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  #1  
Old 01-25-2021, 07:00 AM
rocketman1988 rocketman1988 is offline
 
Join Date: Jun 2010
Location: Sunman, IN
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Default Electric Trim Question

I'm currently flying in Phase 1, and I have seen that the trim is very sensitive at higher airspeeds. I have a VPX, and have activated the speed trim function, and set it for 70% at 120 its or above. It works as advertised ON THE GROUND in simulated airspeed mode.

In the air, however, the trim ceases to function at 120 or above. The indicator shows it is getting a command but the trim doesn't move. I then increased it to 85% and it works but it is still very sensitive.

My question to the group is if anyone has experienced this issue or is it possible that I have a faulty RAC servo? It apparently does not have the necessary power to drive the trim at a reduced voltage in my case...

Thoughts?
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  #2  
Old 01-25-2021, 07:29 AM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
 
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Location: Dogwood Airpark (VA42)
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Adjust the VPX to provide 100% at all speeds and see if that fixes the problem.

Carl
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  #3  
Old 01-25-2021, 09:35 AM
rocketman1988 rocketman1988 is offline
 
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Location: Sunman, IN
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Default trim

Adjusting it to 100% is the problem. At speeds above around 120, the trim is too fast and needs to be slowed down. The problem is, at 70%, it appears that the RAC servo does not have enough power to move the trim tabs.

My question is if anyone else has seen this with their -10 trim. It not, I wonder if my RAC servo is defective...
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  #4  
Old 01-25-2021, 09:44 AM
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Raymo Raymo is offline
 
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70% is apparently not be enough power to move it in flight. My Dynon pitch trim control is set to 50% and works well above 100 KIAS to slow the trim movement.
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  #5  
Old 01-25-2021, 09:59 AM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
 
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Quote:
Originally Posted by rocketman1988 View Post
Adjusting it to 100% is the problem. At speeds above around 120, the trim is too fast and needs to be slowed down. The problem is, at 70%, it appears that the RAC servo does not have enough power to move the trim tabs.

My question is if anyone else has seen this with their -10 trim. It not, I wonder if my RAC servo is defective...
I would first suspect the VPX. Do a run at 100% to find out.

Side note - I ran the trim servo at 100% in my RV-10. I just got use to doing just a bump at speed.

Carl
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  #6  
Old 01-25-2021, 01:56 PM
BobTurner BobTurner is offline
 
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Location: Livermore, CA
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I donít know how the VPX works. Does it send a reduced dc voltage to the trim motor? If so, perhaps there is insufficient torque to get it going. I use the Trio Pro autopilot to send pulse width modulated voltage pulse to the servo. Since each pulse is 12 volts it never hangs up, even at the slowest speeds. It may also be that there is an unusually large mechanical resistance or binding in the physical mechanism.
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  #7  
Old 01-25-2021, 02:45 PM
rocketman1988 rocketman1988 is offline
 
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Default vpx

Why would you immediately suspect the vpx? As posted, it works perfectly at 100%; when the vpx reduces the voltage to 70%, it will not move the trim tabs above about 100 Knots. The vpx has a simulate mode so that you can test it on the ground at the transition airspeed...it transitions correctly at the set speed and the tabs move at a reduced rate as you would expect. In the air, however, I am getting an indication on the trim indicator that there is a command being sent but the trim doesn't move. Given these facts, it is highly unlikely that the vpx is the problem, other than reducing the voltage instead of using PWM duty cycle.

This is why I am asking folks if they are using the RAC servo at a reduced voltage and getting the same or different results.

If people are using the RAC servo on their -10 with reduced voltage and it is successful, that would indicate a problem with my RAC servo, or the linkage between the servo and tabs. I have checked the cam linkage and found it to be free...

I wish the VPX used PWM; that would likely solve the problem...
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  #8  
Old 01-25-2021, 04:53 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by rocketman1988 View Post
Why would you immediately suspect the vpx? .........
....I wish the VPX used PWM; that would likely solve the problem...
Didn't you answer your own question?
On the ground, there is only friction keeping the trim motor from moving the tab; in flight, there is a significant force from the air loads that needs to be overcome. The faster you go, the worse this gets.
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  #9  
Old 01-25-2021, 05:10 PM
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MCA MCA is offline
 
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Bob,

Have you reached out to Chad at VP support?

There are thousands of VP-Xs operating and there's plenty of "power" to move the trim motor on an RV-10 in cruise flight.

This one is a head-scratcher though.

IMO the most likely cause is the mechanical thingy - see if you can swap out the trim motor.
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  #10  
Old 01-25-2021, 05:36 PM
rocketman1988 rocketman1988 is offline
 
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Location: Sunman, IN
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Default trim

I do not think the VPX is the problem as it does exactly what it is supposed to be doing; ate the selected speed, it reduces the voltage to the RAC trim servo to "slow" it's operation. It works perfectly above 85% but when selected below that, the trim motor won't move the tabs.

That is why I posted the question. I am trying to find out if anyone has reduced the voltage on the RAC servo to below 85% and still have the RAC servo be able to move the tabs. It doesn't matter what speed trim system is being used, only that is it reducing the voltage to slow the motor.

Many people use the speed trim function available from several different vendors. If no-one else is having this issue then it would seem that my RAC servo is not performing to standard, ie insufficient power to move the tabs at reduced voltage.

Thanks for the input...
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