musterpilot

G'day guys been a ex retired ag-mustering pilot why do I hear so much about the pounds of pull required by servos to move a control surface
If I had to pull x amount of pounds every time I pulled back on the stick of a a/c I reckon I would be tuckered out within a couple of hours and have arms like popeye
I think we go over board a little bit ,I hear guys say you have to have eg 8 kg of torque to pull a elevator servo boy if I had to pull that much weight on a real a/c without hydraulic assistance I would last about 1 hour flying can anyone explain the reason for giant servos I have been flying r/c since the early 1970's and have always used standard servos even in my large scale planes without a problem any response would be appreaciated

BuschBarber

My Feedback: (2)

Modern Aerobatic aircraft and especially 3D aircraft need plenty of torque to Hold the control surfaces in violent maneuvers. If you are flying a Scale Warbird, then maybe Standard servos will work. 50cc powered aircraft and above need at least 100 oz/in of torque. There is 16oz/lb and 2.2lb/kg. 40% birds are using multiple 200-400oz/in servos. There are calculators you can use to estimate the Torque needed.

If you do not fly in competition, it may not be as important to hold a line in a maneuver. It is important when you lose an aircraft because the servo gears stripped or the servo failed because the load was too great. Also, if you have servos that can easily handle the load, they last longer.

rmh

A LOT of the power used, is simply to fight off the high loadsimposed by engine vibes and the large unbalanced mas in many large models
worse yet the aileron setup on the big aerobats is terrible . the ailerons are not interlinked (side to side) so ech aileron must stay centered under load - through servo holding power
On a full scale aerobat for example the EXTRAS - aileron input by the pilot only requires "two fingers"- the rudder and elevators of course need a firm input

Rodney

With a bit of effort, you can get by with the lower powered servos, even on a big plane. Use boost tabs; keep them at about 6 to 8% of the area of the surface being boosted, to large and they will try to oscillate about the neutral position, to small and not enough boost.

XJet

You confuse force with torque.

The torque is measured in oz-inches so a typical hi-torque servo might have (say) 160 oz/in of torque. 160oz is 10lbs but that's the force the servo exerts at a distance of 1 inch from the shaft-center.

Now look at the control stick on a full-sized aircraft (of a type that has a stick)....

The stick itself will likely be at least 20 inches long so, to create the same toque as your RC servo (10lbs at 1 inch) you'd only have to apply 1/20th of that actual force at 20 inches ie: 8oz or a mere 1/2lb.

As others have stated, modern RC models are grossly overpowered compared to their full-sized counterparts and many types have incredibly large control surfaces with little aerodynamic balancing and no mass-balancing. This means that high torque levels are required for safe operation.

Flypaper 2

I put servo tabs on my old beater 1/4 scale Corby Starlet. Futaba 148s in the wings. 10 % of the ail area. Started moving in one hole at a time on the tab horn. When I hit the magic spot, it was as if I'd put the next size faster, stronger servo in it. As Rodney said don't go in another hole.