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  #1  
Old 10-10-2005, 01:59 PM
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Default Control springs

Quote:
Originally Posted by Terry Schubert
Go look at the pitch trim spring and note how the ends are formed. When spring wire is cold bent sharply, as called for in the plans, the chances of failure go way up depending on the bend radius.
well - whats the solution. obviously the plans method leaves plenty of room for improvement

can i stress relieve in my oven at 500 degrees for a few hours?
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Old 10-12-2005, 10:44 AM
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cmon metal men/women of the forum - no takers on this subject?
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  #3  
Old 10-12-2005, 01:06 PM
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What about replacing the plans trim with the setup that uses the electric trim adjustment motor and the horseshoe shaped composite spring?

Was that on Wayne Hick's site?
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Old 10-12-2005, 01:16 PM
KiloWhiskey KiloWhiskey is offline
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To stress relieve you got to get it close to but do not exceed 1350 degrees F. How close and how long for spring steel, I'm really not sure, textbook I checked last night suggested within 150 degrees, 15-120 minutes (those tiny tables in textbooks are real specific-not), nitrogen atmosphere or vacuum furnace. Go over 1350 and it transforms into an annealed noodle unless properly quenched.
Stupid question time-while making a high speed low pass doesn't a canard have a lot of nose down trim dialed in? So if the the trim spring that's under load breaks, the worst that happens (assuming you're mixing margaritas and waving to the crowd instead of flying the airplane while making a low pass) is that the plane goes shooting skyward in a 6 to 9 G pull up?
  #5  
Old 10-25-2005, 10:32 AM
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mmmmm 1350 max for 15 to 120 mins - i think that is doable either in a kiln or with a temp crayon and a torch.

I don't like being notified with a problem and then ignoring it
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  #6  
Old 10-25-2005, 10:46 AM
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DON'T (unless you have additional info) stress relieve a cold bent spring!

You'll soften the metal which allow the spring to yield at a lower displacement. It won't return to original shape.

Most cold bend springs are made from pre-heat treated steel. The temper is low to provide a high yield strength. Additional tempering will destroy the spring. Of course this is MOST. I have no ideas what this particular spring is made from.

I don't know what the trim spring looks like so I can't comment on it being a bad design or not. Pigtails, swages, flat ends are common designs though.

Mike
Auto engineer in suspensions including coil springs and stabilizer bars.
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  #7  
Old 10-25-2005, 10:54 AM
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When the CP publisher talks - i listen

if i get you a sample - can you figure it out
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  #8  
Old 10-25-2005, 11:26 AM
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Quote:
Originally Posted by Dust
When the CP publisher talks - i listen
CP? Confirmation Prototype?
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  #9  
Old 10-25-2005, 11:39 AM
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The plans trim systems work great with one exception. If you break a spring you get immediate and opposite trim. This one failure mode can be negated by using compression springs instead of tension springs. It's an old trick from the CP (Canard Pusher) newsletters.
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  #10  
Old 10-25-2005, 12:56 PM
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could you splain?
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  #11  
Old 10-25-2005, 02:32 PM
Wayne Hicks Wayne Hicks is offline
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I'm on the road in Utah without my plans, but I'll try.

An excellent example of a compression spring is the one found on screen doors. Most screen doors have a compression spring between two segment of chain. The purpose is to save the door from being ripped off its hinges when the door gets away from you when blown open by the wind.

The top of Figure 1 shows the pitch trim system with compression springs. The bottom shows the tension springs. For the tension springs to work, they must be stretched in order to provide enough tension so that when you move the trim lever, the control horn will move with the spring. They must still have enough springiness to them so that when you move the control stick in the opposite direction, the upper spring can stretch to its full length and the bottom spring can relax fully. This is a compromise. If you "run out" of spring at the top, then the spring will prevent you from getting full range of motion from the control stick. That's very B-A-D. Plus, the tensioned spring -- which was under a fair amount of tension before -- is under ALOT of tension. Even with the control stick in neutral, you can see that if one spring were to break you get full opposite motion.

This doesn't happen if you change out the tension springs with compression springs. The spring is actually being "pushed" rather than "pulled" by the wire dohickeys. When actuated, only one spring is providing force. The other remains slacked. If the active spring were to break, the slack spring is under no force and nothing B-A-D happens.

Does this help?

Figure 2 shows how the compression springs work
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  #12  
Old 10-25-2005, 02:39 PM
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yes - mr metal man - clifford - can you handle this
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Old 10-25-2005, 02:40 PM
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Well, the drawings got reversed on the upload. Figure 1 is to the right and figure 2 is to the left.

The other application for compression springs is with the hidden rudder bellhorns. We normally put a compression spring between the rudder cable and the rudder pedals. The RAF plans show how to make the dohickeys from music wire. The compression spring is stronger than the force required to move the rudder in flight. When you step on the rudder pedal, the spring doesn't compress, it pulls the cable, the rudder deflects outward. Once the rudder is fully deployed and the bellhorn hard-stops inside the winglet, the spring then compresses to allow further movement of the rudder pedal to engage the brake cylinder without further movement of the rudder cable.

Clear as mud, I'll bet.....
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Old 10-25-2005, 02:43 PM
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Actually - when i reviewed the pics - they reminded me of the hidden belhorn system

thanks Wayne or as they say in Lithuania - ahchoo
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  #15  
Old 10-25-2005, 03:28 PM
Marc Zeitlin Marc Zeitlin is offline
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Quote:
Originally Posted by Wayne Hicks
..... Even with the control stick in neutral, you can see that if one spring were to break you get full opposite motion.
Not quite. The springs don't control motion - they add force bias. The only way you'd get full opposite motion is if you let go of the stick. It is possible to fly the plane with a broken spring - it just takes a lot of stick force to do so.

Quote:
Originally Posted by Wayne Hicks
This doesn't happen if you change out the tension springs with compression springs. The spring is actually being "pushed" rather than "pulled" by the wire dohickeys. When actuated, only one spring is providing force. The other remains slacked. If the active spring were to break, the slack spring is under no force and nothing B-A-D happens.
Sorry, Wayne, but that's just not true. Whether you have compression springs or tension springs in the trim system is completely transparent to the user - the actions of the system are exactly the same.

When installed, BOTH springs (whether tension or compression) are <stretched/compressed> by an equal amount when the trim system is centered, and when the system is at FULL trim deflection one way, with FULL stick deflection in the other, ONE spring will be fully <stretched/compressed> and the OTHER spring will be JUST BARELY becoming slack.

BOTH springs are always "active", and either stretched or compressed to some degree. Because of this, the trim system would act exactly the same if a compression spring broke as it would if a tension spring broke. If any trim system is set up in a way that does not conform to this (whether with tension springs or compression springs), it's set up incorrectly.

The reason to use compression springs is that they have a failure rate that's orders of magnitude less than tension springs, NOT because the trim system acts any differently, or works any differently.

While I've never had my pitch trim springs break, I have had the manual landing brake system return springs break three times - long, highly loaded tension springs.

And for Mike, Terry Schubert publishes the CSA Newsletter, NOT the Canard Pusher. Very different things.
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