Jetdesign

My Feedback: (8)

I just read a report posted in the pattern forum. A pilot flew a 2 meter pattern plane through the FAI sequence and recorded the draw between each individual servo and the receiver. The highest recorded draw by the rudder during a knife-edge loop, translated to only 21oz-in of torque! There are not many replies to the thread, so I wanted to hear some other input on this test/information. Is there really no need for more than 90oz-in servos in my planes?

Here is the [link=http://www.rcuniverse.com/forum/m_8189267/tm.htm]thread[/link] and here is the [link=http://www.ofremmi.info/F3A/Technic/Flight_testing/servo_flight.htm]report[/link]

I was just about to get some 130 oz-in servos for my 120 Yak rudder and elevator, but now I might just get some more $30 DS821's?!

The Ghost

Its not so much the moving power of the servo, but the holding power from the servo, or the ability of the servo to hold the surface in the position required which is more that the amount required to move the surface.
I have done AMP draw tests with servos and there was more current draw to hold the servo in a position than to move it to the position in the first place. Yes it was loaded up with tension in the first place before any movement.

Cheers

ChuckW

The person who did the actual testing would be better qualified than I am but I don't see how you are coming up with that torque figure. The testing looks useful to me for determining demand on a battery during flight but doesn't necessarily translate into torque requirements. I know there are some known loads applied and current measured but, while I am not 100% positive, I imagine it gets a little more complicated than that. I've done a lot of current monitoring and other tests on electric forklift trucks and it never turns out as simple as it seems on the surface.

Sounds like a good project too me... devise a way to measure actual force required to move a control surface and hold it there on various airplanes at various speeds using various linkage geometry, etc. Maybe in a wind tunnel? Then test various servos in the application and see how they compare?

There's a lot of things in this hobby (and the world in general) just accepted as fact. Most likely is true but I'm sure some of it may turn out to be a little different if actually put to the test.

whstlngdeath

My Feedback: (15)

I would think that the area of the surface being tested has a huge bearing on the amount of torque/current draw required of the servo. A 30% YAK rudder is much larger than that of a comparable sized pattern plane. The holding force of the rudder servo in the YAK would have to work harder to hold that rudder than would a servo holding the pattern plane rudder during a similar maneuver, such as a knife edge. Consider this analogy: a person trying to hold a standard size house door from flinging open in a 40mph wind, compared to the same person trying to hold a barn door in the same wind. The barn door would require much more force to hold than would the smaller door. I guess what I'm saying, is that a better test result can be obtained from testing different aircraft with different flight characteristics.

Jesse

ChuckW

Agreed, the bigger the surface area exposed to the airflow, the more force acting on it and the more force from the servo and linkage required to overcome that.

Jetdesign

My Feedback: (8)

My Yak is still in the box, not sure how much bigger the surface is; Venus is not small, but not 3D.

The author of the report talked about comparing the electrical load of the servo during flight to a static load on a bench - that's where I got the 21 oz-in (converted from 1.5 kg-cm):

and he measured 200-250ma draw for stall turn.

I don't mean to be doubting the masses, but the report does have me thinking. Even if the load was 4 times what he tested, it still would be under the 88oz-in of a DS821.

Last thing is I have a hard time wrapping my head around the 'more power to hold the servo' idea. Everything I've been learning in school for the past 3 years talks about forces being greater during acceleration. I haven't taken electrical classes yet but I'm not sure if it's an electrical thing. For the barn door example, I think it would be easier to hold the barn door steady than to try to move it against a heavy wind. If it wasn't finals week I'd definitely be drawing a good FBD right now

Does anyone have any experience with servos failing under load simply due to too high of a load?

The Ghost

YES, usually just stripped gears, but have had the little metal gear on the motor come loose with metal gear sets.

Cheers

ChuckW

Nothing wrong with thinking, it would definitely be worth looking into. Even if your thought on the torque requirement turned out to be wrong, something else useful might be discovered.

If that is in fact the case, I think this has to do with the fact that a servo contains an electric motor. Any electric motor will use more current at zero or low RPM (with a load) than when they are rotating at faster speeds.

On a brushed motor the current passes from one brush, through the commutator then through the other brush. If that commutator is not turning, quite a bit of current can begin to flow. Once it is rotating, brush to commutator bar contact is shorter in time so less current flows. I'm sure RPM and brush to commutator contact frequency come into play as well.

Brushless induction motors act in a similar manner because, from a dead stop, more current has to be applied to them to develop a magnetic field strong enough to get the rotor and load attached to it rotating. Once it is turning, the current demand drops.

Ever listen to a big fan or other electric motor when you start it up? It kind of hums and strains at first until it gets going. If you graph the current, it has a sharp initial spike then drops in a smooth curve to a reasonable level as the motor begins rotating. Prevent that motor from rotating somehow and the current spikes back up quickly and stays there. I've seen this dozens of time testing electric lift trucks while driving them, climbing on ramps and then staling them on ramps.

