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Takeoff Roll Got My Attention

N208ET

Well Known Member
I had a, hummm, interesting situation yesterday. I flew to a local airport, landed, was out of the plane for about 15 minutes doing my thing. I get back in and fire up, taxi out to the runway, all is good. Do a quick runup, roll out on the runway and start the takeoff roll. My plane has always wanted to roll left when I put in full power on the takeoff roll, so I have gotten into the habit of compensating with right aileron, a lot at first and slowly decreasing defection as speed builds up. This is where things get a little hairy. I go to put in right aileron, and the stick won't move. Mind you the power is still in and I'm in the early stages of the takeoff roll. I pull the power out and as I roll down the runway start playing with the stick, elevator travel is fine, it just won't move left or right. I taxi off the runway and keep playing with it the entire way. The stick will move about 1/2 to 1 inch left and right, that's it. I shut down the plane and the problem immediately goes away, the stick moves freely. I have a dual axis dynon autopilot with the servo out in the wing. I have checked the entire control system and I can't see anything that would have caused this situation. When I engage the autopilot test function on the ground, the stick is still able to move 1/2 to 1 inch. Is it possible that the servo caused this? I have had to replace a shear screw on this servo in the past and had no idea that it had sheared until I went to use it one day. I don't think this has anything to do with the situation at hand, but ya never know.

One things for sure, this will freak ya out!

Randy
8A
 
Before the takeoff, were the controls clear? It sounds like the servo might be involved... but?
Foreign object tossed up into an aileron from the ground? Just wondering.
Good thing you shut it down right away. Surprises like that can spike the heart rate for sure.
 
Definitely something you need to fully investigate before further flight. Are any bellcranks able to go over-center or anywhere close to over-center?

My first thought was maybe the roll servo was engaged and you had to overcome its force causing it to slip before it would move. On a recent flight I had the AP engaged and, not realizing that I had it engaged, I tried moving the stick. It sure gets your attention when something resists your control inputs!

Good luck with your inspections and please let us know what you find.
 
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Randy, it sounds to me like the autopilot was engaged on the ground, before your takeoff roll. Once you shut everything off, the autopilot had no power, so the stick was free to move.

Just another pre-take off item to check.

Best,
 
Definitely sounds like the autopilot is engaging some how. Do you have a separate fuse for your autopilot? If so, pull it and re-test. I bet the problem goes away.:)
 
Before t/o?

As mentioned above: it would be really interesting to hear how the controls where during the "before takeoff check"?

For example: did they feel normal or binding during the check?
 
I get back in and fire up, taxi out to the runway, all is good.

Did you notice if the stick was free during taxi???

I go to put in right aileron, and the stick won't move. Mind you the power is still in and I'm in the early stages of the takeoff roll.

I shut down the plane and the problem immediately goes away, the stick moves freely.

Did the problem go away after shutting down the engine, or engine and electrical system??
 
I agree with the above. How were the controls on the pre takeoff check. I think the control check before takeoff is the most critical check we can do.

George
 
Good question

And I tried to remember that myself. I am in the habit of rolling the stick around before takeoff but to be honost, I'm not sure that I did that. My preflight was a little less than ideal since it was the second flight of the day, and I had only been on the ground 15 minutes. No excuse I know, but a fact none the less.

When I shut the engine down, I shut everything off. I wish I would have turned off my servo power switch, that would have gave me an immediate answer. But before I did shut down, the stick wouldn't move but that little bit right and left. After shutdown, free motion. I checked and double checked the entire control system, looked closely at the ailerons for any sign that something got lodged and came up with nothing, everything looks good. So I went out and ran up with the servo power off, everythings perfect. The plane has over 400 hrs on it, the servo system around 100. I am going back out today for another inspection and play with the servo some more. Testing the servo on the ground produced normal results. But I keep asking myself, I definitely didn't engage the servo, and the autopilot isn't suppose to function below a certain airspeed, 100 mph is what I have the lower limit set at. Is it possible for the servo to even do this? I'm heading to the airport now and will report back any findings.

Randy
8A
 
I am in the habit of rolling the stick around before takeoff but to be honost, I'm not sure that I did that.

So I went out and ran up with the servo power off, everythings perfect.---------- Testing the servo on the ground produced normal results.

Is it possible for the servo to even do this?

I suspect the AP is going to end up being the culprit here, but the real questions are going to be why and how did it happen.

If you did in fact check control freedom prior to your application of take off power, as per your stated norm, then it would appear that the high power application is what caused the AP to engage.

High power equals high vibration levels.

I would then suspect a wiring issue-------bad switch, loose connection, chaffing wire etc.

This is a pretty loose logic chain, but it is in line with what you have mentioned so far.

Good luck with your investigations today.
 
I am going back out today for another inspection and play with the servo some more. Testing the servo on the ground produced normal results. But I keep asking myself, I definitely didn't engage the servo, and the autopilot isn't suppose to function below a certain airspeed, 100 mph is what I have the lower limit set at. Is it possible for the servo to even do this? I'm heading to the airport now and will report back any findings.

Even if the autopilot is engaged, it should be possible to apply enough force through the stick to override it and fly the aircraft. If your autopilot servo is so "strong" that you cannot override it, I'd disable the autopilot. There are failure modes, or pilot misselection errors, that could lead to the autopilot being engaged when you believe it is not engaged. If you can't override the autopilot and fly the aircraft, then these situations could lead to an accident.
 
