platnuim1

My Feedback: (62)

Looking for suggestions on minimum torque (oz-in.) for a 2 meter pattern plane.

I realize that each manufacturer measures their numbers differently, but an approximate figure would do.

Ail
Elev
Rud

Thanks in advance!!!!!!

nonstoprc

My Feedback: (90)

From my experience, the following is about right.

Ail : 100-120 oz-in per aileron half
Elev : 60 oz-in per elevator half
Rud : 120 oz-in

platnuim1

My Feedback: (62)

Thanks nonstoprc

Jetdesign

My Feedback: (8)

You can use something like the JR DS821 on ailerons - 88oz-in at 6 volts I think. They are a good entry-level servo - nice centering.

S.A.D.

Are you going to use your new 8FG?

platnuim1

My Feedback: (62)

Is this Ron?

Doug Cronkhite

My Feedback: (34)

I'm going with the following from Airtronics

Ailerons 94774 x 2
Elevator 94761 x 2
Rudder 94780 x 1

S.A.D.

it might be. a little birdy told me you got an 8FG.

dakoris73

Hey Guys;

I'm still new to the pattern scene, and I'm picking up an older Focus Sport pattern plane with an electric setup. However, I'm not sure what servos to consider, and wondering if regular standard servos would work, or is it better to spend the extra money on the higher priced servos? My budget is fairly limited and not sure I would be needing digital servos at this point. I'm still working on the Sportsman and possbility Intermediate sequences this year, but still want something that will help me be bit more competitive.

Thanks for the advise, and hoping to have a good season this year.

Mikey

serious power

Hi
So that is; 220 oz/in for aileron.
;120 oz/in for elevator.
;120 oz/in for rudder.
I don't get that.
Why nearly twice the power for aileron and why as much for elevator as for rudder.
The rudder has to move and hold a big flank of rear side area I'd have thought.

Brian

nonstoprc

My Feedback: (90)

These numbers are based on the servos used in my planes.

You need in total:
two aileron servos, each is at 100-120 oz-in.
two elevator servos, each is at 60 oz-in
one rudder servo, at 120 oz/in

EHFAI

Here're some data that may help you decide. I consider fast / constant servo speed critical for precise flight performance and choose the fastest servo with sufficient torque. I wanted more information as to what "sufficient torque actually was, so I designed an experiment in an effort to find out.

Peak current draw was measured in flight on each servo during high load maneuvers, snaps, knife edge loops, high speed rolls, etc. Servos were ofboth brushless & digital variety. Servo voltage was regulated 6v from a LiPo source and the aircraft was a typical 2m E powered pattern model. These data were then used to measure servo speeds under load at the same current draw on the bench using a rig built for this purpose.

A significant observation is that the servo speed is relatively constant until current draw reaches 60% of stall current. Probably a good target is 40% of stall current - here'redataobserved in this testing.

On elevators, 2 X 60 oz in servos exceeded 60% of stall current (SC) and slowed measurably - changing to 75 oz in servos dropped the current draw to around 50% SC and speed is maintained.

On ailerons, 2 x 128 oz in servos drew 30% of SC.

On rudder, 1 x 300 oz in servo drew 20% of SC.

Basis these results, it appears thatminimum digital servo torquefor this ariplane might be 150+ oz infor elevator, 75+for each aileron, and 100+for rudder to prevent servo slow down during stressful maneuvers. Be aware that linkage ratios will factor into this and that for this test ariplanethe elevator linkage amplifies surface movement to increase travel speed (which will also increase servo load.

Generally it's believed that stronger servoshave more "holding power" and I didn't address that issue in this experiment. My "feel" over the years is that the digital servosprovidemore solid flight which may be related to better holding power.It was noted that servo current draw isextremely lowin level flight incalm and veryrough air.

petec

My Feedback: (58)

You don't need digitals to get started, but there are some pretty cost effective digitals available these days. The most important thing is centering on the servo you are using. If the servos do not return back to center when you let go of the sticks you will spend a good amount of time chasing your plane around the sky. My first plane for pattern was an Excelleron 90 and my budget was even tighter than it is now but I thought new servos would be best. I bought new Hitec servos for it even though I was warned about the centering and learned the hard way. Next plane was upgraded to some used Futaba and JR servos that were suggested, analog servos like the 9202, and the trimming issues disappeared.

