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Hitec 605BB servos are not suitable for the H9 1/4 scale Cap!

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Old 07-29-2003, 02:32 PM
  #51
bgi
 
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Default Hitec 605BB servos are not suitable for the H9 1/4 scale Cap!

For you guys who want to max out the use of the torque you paid for AND keep the control surface arm long, there is a tool which makes all the calculations easy. You input the parameters:

Max control surface deflection in degrees.
Max servo arm deflection in degrees.
Control surface arm length.
Control surface arm distance fore/aft from hinge centerline
Servo distance from hinge centerline fore/aft and up/down

It will calculate the sero arm length required to achieve the desired surface deflection with the specified servo arm deflection.

I agree that the longer servo arm is good up to a point so you can get the torque and resolution you paid for. But if it's too long you lose torque out on the surface as well as resolution. This tool tells you the exact length you need to meet your criteria. If your control surface clevis isn't centered over the hinge centerline, it will also tell you the ideal servo arm angle to achieve balanced throws AND display the linearity of the throws.

It also performs a number of other really cool calculations, like the torque required of your servo based on control surface geometry, linkage geometry, and airspeed.


Go to this thread post 10 to download from RCU.

http://www.rcuniverse.com/showthread...88&forumid=230


This thread has some other links relative to this work in post 7.

http://www.rcuniverse.com/showthread...67#post1225441


A couple improvements which would be nice would be some way to add a counterbalance to the equation and adjust for altitude and humidity changes. I suppose for a counterbalance, you could do a manual adjustment by removing the counterbalance area and an equivalent portion of the main surface from the inputs..
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Old 07-29-2003, 02:44 PM
  #52
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Default resonance

Quote:
Originally posted by John Weber
I think RysiuM is on to something here also. I am running 605's on a Great Planes Giles 202 with Saito 180 power and the APC 17x6 prop with no problems thus far. At certain rpms though, the engine/rpm/airfame will come into resonance and the ailerons will go absolutely nuts! The entire airframe is experiencing resonance but the effect is magnified at the ailerons. Also I noticed the wheel pants will come into resonance as well at the same rpm. Increasing or decreasing rpm will stop the resonance. I am not an engineer, but I am an airframe and powerplant mechanic on full size aircraft as well as an aircraft inspector (IA). Full size propeller driven aircraft will often have a yellow cautionary range on the tachometer. This is a critical vibratory range where the engine/propeller combination will come into resonance and ultimately result in a crankshaft failure. It seems to me that operation in this range on a model aircraft could be causing some of the problems that we are attributing to flutter. The gears are actually fatiguing over time from what is in effect a low amplitude, high frequency flutter mode induced by propeller/engine/airframe resonance. Just my thoughts on what is an intriguing puzzle.
You have nailed the source of the problem. Just ask Lockheed. They lost a couple of turboprop airliners due to this.
BTW flutter is the ailerons resonating. You can minimise this by balancing the ailerons.
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Old 07-29-2003, 03:04 PM
  #53
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Default Hitec 605BB servos are not suitable for the H9 1/4 scale Cap!

Roger that!! I believe Lockheed engineers termed the phenomenon "whirl mode" and it resulted in the loss of two Electras. Apparently propeller/engine resonance at certain airspeeds aided by gyroscopic effects would induce a resonance that would excite the wings natural frequency creating a flutter. Both aircraft lost their wings in midair with predictable results.
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Old 07-30-2003, 03:25 AM
  #54
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Default resonance

"Apparently propeller/engine resonance at certain airspeeds aided by gyroscopic effects would induce a resonance that would excite the wings natural frequency creating a flutter."

This is not quite correct. It should be:
Apparently propeller/engine vibration at certain airspeeds aided by gyroscopic effects would induce a vibration that would excite the wings natural resonate frequency.

However, there is more to it. At the time a pooly understood weather condition called clear air turbulance (CAT) induced the problem. The result was the FAA restricted the Electra to 300mph until the problem was fixed. That was the end of the Electra.
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Old 07-30-2003, 12:30 PM
  #55
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Default Hitec 605BB servos are not suitable for the H9 1/4 scale Cap!

Thanks for the corrections Dirty Birdy, I was trying to write that post and teach a class on gas turbine engine maintenance at the same time and got a little distracted. Anway, engineers at GM's Allison Division concluded after exhaustive research that yaw induced cyclic vibrations from the Electra's big fourbladed propellers excited the wings natural resonant frequency and led to wing flutter. A contributing factor according to an article posted in Air and Space magazine:
http://www.airspacemag.com/ASM/Mag/I...FM/Hammer.html

was prior engine mount damage. Apparently engineers redesigned the engine mount and the problem was solved. No mention is made of CAT in the article but it could have been a contributing factor in setting up the initial yawing motion that triggered the whirl mode.

J.W.
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Old 07-30-2003, 05:15 PM
  #56
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Quote:
Originally posted by John Weber
Thanks for the corrections Dirty Birdy, I was trying to write that post and teach a class on gas turbine engine maintenance at the same time and got a little distracted. Anway, engineers at GM's Allison Division concluded after exhaustive research that yaw induced cyclic vibrations from the Electra's big fourbladed propellers excited the wings natural resonant frequency and led to wing flutter. A contributing factor according to an article posted in Air and Space magazine:
http://www.airspacemag.com/ASM/Mag/I...FM/Hammer.html

was prior engine mount damage. Apparently engineers redesigned the engine mount and the problem was solved. No mention is made of CAT in the article but it could have been a contributing factor in setting up the initial yawing motion that triggered the whirl mode.

J.W.
I got my information from an article in Aviation Week or it might have been Newsweek shortly after the incidents. The name of the article was "Caught in CATs Claws".
I found I had to take the AC from Chicago to Minneapolis or take a bus. The airline assured me that with the restricted airspeed I had nothing to worry about. I did not enjoy the flight.
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