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AP question in regards to trim...

W

wickedsprint

Well Known Member
Do the servos act directly on the control surface linkages, or do they manipulate the trim...and if the former,will not having the pitch trim set perfectly result in extra workload/wear and tear for the servo..ie..do you set the trim close..then let the AP take up the slack?
 
I'll go out on a limb and say that MOST of the popular RV autopilots (Trutrak, Trio, etc) use a servo operating the control surface push rods themselves. I think it would be much harder to tune one to work on the trim tab and fly accurately.

Paul
 
For all intents in purposes, it is not possible for an autopilot to fly an airplane with Trim.

Pilots have landed airplanes with elevator failures with trim, but it will not be smooth.

The TT autopilots that have VS control have trim sensing servos that annuciate the need for trim.

The simple alt hold autopilots do not annunciate trim, and as fuel burns off the airplanes CG moves, it is good to turn off the alt hold, retrim, and reengage the Alt hold.

Tailwinds,
Doug Rozendaal
 
Some certified airplanes actually fly the autopilot with trim. I know that some 172s use the trim to hold altitude. Some of the late model 172Rs for example. Engage the altitude hold on the ground and the trim wheel will start spinning.

There IS an autopilot for homebuilts that uses the elevator trim, but the RVs are far too pitch sensitive and the elevator trim motors far too slow for it to work on our planes. I would imagine on some slower homebuilts it work work well. It's called the EZ Trim.

In general though, there are three types of autopilots used in RVs. All of these autopilots use a servo for roll and a servo and pitch, connected to the control surfaces using pushrods.

"Dumb" autopilots:
These are systems that completely ignore trim. Most will fly the airplane completely out of trim but the servos will be working harder in that case. One should periodically disengage the autopilot and ensure that the airplane is still in trim. This is the most common arrangement among most of the autopilots being installed in RVs these days.

"Trim Sensing" autopilots:
These autopilots generally contain an annunciator of some sort (like an LED light). They will alert you when the airplane gets out of trim. The autopilot knows this because of the load on the servo needing to keep the airplane flying on course/altitude. These autopilots are *not* connected to the trim system of the airplane, so they work with manual or electric trim.

"Automatic Trim Compensation" autopilots:
These autopilots are connected directly to the electric trim system and they will automatically keep the airplane in trim.
 
B25Flyer said:
For all intents in purposes, it is not possible for an autopilot to fly an airplane with Trim.

Tailwinds,
Doug Rozendaal
Click to expand...


Actually this is not true.
Ther have been designs to run the mac trim servos in planes, and in RV's.
This little booger was specifically designed to run the old ray allen electric servos in our planes. I flew one in a long EZ years ago and it worked just fine. I bought one and installed in my RV-6 and it was terrible. it was a laggard. I spoke to 1 man with an RV-6 who said his worked just fine. So results varried.

I did find that the unit was designed to apply voltage to activate a servo, which is opposite of how the Mac servos operate with a dual stick setup with the mac relays in there. . Here is the relay circuit I built to convert the voltage from the Alt. Hold unit to supply ground to the Mac relay system.

I do not know if this is still sold today. My experience was 8 years ago.
Best,
 
Our Aircraft Ops folks at work are fooling around with an autopilot for the T-38's (to go along with the new glass cockpit), and I understand that they are trying to make it fly with trim - and having a devil of a time getting it tuned to work smoothly and accurately enough with such a fast and pitch-sensitive airframe. Interesting project though!

Paul
 
wickedsprint said:
With that being the case, I assume you have to trim the airplane before coupling the AP?
Click to expand...
The Dynon AP connects directly to the controls, similar to TruTrak.

The Dynon AP can be engaged in any flight attitude and any trim condition as long as the airspeed is greater than 1.3 times the stall speed.

They mentiond on their web site the following:
Dynon said:
Trim sensing is something that we're taking a hard look at, but we don't have an announcement to make at this time on the availability of this feature.
Click to expand...

That's all I can say about their auto pilot so please don't ask, check thier web site for details. I will say that when they release it, a lot of customers are going to be very happy, not so much for their competitors.
 
Flying with trim

It is very possible to fly with trim. As a matter of fact, the DC-9 family of airplanes is controlled by servo tabs which are nothing more than trim tabs. Under normal operation, the ailerons and the elevators are controlled by these tabs. The left and right elevators are not connected. This is why you may see the elevators split during taxi.
 
