RC Radios, Transmitters, Receivers, Servos, gyros Discussion all about rc radios, transmitters, receivers, servos, etc.

Servo sizing question

Reply

Old 02-28-2019, 08:39 AM
  #1  
MishaParem
Thread Starter
 
Join Date: Dec 2005
Location: Fremont, CA
Posts: 16
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Servo sizing question

Hello everyone!

It's been a while since I've flown RC planes. Since last I flew I've gone to college and gotten my BS in Aerospace, and received some work experience in the field. It's made me question some previous decisions I used to make as a young pilot, and getting back into the game, I've realized a lot has changed regarding radios, receivers and servos. Back in the old days (yey, I get to say things like this now! =), I didn't have 2.4ghz radios or digital servos.

But, cutting to the chase - I used to use standard or high-torque servos on all my planes, but I'm wondering now - if I wanted to build a slow, stable plane for aerial photography and as a flying test bed for my experiments, could I in theory use gimpy microservos on the control surfaces? I don't really need quick response (except maybe during landings in gusty wind, yikes), all I need is a slow steady response to pitch, yaw, and roll. This means I can use a short arm on the microservo, a large arm on the control surface. My main concern, is that the aero loads (gusts/turbulence trying to deflect control surfaces) might strip the servos, or the servos might overheat trying to apply the input I gave them, against aero loads that are greater than their torque rating (assuming the gears don't strip first).

Anyone ever tried this?

Best regards,

Misha

PS the reason I want to use the smallest servos that can do the job, is that I want to leave the maximum amount of available weight for payload. Yeah, I know RC planes have ridiculous power-to-weight ratios compared to actual planes, but if I use four microservos instead of four standard servos, I can squeeze in a bigger battery (for instance) with all other things being equal.
MishaParem is offline  
Reply With Quote
Old 03-01-2019, 06:37 AM
  #2  
Hemikiller
My Feedback: (125)
 
Join Date: Dec 2001
Location: Killingworth, CT
Posts: 694
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default

Durability of the servo gears is the primary issue. While the torque rating may be similar, the shear strength of the smaller gears is a small fraction compared to that of a standard size servo.
Hemikiller is offline  
Reply With Quote
Old 03-01-2019, 02:18 PM
  #3  
Truckracer
My Feedback: (19)
 
Join Date: Apr 2004
Location: Des Moines, IA
Posts: 4,887
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default

Originally Posted by Hemikiller View Post
Durability of the servo gears is the primary issue. While the torque rating may be similar, the shear strength of the smaller gears is a small fraction compared to that of a standard size servo.
+1 to the above. Plenty of torque available in many of the modern micro servos but they may not be durable enough for your application. Will they have strong enough gearing to resist flutter, etc. You will have to make that call.
Truckracer is online now  
Reply With Quote
Old 03-06-2019, 11:11 AM
  #4  
MishaParem
Thread Starter
 
Join Date: Dec 2005
Location: Fremont, CA
Posts: 16
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default

Originally Posted by Truckracer View Post
+1 to the above. Plenty of torque available in many of the modern micro servos but they may not be durable enough for your application. Will they have strong enough gearing to resist flutter, etc. You will have to make that call.
Just an update - approximate info, subject to change:
max takeoff weight: circa 8kg (eyeballed, didn't really calculate as I won't be lifting that much.... yet)
design takeoff eight: 2kg (preliminary)
target empty eight: circa 1-1.5kg, haven't weighed the materials yet though.
cruise speed: 10m/s
wingspan: 2.5m
chord: 0.3m
ailerons: 0.4m x 0.03m (x2)
elevator: 0.4mx 0.03m
rudder: 0.2m x 0.03m (x2)

I ordered 9g servos, and a colleague donated some standard servos. I'll most likely stick to the 9g servos though - at 10m/s there's just not going to be that much buffet/flutter to begin with, and since I'll be using small deflections on the control surfaces (big arm on the surface, small arm on the servo), 9g should be more than enough.
MishaParem is offline  
Reply With Quote
Old 03-12-2019, 07:41 AM
  #5  
Dansy
My Feedback: (53)
 
Dansy's Avatar
 
Join Date: Mar 2002
Location: Prescott, Ont.
Posts: 1,336
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default

Some guy's are starting to run the MKS HV69 (a little bigger then micro) in each elevators on 2 meter pattern plane, those can also run ailerons on 2m bipe......In a slow plane like you describe these would be an overkill....so you would have a great cushion to protect your photo equipment. If I was building a new 2m pattern plane....I would used those in the elevators and a pair of HV747 on the ailerons, with a full size on the rudder.....

Advancement in servo's (not just MKS) has really been coming out in the last decade.

PS: I'm not sponsor by anybody....
Dansy is offline  
Reply With Quote

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service