I'm re-configuring the nose with the new DX-18 and telemetry doo-dad. Since I have her up on the jacks I'm redoing the servo tray and internal layout, I'm bit fussy, ballast never really fit correctly, battery access is currently a pain.

I'm going with 809 for the rudder for sure, considering the same for the elevator with an 809 as well (currently both have 761's)

So I've seen talk of 809's on X 4.0 flaps, and I recall Darryl is using them all around. Seems it ought to handle a very light, sub 60 oz Supra elevator servo.

Comments?

Dan in Dallas...

I wouldn't. The 809 is a great aileron and rudder servo, but a little insurance on the stab is usually a good thing. Others here will undoubtedly say otherwise, but what is there to gain?

Tom

Originally Posted by Hostage-46

I'm re-configuring the nose with the new DX-18 and telemetry doo-dad. Since I have her up on the jacks I'm redoing the servo tray and internal layout, I'm bit fussy, ballast never really fit correctly, battery access is currently a pain.

I'm going with 809 for the rudder for sure, considering the same for the elevator with an 809 as well (currently both have 761's)

So I've seen talk of 809's on X 4.0 flaps, and I recall Darryl is using them all around. Seems it ought to handle a very light, sub 60 oz Supra elevator servo.

Comments?

Dan in Dallas...

i thought he was using them all round on a vtail model, tbh, not a Xtail. Correct me if I am wrong.

It's certainly going to result in an absolutely tiny overall weight reduction as the servos already sit well in front of the CG.

I agree with Tom... they're too light for my taste on elevator...

Jack

Rudder usually requires more torque than the elevator.

Alan

OK fair enough in the elevator....

Originally Posted by jh2rc97

Rudder usually requires more torque than the elevator.

Alan

Hmmm I beg to differ, but do tell.

It's not working against a power plant, that is, no pfactor or torque to work against. Deflection is minimal, especially at higher speeds.

I've never run an 809 on a full flying stab. For a Supra, the 809 is probably up to the task. Keep in mind a full flying stab is aerodynamically counterbalanced. But, since I will size an elevator servo arm for the throw I need... in excess... I tend to run an elevator servo with more torque.

Yes, I run 809's on the V tail of my Vixen and am having good success with them there.

D

I've been using 802 plastic gear servos on my rudders for the last two years, 2 Xplorers, and a Satori, no issue's.

Originally Posted by Hostage-46

OK fair enough in the elevator....



Hmmm I beg to differ, but do tell.

It's not working against a power plant, that is, no pfactor or torque to work against. Deflection is minimal, especially at higher speeds.

Lets start with the stab. Since I don't have mine here with me to measure here are the rough numbers I am using based on Vladimir's website.

1. Stab length 690mm.
2. Cord used for calc 58.5mm - this is based on being a full flying stab with the pivot point which looks to be at about 25% of the chord of the stab so the front 25% cancels out the second 25% there by giving us 50% of the chord as being loaded which is 1/2 of 117mm.
3. I use a max of 12mm of throw on the stab which is roughly 10 degrees.
4. Assuming most of the servo throw is used to get this 10 degrees of stab throw I used 25 degrees of servo throw out of the possible 30 degrees.
5. And I used a max speed of 120 mph which might be a little high but helps include a measure of safety.


Using a servo_caculator spreadsheet I found a few years back (which I can not find a name to give credit to in the spreadsheet) and just going with a simple setup these numbers show a need for 19 oz-in of torque for the stab.

Now for the rudder:
1. Length 350mm.
2. Average Chord 110mm.
3. For 20mm of deflection would be ~10 degrees and for 30mm would be ~13 degrees.

All this plugged in for 20mm deflection is 27 oz-in of torque and for 30mm is 47 oz-in of torque.

Of course these are theoretical calculations and the best things to make it easier on the servos are using the max throw of the servo to get just what is needed in control surface deflection and reduce as much friction as possible on the control rods, hinges, and anything else.

Alan

Alan

I have to tend to agree with you on this. I ran the Futaba s3156 servos on the elevator and rudder on my xplorer. My spread tow aspire uses one for rudder and ailerons. And I have never had a problem. If I ran airtronics I would not hesitate to run the 809 on elevator.

Joe

Gentlemen,

Please calculate in maybe 4 to 8 g's for stuffing the nose of your airplane on landing. Air loads and landing loads are two different animals.

Or a quick test is drop the nose of the sailplane vertically from 3 to 6 ft and see if the gears of your elevator servo survive, and you check them before the next flight.

It is not gravity but the stopping distance that is the b****.

What's the harm in a few missing teeth in the elevator servo gear train? There is always a chance that you'll spot it before your next zoom.

Exactly. I see nothing to gain here and everything to lose. Why do it (tiny elevator servo) just because you can?

Tom

I am always in for over engineering until weight or size becomes an issue. I use DS3717HVs in mine but I took the post as more of a question of if the 809 would work. For the elevator if we are looking at landing loads and assume an 8g load that is in the direction straight down the boom of the Supra. Then the load on the servo is mostly the weight of the pushrod times 8 on the control arm. I don't know right off but if the weight of the pushrod and clevis is an ounce then that is 8 oz of force and if the control arm is less than an inch long then that is less than 8 oz of force on the gear train. Personally I don't dork them in and even for testing would never drop a Supra on it's nose even from five or six inches.

Alan

I agree that having a bit of extra margin for safety on the stab is a good thing.
Although, it sounds like the OP is looking for space over weight savings.
Everything is good until you have a failure, and they last place you want one is on the pitch control.
I use the 11mm servos all the time for V-tailed F3B and F3F planes. They are never full flying tails, and normally don't get "doinked" as much.
They seem to work fine, and do save space for that application.

My advice? If you DO run an 809 for the stab, I would think about swapping it out every season, or taking it out regularly for inspection.
Also, you may be able to build in a little shock protection, depending on how you set up the linkage, that may help spare the servo gears from shock.
Example: a ball link bolted through the servo arm with a ball yoke snapped on it may offer some protection from stripping gears, since the arm may bend a bit with the pushrod offset from being in line with the servo arm.
Just some thoughts.

R,
Target