Hi Guys,

This has been hashed and re-hasned many times over the years as far as an AP flying the plane with a trim tab. To be quite blunt, it has never worked in an application like an RV as an AP that can move the controls. The AP will always be behind the movement of the actual flight control itself, so on and so forth. No big deal in a 172 that is relatively slow, and relatively insensitive in pitch. The DC9 isn't even worth comparing, the entire flight control system couldn't be any further away than that of an RV (while the elevators in a DC9 may be controlled by servo/anti-servo tabs, the trim is NOT - it's a jackscrew that moves the entire horiz stab - and when that breaks...well, we know what happened there).

There are 2 well proven autopilots out there for RV's that are very reasonably priced (TruTrak and Trio) as well as BMA with their own. Dynon will be selling theirs sooner or later, but at the moment we don't know much about it - other than what little anecdotal responses you occasionally see on the forums from them or some of their beta testers. TruTrak and Trio both have nice optional little boxes that will automatically trim the plane (hooks right into your electric trim system) for you as fuel is burned, or for climb/descent, etc.. and they work marvelously. Of course you need a trim sensing servo to do such a thing.

Anyway, neat stuff all the way around. Once you fly with an AP, you'll never want to go back...no matter which one you choose! Heck, even the NavAid's were pretty darned good in their day!

Cheers,
Stein

BTW - interesting factoid. As of OSH this year, TruTrak has shipped more than 10,000 servos! Can you believe that?!?!
 
Yep . . .

wickedsprint said:
Do the servos act directly on the control surface linkages, or do they manipulate the trim...and if the former,will not having the pitch trim set perfectly result in extra workload/wear and tear for the servo..ie..do you set the trim close..then let the AP take up the slack?
Click to expand...
If the trim force is comfortable for you, it's great for the A/P. As long as the trim is set so that the control force is below the breakout force of the A/P system, the servo 'takes up the slack' as you put it. Trim sensing servos monitor the constant load, and when it gets too high the 'trim' light is turned on.
Is it 'extra' wear and tear if you are out of trim? Probably not. A constant load is less wear/tear than constantly changing loads - like flying in turbulence, and the servo should be designed for many (many, many) more hours of use than we'll ever give it...
 
Kahuna said:
Actually this is not true.
Ther have been designs to run the mac trim servos in planes, and in RV's.
This little booger was specifically designed to run the old ray allen electric servos in our planes. I flew one in a long EZ years ago and it worked just fine. I bought one and installed in my RV-6 and it was terrible. it was a laggard. I spoke to 1 man with an RV-6 who said his worked just fine. So results varried.
Click to expand...

Even the little booger website says it won't work in turbulence. But I will amend my statement:

For all intents in purposes, it is not possible for an autopilot to fly an airplane well with Trim.

Tailwinds,
Doug Rozendaal
 
Don't know about the others, but the TruTrack has no idea of whether the aircraft is in or out of trim prior to engagement - and pretty much doesn't care.

If the airplane is severely out of trim, you will hear the affected servo (roll or pitch) start to "pop" or "click" (they are stepper motors, and each click represents being force one step back vs the position demanded by the autopilot).

TruTrack (and, I presume the others) recommends that you configure the maximum servo force such that it can be easily overcome by hand in the event that the autopilot goes haywire and cannot be turned off. It provides a visual indication of out-of-trim, which never seemed quite right to me in my previous airplane (haven't finished installation on my new plane).

However, the "click-click-click" of the straining stepper motor is both audible and can be felt in these small aircraft - and thus makes a dandy (if unintended) annunciator for a badly out-of-trim condition.
 
Thanks for the replies, is there some sort of failsafe in case the AP has a runaway? Do you shut it off..or can you overpower the servo clutches?
 
You can and should be able to overposer your AP at any time. They all also have a disconnect switch that can be mounted on the stick, and also you should have a switch to shut if off if need be. Some servos have clutches, most don't....you simply just grab the stick and put it where it needs to be or shut the darned ap off!

Not an issue to worry about.

Cheers,
Stein
 
Actually, there are several fail safes on these autopilots.

1. If you have performed setup correctly, the maximum torque supplied by the autopilot to the controls should be easy to overcome.
2. Fast disconnect switch, if installed.
3. On / Off switch on the autopilot head.
4. Pull the circuit breaker.
 
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