Back to your original point though... I wonder if anyone has actually measured the force required to move the control surfaces on various airplanes or do we just automatically assume that we need the biggest, baddest servo we can find? It would be interesting to find out.

-pkh-

My Feedback: (2)

I read his report, and he is ignoring the peaks, where the servos must make a fast transition. I see peaks of up to 700mA in his data plots, so why he thinks he only needs the torque corresponding to 400mA makes no sense.

I believe the torque ratings on servos are measured at the stall point, so if you need only 21oz-in of torque to hold it static, you'll need more torque than that to move it.

ChuckW

Yah, I saw the 700mA on the aileron and around 650mA peak on the rudder too.

I think the best way to determine what servo is actually needed to move a given control surface on a specific airplane at a given airspeed is to actually measure required mechanical force. Then take a typical control linkage that would be used and determine what the servo torque is required duplicate the necessary force using that linkage. There's a lot of factors involved.

Jetdesign

My Feedback: (8)

Sounds like I'm still getting the 8411's

ChuckW

I just looked at the Yak manual. If it says 130oz-in servos, I'd trust them on it. Does it have to be digital? Maybe, maybe not. Hitec has some good options either way:

http://www3.towerhobbies.com/cgi-bin...?&I=LXUZ89&P=0

http://www3.towerhobbies.com/cgi-bin...?&I=LXUZ80&P=0

Not surehow these compare in cost to the JR's.

Jetdesign

My Feedback: (8)

Yep. 130oz is 'recommended' in the manual.

The pattern side of me says Hitec's won't work. I could get a programmer, but for the cost of that plus servos I might as well just stick with JR. I've already had centering issues with analogs so not interested there.

Thanks for the thoughts though. I"m taking my EE classes in a year from now; I wish I could incorporate a study like this into the class, but at least I should have the understanding to do such a study. That is, if I'm not trying to redesign airfoils

ChuckW

Just out of curiosity, why wouldn't the Hitecs work? I see a lot of very good flyers with high-end airplanes using them. Not doubting you, just wondering. All my stuff is just sport and general aerobatic so I haven't checked out everything out there.

I've also had centering issues with some of the basic analog servos but the higher quality ones seem to work very well. I do hear that the digitals hold better though under a load. I have a number of digitals but I've never really taken the time to evaluate any differences in performance.

Clint H

My Feedback: (1)

Something else to consider is the amount of travel required. A pattern aircraft doesn’t need the extreme throws that a 3D aircraft would. A long servo arm is usually needed in order to achieve these large control surface movements. Torque is force applied on a lever arm; so, if you change the lever arm, you change the needed torque. Correct me if I’m wrong, but I believe a 100-oz/in servo can deliver a force of 100 oz at a moment arm of 1 in. If that arm is increased to two inches, the servo will only provide 50 oz of force but at a greater range of travel.

-Clint

Jetdesign

My Feedback: (8)

Hitecs have a deadband that is set to not center as accurately as JR and Futaba servos. I've been told that this is done on purpose for surfaces that require two servos, so the servos are not fighting each other over a center position. This is adjustable with a servo programmer. Hitec doesn't make a 'bad' servo, but the experienced pattern guys I've been working with say there's a noticeable difference between the brands, and most of them won't use Hitec servos on competitive planes. The Yak's not a pattern ship but I figured I'd still be working on precision with it.

I guess I could decide to just set it up completely different and not worry too much about it.

Clint: Excellent point. The large servo arms and horns definitely amplify the amount of torque needed. The test was done on a pattern ship, and usually they've got the control rods mounted as close to the servo as possible to reduce slop and increase accuracy.

ChuckW

I was not aware of that. I guess it probably isn't noticeable for most types of flying.

As for the control horns, that was my point about various linkage designs. They could either give you a mechanical advantage or perhaps even a disadvantage depending on how it is set up. That will dictate servo power requirements as well.

Like ClintH said, with the linkage on the very tip of the servo horn and closer to the hinge on the surface, you'd get maximum travel, ideal for 3D but you'll be at a mechanical disadvantage and require a stronger servo. flip that arrangement around though with the linkage close to the servo shaft and at the tip of the surface horn and you have a mechanical advantage. Less servo power will be required to do the job but there will be less throw.

For every advantage, there is generally a corresponding disadvantage. Designing anything is sometimes a bunch of compromises to ultimately achieve a goal.

Jetdesign

My Feedback: (8)

Yeah a lot of guys said you wouldn't notice unless you switched from a Hitec to something like a JR, then you'd feel the difference. I can't believe how hooked I got on pattern - I used to love burning holes in the sky, now everything I do is pattern based. I thought about just having the Yak setup for 3D/fun, but I'm really just interested in having two planes setup to fly precision. Hopefully one will be for pattern and one for IMAC (even though it's small).