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Author Topic: How to set models to turn  (Read 213 times)
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Ara Dedekian
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« on: March 18, 2020, 03:26:22 PM »



      The photo below shows what I've resorted to on my planes that won't turn within the confines of a small gym.  But some of my planes turn nicely indoors without such a shim.  Applying rudder seems to cause a stall. Without drag flaps and other protuberances, will unequal washout create a turn? Is that the difference between models that turn and those that don't?

      Ara
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lincoln
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« Reply #1 on: March 18, 2020, 07:44:18 PM »

Ara,

I should probably refrain from answering, since I'm sure you have more stick and tissue experience than I do. However, my social intelligence is low enough that I'll jump right in anyway.

Are you sure that stall isn't a spiral dive? If it's a spiral, maybe a bit of washin on the inside wing and washout on the outside wing would help.

Does the effect of the thrust adjustment run out as the motor runs down, so that the model straightens out and runs into the wall? Rudder adjustments don't have this problem.

A spiral dive may be more likely if the vertical stab is too big or there isn't enough dihedral.
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OZPAF
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« Reply #2 on: March 18, 2020, 08:09:13 PM »

I'm not an experienced indoor modeller either Ara, but I'm curious about the size of the prop relative to the wing span. Natural left turns are largely torque induced due to the prop size - increasing with diameter. I would think that 40% of the wing span would be safe.

Also how fast does it fly? Thrust line changes are less effective than surface adjustments at speed.

I also agree with Lincoln re the possibility of a spiral dive due to a possibly lack of dihedral.

Good luck

John
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Yak 52
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« Reply #3 on: March 19, 2020, 05:16:17 AM »

Does the effect of the thrust adjustment run out as the motor runs down, so that the model straightens out and runs into the wall? Rudder adjustments don't have this problem.

Generally indoors a left turn trim is achieved with left rudder against right thrust.

Is it possible to provide more information about which specific models you have trouble with? (P51?) And how much space you have?

I suspect that if you have some apparent stall behaviour associated with rudder position, it may be that you have low yaw stability (aka weathercock stability) from too small a rudder or too large a propeller diameter. This can permit the model's nose to wander off in yaw to the point where one wing snaps into a stall. If so, increasing rudder area or decreasing prop diameter might make a difference.


Jon
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Ara Dedekian
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« Reply #4 on: March 20, 2020, 10:00:15 AM »

      Thanks for the tips.

      Adding to the dilemma, my two best turning indoor flyers have nothing in common! The Comet Spitfire dimer took a reduced rudder and wing mods to begin turning and then needed the offset shim to stay within the confines of the smallish gym. It has a 16" wing span @ 22 grams ready to fly with a 5-1/2" Delta Dart prop (1:1.2 ratio).

      The Walt Mooney Back Bay Bostonian took merely a 1/16" shim to keep it inside the gym. It has a 15.75" wingspan, 24.8 grams ready to fly with a 5" Guillow prop (1:1.4 ratio)

      Both the dimer and Bostonian are majestic, slow flyers. The Mustang, in my initial post, is also a great flyer that I've flown in a large gym and outdoors but will be close inside a smaller gym.

      My experience with rudders has been disastrous which is why I try not to use them to induce a turn. When I find the right trim on a model and then apply rudder for it to turn, the model stops flying. I attribute that to the lack of coupling between dihedral and rudder as you folks have suggested. I know from my three channel radio assist Old Timer models, rudder only causes a yaw without gobs of dihedral and I'm limited by the FAC rules and scale proportions as to how much dihedral I can put in stick and tissue builds.

      
      Off topic:
      My biggest disaster with rudders was my outdoor Bell/Golden Age Reproduction Hughs H-1. As I walked out to the flight line with it at Geneseo some years back, Dave Mitchell, the first person to see it, asked what I did to counteract the spiral instability inherent in the design. Of course, my reaction was, "what the ---- is spiral instabiltiy?" He was right, the model just wouldn't fly, and fortunately I had built in adjustable rudder and elevators with copper hinges. Removing the rudder, leaving just the verticle stab, brought it into balance with the shallow dihedral and created some stability. I built the model with both the short and long wing (as Hughs did) held on with magnets and when I switched to the short wing with more dihedral, it took off like a bird. I than reattached the rudder as a floppy rudder with Esaki hinges control line style and can only fly it with the short wing.

       Back on topic:
       So is the answer prop size reletive to wingspan to induce a tight turn? I'll have to try left rudder against right thrust as Jon suggested.
       I've also learned that to keep the turn through to the landing, the front end has to be tight. Using dowels to hold it in place works better than friction so it won't fall out as the winds run down.

       The other aproach I'm trying is a Gurney ( as in Dan Gurney the race driver) flap on the Comet Great Lakes Trainer. It's the 1/16 strip under the left wing that's supposed to improve the airflow and create more lift. It's not meant to be a drag flap.


       Ara
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