You'll find some good deals on used servos on the NSRCA list as a lot of guys will send theirs in for service pretty regularly so you wind up with a basically new servo for a lot less. If you want to look at new digitals for a very good price point check out the Airtronincs offerings.

serious power

Hi,
Interesting data.
Not what I would have perceived - rudder v elevator load wise.
Would it correct to say a graph of load v current draw would give an exponential curve with a pretty severe curve occurring near that 60% figure.
Nice to have some reference data.

Brian

OhD

Hi Earl,

We've talked about this before and I thought I understood your methodology. Now I'm not so sure. I would expect the peak current to occur at the instant the servo starts to move because it is truly stalled. This peak is pretty much independent of the load. It only lasts for milliseconds so your data rate may nor be fast enough to catch it. The current of interest occurs while it is moving and while it is "holding" against the load. This should be lower. Is this the peak current you measured? Can you see in the flight data how long it takes to get to the position where it is holding or are your speed observations made on your bench tests? I'd love to see a trace of your flight data.

Jim O

nonstoprc

My Feedback: (90)

I found this page interesting as it contains testing data for Futaba S9650, a popular elevator servo for e-powered planes. The max current drawn in the test case (2kg-cm) does happen when the servo tries to move. See the peak point (along the pink current draw curve) in the last chart.

http://www.ofremmi.info/F3A/Technic/...ba%20S9650.JPG

On the other hand, I think the stall current should be higher than the "start peak current", from my experience setting up multi-servo per control surface for a giant scale plane.

Earl,

I also wonder the need to increase the speed of elevator servo(s) in your experiment. It is quite common to restrict ATV for the elevators and even dial in negative exponential on top of it, to allow the elevator servo

1. to travel in +/- 12-15 degree max;
2. to be less throttle sensitive around center (= slower speed).

OhD

Earl, does your flight data look anything like Ola's bench data? His looks filtered and of course has a constant load, so the current looks fairly constant during the travel as does the speed. I would suspect that is not typical of flight loads but there are many variables.

Jim O

EHFAI

The question I originally set out to answer was "Are the servos strong enough so as not to result in control surface slowing while moving for a snap roll". My goal was to measure the servo current draw during this condition and use that info to study the speed of the control surface on a bench rig.

The elevator / ail snap set-up measured was a DR step function, stick activated, with minimal AFR limiting and no expo - pretty simply on/off (switch safety) max travel, which then transitions to normal behavior after the initial "hit". Initial observations were that current was high on the elv and "normal" on everything else. Knife edge loops were then flown to observe max rudder current draw which was relatively low. Measurements during normal F3A sequences were considerably lower, in fact, lower than I expected.

The same servo was then mounted on a rig and connected to linkage geometry / control surface dimensions as the airplane. The same current sensor was installed and the servo driven from the airplane RX using the same TX program used in flight. A variety of springs were attached to the "control surface" until the current signal / amplitude matched that observed in the airplane.

Another rig fitted with linkage / pot so as to provide a voltage output was connected to the "trailing edge" of the "control surface". The voltage output from this rig was fed into a PC stripchart (poor man's storage scope) which provided a means to measure travel vs. rate and can be expanded sufficiently to measure / observe the time from start / stop of movement. With this it was apparent that the rate slowed noticeably with the elevator servo and not with the other servos during the snap inputs. Changing the elevator servo to one 20% or so more powerful eliminated the slowdown. That's really the info I was interested in.

Jim, as I recall the current draws measured in "normal" flight maneuvers were consistent with Ola's data. The rig draws were higher, as the load was set for max observed flight current. I'll send you the flight current files, I don't recall the details (been a year or so). Most of the rig current set-up was done with real time observations that I didn't record. When I get a chance I'll post some of the chart recorder files.

Also interesting to set the rig up for "normal" loads and explore the different starting / speed characteristics of different servo types - but that's another story