Are these servos good enough ?

[NigelH]

I've just bought a second hand E-Flite Beaver

 

http://www.e-fliterc.com/Products/ProductGallery.aspx?ProdID=EFL4525

 

The manual asks for six MN48 servos, which is more or less an HS-225.

 

http://www.e-fliterc.com/ProdInfo/Files/EFL4525_DHC-2_Beaver_Manual-LoRes.pdf

 

I intend to buy a couple of HS225s for the tail but the wing is already full of Turnigy 1160A servos

 

http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idproduct=7562

 

One UK vendor actually states -

 

Radio: 6+ Channels with 6 mini servos

Servos: (6) JR Sport MN-48 (US Spec) or JR NES331 or Hitec HS81/82/85

I know that the new servos are good for the tail but I wonder if those in the wings are up to the job as I don't need to be unnecessarily spending extra money ?

Edited By Nigel Hunter on 13/09/2010 19:22:25

[David Ashby - Moderator]

The Wing Turnigies are 2.1kg torque units Nigel so yes, they'll be fine for an easy going flyer like the Beaver.  

 

I've had 2kg torque units in my 50" span electric aerobats these last few years and they've been comfortably up to the task.  

[NigelH]

I was looking at those servos on Hobby King again today and noticed that everyone was saying that the gears were stripping. I'm wondering if I should replace them with the HS81

 

http://www.inwoodmodels.com/product_info.php?products_id=3800&osCsid=c5a8edf4d5a5c003a5b42ae3ed0890d0

 

The Beaver has flown ok but I'm not sure if it's wise to fly a 6.5lb model with those HK servos on the ailerons and flaps.

 

Edited By NigelH on 05/02/2011 15:52:21

[Biggles' Elder Brother - Moderator]

Aileron servos are a difficult topic. I had a lot of problems with the aileron servos on my Pitts special. It not large - only 50" span or so - but it was stripping servo gears like noone's business - and I'm not talking throwing it about, I could strip mini-servo gears just with some very mild aerobatics! Eventually I bit the bullet and installed metal geared servos - HS82MG's - and the problem has gone away. Note that the HS81MG is no longer available, replaced by the HS82MG.

 

Now OK on the Pitts the servo is driving two ailerons (top and bottom) and it is a much more aerobatic model than a Beaver. Its also quite a heavy model, so that doesn't help.

 

You have them on the flaps as well - how much flap are you deploying to land? If its over 60 degrees then the forces on them would be very high - even on a Beaver - especially one weighing in at 61/2 pounds.

 

My experience is that you would be unlucky to lose both aileron servos at the same time, and a model is perfectly flyable on one aileron. So I'd be tempted to stick with the ones on here but watch them carefully for the first few flights. If one goes obviously replace the pair. Regarding the flaps I'd would probably replace the plastic geared servos if I was going to use larger flap angles on landing. Loosing one flap on approach would cause you big problems! Better to be safe than sorry.

 

BEB

[Tim Mackey]

Most online servo calculators suggest that the models maximum speed, and the actual size of the control surfaces are the key factors in deciding torque required, rather than the models weight.

Here is an example - its the basis I use when deciding on servo selection.

[Tim Mackey]

I dont want to hijack the thread - so I am starting a new one to discuss "choosing the right servo". I did do something about servo selection some time back, so I ll try and find that too.

[Biggles' Elder Brother - Moderator]

Totally agree on your point on speed and control surface size Tim - put with flaps weight does come in to it because on decent it can be the factor that is deciding the airspeed. Heavier models tend to come in steeper and so faster - increasing the force on the flaps especially at high angles where their role is chiefly a retartation function.

 

BEB

[Tom Wright 2]

Yes speed and control surface area are the main factor here,I have at least 50 of the TG9e servos ,many are in 6ft models with very large ailerons ,but all the models are slow flying with wing loadings under 16oz/sq/ft. I have never had a in flight failure. {so far!}

[Tim Mackey]

Interesting mention of the wingloading there Tom - I wonder if that should be factored in when choosing a servo - the charts I use never mention it!

[Tom Wright 2]

Perhaps most think heavy a/c =high wing loading ,light a/c = low wing loading,or big a/c =high wing loading etc .But of course we know it ain't necessarily so.

This could get complicated as a light a/c with a low wing loading can be capable of high speed depending on the drag,and aoa,such a a/c may need better than 9gm servos.

Edited By tom wright 2 on 05/02/2011 19:43:04

[Steve W-O]

Posted by Tim Mackey - Administrator on 05/02/2011 18:35:48:
Interesting mention of the wingloading there Tom - I wonder if that should be factored in when choosing a servo - the charts I use never mention it!

 

 

I think it could be one valid factor. It seems that most of the calculations give the force on a control surface as if that surface was mounted on a a solid structure with air moving over it.

A lighter, or lower wing loaded plane would be deflected by the force of the air flowing, so the force on the control surface would never reach the figure calculated as if it couldn't move.

[Braddock, VC]

My own view is that speed and size of control surfaces are two of the main issues, also the maximum angle of the surface may have an impact.

Everytime I've had the inkling to use servo sizing software futaba 148s/3003s/3001s and 3004 have been well able to provide the necessary output.

Some models I choose metal gears simply because they are more resistance to damage during transport. I've noticed that the motor drive gear is almost always nylon on metal geared servos so if you do manage to strip the gear there's a higher cost for a replacement set as opposed to the standard nylon gears so I've started using the hitec karbonite geared servos from giant cod.

Incidentally the servo spec for my chris foss xtrawot is for standard servos all round and that is straight from CF, for a 71" span 11pound model with a 180 2 stroke up front.

[Myron Beaumont]

For those that design their own airframes ,a good way of relieving stress caused by moving surfaces is of course to have a proportion of the moving control ( eg rudder commonly )projecting out in front of the hinge line (full chord that is at tips) to help out the balancing of the aerodynamic forces involved .I would think about 5 to 10 % of the area as a general rule of the moving component , With ailerons a small hanging weight on a stalk projecting forwards from the hinge line will help prevent flutter that is more likely with my suggestion IMHO & experience

If It's good enough for full size light aircraft ,it's good emough for us surely

Myron (4 small steps without crutches -4 giant leaps for a terrestial modeller)

[Tom Wright 2]

That seems to be a neat summary Tim,this fits the test flying i have done with large but light a/c using 9 gram servos.

 

Good point Myron ......reducing control surface loads that way will reduce the load on the servo.Cor can we use even smaller servos.

Edited By tom wright 2 on 05/02/2011 20:25:02