Determining Servo Sizes

[atjurhs]

Hey Guys,

The question at hand is how big of a servo do I need, without going too big?

We all want to keep our planes lite, so we don't want to use too large of servos, but we also don't want to use too small of a servo either else we will strip gears or have diminshed control surface performance.

Sure, if the plane you're building has been around for a long time then you can get in line behind all the other ducklings and follow the well established path. Good thing Dr. Drela didn't take that approach! And what if your plane/design hasn't been around for a while?

So, unless somebody already knows of an existing computational tool, I'm thinking it's just physics, and we ought to be able to calculate how much torque is required. The parameters involved should include:

- area of the wing and the deflected surface
- degree of deflection
- servo arm lengths on both the servo and the control surface
- speed of the plane and/or force under tow

Any other ideas as to what other parameters?

[BMatthews]

Do a google for "servo load calculator". There's a few sites out there where you input all that stuff and it tells you how much torque they require.

But you don't want to run the servo that close to the stall point. It's hard on the servo and you're risking early failure. My own rule of thumb is to take the numbers the calculator gives you and double it. That's the minimum rated output torque you should look for.

This also assumes virtually zero friction and binding in the control system. For direct coupled short rods you can expect it to be negligable. If you're using cable in sleeve systems I'd add in another 50% for the servo.

[atjurhs]

All of the references that I found all point back to the same website at "Electric Flight at Colorado". The website does point out the theory for it's derivation, but it assumes a control surface deflection while the plane undergoes a constant velocity.

The situation that I believe to be the most stressfull for full-house sailplanes is the launch. Build up as much tension as you can physically hold, deflect your flaps and ailerons to 70degrees or so, and let go. That's a huge amount of force applied in a very short time span. This sort of scenario is not answered by the Electric Flight Website.

Time for a little more physics to be applied I think.....

[O.L. Adcock]

If it's the same site I looked at a few months ago, it suggests servos WAY stronger then common use suggests is adaquate. 40+ in/oz should be more then enough on any thermal ship. More never hurts. Metal gears is a good idea for flaps however I used plastic gears for years and never stripped one......O.L.

[BMatthews]

Just replace that constant velocity with the expected max velocity you think you'll encounter during your launch. I don't see what the issue is.

I always assumed in looking at and playing with those calculators that you would automatically just input the expected maximum velocity. The servo doesn't care if it's steady state or a momentary max. It's still got to be able to deal with the max loads and stay within a decent safety zone.

[nuevo]

The situation that I believe to be the most stressfull for full-house sailplanes is the launch. Build up as much tension as you can physically hold, deflect your flaps and ailerons to 70degrees or so, and let go.

70 degrees?? Yikes! :eek: I hope that's a typo.

the spreadsheet I use is attached to this post.

[atjurhs]

Just replace that constant velocity with the expected max velocity you think you'll encounter during your launch. I don't see what the issue is.


The issue is the plane's wing, ailerons, and flaps are experiencing enormous amounts of load (forces) while at an extremely low velocities.

Just think about a typical launch scenario. Ailerons and flaps are significantly depressed (ok, mabe not to 70degrees, but I'd guess at least to 50), tension is built-up in the tow line till you just can't hold it any longer, and then you stand on that pedal. I know I've broken several tow-lines that were rated for 200lb. or more. So what is "the expected max velocity you think you'll encounter during your launch ". I'd say it's small in comparisson to the forces involved. What I'm saying is that during launch conditions, the velocity is not the issue. It's the force. The same thing is true on landings just in the opposite direction. You go from say 10mph to 0mph in a very short distance with the flaps fully deployed. Yep, it's going to strip them. Why, not from the velocities, but from the forces.

Jon, thanks for that spreadsheet! I didn't find it in my searching last night. I'll see if it adresses and can model a sailplanes's launch conditions.

[nuevo]

On terminology, I assume 0 deflection is ailerons/flaps straight back in their normal flying position. That said, I consider typical trailing edge deflection to be 20 degrees on launch. Think about what flaps dropped 50-70 degrees look like.

Assume 55mph during launch, before the zoom.

The velocity is very much an issue, as it is a large determinant of force. Think about it this way. On landing, how much force is at a 90 degree flap at zero velocity? Almost none, since the flap is just hanging straight down. Now how much force when the plane is going 25 mph? I'd say quite a bit more. Experiment with the spreadsheet to confirm what I am saying.

[TheNightowl]

I have to say that I found the site, tried to look at what I need, but I'm stuck in that there are no clubs anywhere close, I'm new to sailplanes, and I have no idea what velocity to put in to figure calculations. Closest I could come to "guesstimating" it is when people talk about turning upwind and returning to the field when they're flying in 15-20 mile per hour winds, and assuming SOME forward momentum of say 5-10 m.p.h. Does anyone know of a site, book, experience that says for instance an Oly II flies 30 m.p.h. or a Gentle Lady flies 20 m.p.h.? I just have no good reference point to start from.
Nightowl

[O.L. Adcock]

Quote:
Originally Posted by BMatthews
Just replace that constant velocity with the expected max velocity you think you'll encounter during your launch. I don't see what the issue is."

Me either??? Nuevo, I played with some numbers in your program and it appears to be well within reason. I know from experience that you can fly 1/3 scale ships that weight 15-18 pounds on "standard" servos with no problem as well as the hottest "thermal" ships. Get into slope racing and DS, the dynamics change by the square of the velocity as well as the "way" the planes are flown.....O.L.

[nuevo]

BTW, the spreadsheet was written by Craig Tenney. A google search for Servo Calculator Craig Tenney (http://www.google.com/search?q=Servo+Calculator+Craig+Tenney) turns up a few more copies.