If it is the servo, the autopilot should be configured that the pilot can over power the servo. Each AP is different so I'd refer to the manual or the the manufacturer for settings.

Even if the autopilot is engaged, it should be possible to apply enough force through the stick to override it and fly the aircraft. ...
You are both correct and the Dynon AP is designed so you can override it. (I have the SV42 roll servo in my -9, which is the mid-sized servo.)

However, it bent the attach plate, like happened to me, it will jam the controls.
 
Check and see if the pin that locks the cable on the servo wheel has popped out. This happened to me on my elevator servo and it locked it up just like yours. I sent the servo back and found out this was a known problem a few years back however corrected.
 
One additional thing to note. The software in some installations that prevents servo activation on takeoff is dependent on GPS data. If a good lock on GPS is not present... the safety feature will not work.
I trained special warfare pilots in C xxx aircraft at one point. The certified servos locked up... and the disconnect button did nothing also.
Pilot pulled the wrong breaker (down by his lower left leg) and that confused things even more. Had to land with unusual forces to over ride the clutches. It was important to keep a light but firm touch.... knowing that the servos might disengage just as quickly as they locked.... right at touchdown.
Other testing with The Chelton Autopilot (later pulled from the market... and sent to the shredder) drove the elevator trim tab. That always gave me concern, regarding a run away trim motor. We are all test pilots to some degree in the RV series.
Please.... always waggle those sticks before takeoff. Even if just a stop and go. And check trim positions... Cheers guys/gals....
 
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The recent high winds where I have been this week are a reminder to me.
Never let go of the controls. Never leave the gust lock off... even to walk over and do a quick task. Always waggle the stick before flying. When the autopilot gets power on the ground, hold tight. You are in charge... not the little motors.
 
The OP has a non-standard installation that will produce more than a 50% increase in force over the standard installation, using the same hole in the servo arm. Given the same geometry and test routine, would the SV32 have been capable of damaging the steel brackets? Who knows. But thankfully this is the first case we've heard of.

Actually the OP was a totally different person and problem---------major thread drift has occurred here.

As to the problem in Bills plane------well, isnt that the whole purpose of doing a beta test??? find any issues and determine how to deal with them. Seems like this is all going to be a good thing in the long run.
 
My plane has always wanted to roll left when I put in full power on the takeoff roll, so I have gotten into the habit of compensating with right aileron, a lot at first and slowly decreasing defection as speed builds up.

Just to back up a bit, I'm a little confused.

You compensate "on the roll" with right aileron?

Isn't this a rudder input situation?

I ask because I often end up left of centerline after I've left the ground and compensate THEN with aileron. I haven't had a chance to practice lately but I'm pretty sure I'm not putting enough rudder in on the roll and probably am also not coming straight back with the stick on rotation.

I realize this is not the point of your post but I was just curious.
 
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Just to back up a bit, I'm a little confused.

You compensate "on the roll" with right aileron?

Isn't this a rudder input situation?

I ask because I often end up left of centerline after I've left the ground and compensate THEN with aileron. I haven't had a chance to practice lately but I'm pretty sure I'm not putting enough rudder in on the roll and probably am also not coming straight back with the stick on rotation.

I realize this is not the point of your post but I was just curious.

I notice that no one has answered your question. You are correct that rudder is the proper input to keep straight on takeoff roll. Some aircraft can use the ailerons similarly, due to adverse yaw. The T-38 is one example. Maybe there are general aviation aircraft with similar characteristics, but I have not found my RV-8 to be one of them.

The normal use of ailerons on takeoff roll is to counteract crosswind, i.e. aileron into the wind. IMO, the problem of always using right aileron is that it is exactly the wrong input for a left crosswind.

-John
 
Yup

I use right rudder to keep it straight but also start with right aileron to counteract it's tendency to want to roll left, it's done that since day one.
Maybe the 8A being narrower has something to do with it, I've never talked to other 8A pilots about it.

Randy
8A
 
Three things:

1) The Dynon AP will engage on the ground, however it will automatically disengage if you get to 20 knots. This is based on AIRSPEED, not GPS speed. The purpose of this is to allow you to test function on the ground, as well as use the AP in a situation in flight where your airspeed is zero. Between 20 knots and your minimum AP speed, the AP will not engage.

2) If this is a SkyView, you have a datalog in the system that shows if the AP was engaged when you rolled out.

3) While there is a chance a chafing wire or something else could do this, you would need to short the button for 2 seconds solid to ground to cause an engage, and then release for the servo to put out any force. Even a momentary short after that would disconnect. So it seems unlikely. No Dynon AP has an external button that can be pressed for only a moment to engage.
 
I use right rudder to keep it straight but also start with right aileron to counteract it's tendency to want to roll left, it's done that since day one.
Maybe the 8A being narrower has something to do with it, I've never talked to other 8A pilots about it.

Randy
8A

How does it roll left with both mains on the ground?

-John
 
It doesn't

I said it wants to, I didn't say it rolls on its side. Just because the mains are on the ground doesn't mean they are both putting equal pressure on the runway, that's what starting out with right aileron does. Stick is also back till the nosewheel comes off then I just keep it a inch or so off the ground until the plane flies off, I don't rotate at a certain speed. It's total soft field technique every time. There is right rudder as per keeping her straight down the runway, whatever it takes.

Randy
8A
 
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