, that is.)

If I want the most time I can get, I'll generally wind as tight as I can, then back off enough turns so that it won't quite hit the ceiling. If the motor is right for the model, the turns left at the end may be about as many as the turns I backed off after winding. I've found the motor is close to the right size when I wind it all the way up, then back off half the turns, and the model flies level, without climbing or descending, at least at first. I suspect you don't have a rubber stripper, so you may have to use lower pitch for the propeller if there isn't enough climb, or else a prop with a SLIGHTLY smaller diameter.  Or you can mess around with the motor length. A longer motor will keep the model from climbing quite as much. 

BTW, I seem to recall that, in a blimp hangar, you may be able to use a slightly thinner motor, since you don't have to back off nearly as much,  but I've never flown under any ceiling that high.

That's probably more than you want to know. Can you tell by the length of this note that I'm supposed to be doing something else?


Here's a link for a more recent edition of Winning Indoor Designs:
https://indoornewsandviews.files.wordpress.com/2012/10/winning-indoor-designs.pdf
You can probably learn a lot from it, and from the INAV site it's found on.
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Tapio Linkosalo
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« Reply #2 on: December 04, 2017, 03:59:06 AM »

There are two types of duration models. One, where the motor weight is limited (F1D, F1M). For these it is common that the hook distance is about the same as the motor length, as it is no good to deviate in either direction. If the hook distance is shorter, then the motor tends to bunch more and unwind more unevenly. If, on the other hand, the hook distance is longer than the motor, you get less turns in it.

Then there are the models with unlimited rubber. Here is is common that the motor weights as much as the empty airframe. Such motors tend to be long, and making such long hook distance would be impractical. So the motors are much longer than the hook distance. Here an optimization comes into play: The longer the hook distance, the less bunching of the motor. But the long stick needs to be heavier to be sufficiently strong, and also longer motor (mind you, it may be up to 50% of total flying weight) also means larger longitudinal inertia, which is not good for the model stability.
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Olbill
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« Reply #3 on: December 04, 2017, 10:40:42 AM »

Hook to hook length doesn't need to be the same as the motor length. As you have figured out you're not going to use all the turns in the motor so the unused turns will help keep the motor from tangling.

In many events there is a restriction on MS length. For instance in Limited Pennyplane the distance from the front of the bearing to the front of the rear hook is limited to 10" max. But LPP motors can be up to 24" long.

When you don't have a rules based restriction on motor stick length it adds another variable to the model design.
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ddock
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« Reply #4 on: December 04, 2017, 02:05:02 PM »

Had a Great day at the test field yesterday...Getting ready for the King Orange in Florida. Re trimmed class A Jay's Bird that I folded the wing on at the Nats (AFTER 3 maxes) testing 6 second engine run with short DT...DAAA TOO short!! Flew pretty good. Got out class B Provincial with Green Head 201 that I had never run. Not bad ,kept feeding a little incidence into it and left thrust ...got it going pretty good. Got out the modified Satelite 450 with the Conquest 15 which all all over the sky in Muncie. Two perfect test flights and put it away...Have no clue what it was doing at the Nats other than it was the Nats!
Soooo for me, a 3 for 3 flying session makes for a fun drive home. Smiley
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jdpsloflyer
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« Reply #5 on: December 04, 2017, 06:37:08 PM »

how can I delete a post i did?
« Last Edit: December 04, 2017, 08:14:39 PM by jdpsloflyer » Logged
mkirda
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« Reply #6 on: December 07, 2017, 01:08:29 PM »

Is there any reason not to make the motor stick 10" long? (Going to an extreme here)

Thanx all!

Flew a beta SO design last night with a 12" hook to hook distance.
Could even be longer as the best times were with motors around 19" in length.

Regards.
Mike Kirda
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jdpsloflyer
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« Reply #7 on: December 07, 2017, 01:30:43 PM »



Flew a beta SO design last night with a 12" hook to hook distance.
Could even be longer as the best times were with motors around 19" in length.

Regards.
Mike Kirda

Details?  Winds, weight, model (pictures)? 

My kids are just finishing their first kit (Lasercut planes) and I would like to know what other teams are getting out there. They have not trimmed or flown yet (next week).

I also made an assignment for the more advanced students to use their hand drawn plans I assigned in September to construct a plane over Christmas break.  Looking forward to it.
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mkirda
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« Reply #8 on: December 07, 2017, 01:47:14 PM »

Details?  Winds, weight, model (pictures)? 

My kids are just finishing their first kit (Lasercut planes) and I would like to know what other teams are getting out there. They have not trimmed or flown yet (next week).

I also made an assignment for the more advanced students to use their hand drawn plans I assigned in September to construct a plane over Christmas break.  Looking forward to it.

They will be forthcoming as soon as I can get to it.
Maybe over the weekend. A kit could done similar to John's.
It is really simple, yet flies really well.

Mike
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jdpsloflyer
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« Reply #9 on: December 07, 2017, 07:23:16 PM »

We are using John McGrath's Fruit Bat kit.

To be safe, I got extra 2/56 nylon bolts, washers and nuts.  Since there are 2 bolts in the kit and I have lost one myself on the floor, I have replacements when it happens.  The washers are for shiming the wing posts when corrections for warp is needed (or washout, etc.) and the bolts are for when the threads through the fuselage no longer hold.  this adds minimal weight since the parts are so small. 

Our first plane will use the stock red prop.  this plane is to learn building, trimming, and flying.  The kids do this to destruction, repairing their planes until they no longer fly well.  Then we go into another set of the same planes to hopefully get us to contest time.  I bought 30 kits to last us this year (I hope).

I have a supply of balsa from CLG 2 years ago for those who want to build their own and experiment.

Jerry
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mkirda
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« Reply #10 on: December 08, 2017, 12:23:13 PM »

Here is a pic. Taken from the balcony at Racine Memorial hall as it flew by.

Regards.
Mike Kirda
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Re: Should the distance from the prop hook to the back hook, equal the relaxed lengt
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