72" B-17 Servos

[chris henton]

Hello all,

 

I am new to the traditional building side of things and am currently building the Tony Nijhuis 72" B-17.

Could sombody recomend the make and type of servo that would be suitable?

Just mid ranged brand's please as i am still waiting to win the lottery!

I have purchased Air retracts and also require a servo to operate this aswell as all the usual functions.

 

Thanks in advance.

 

Chris

[Tim Mackey]

Take a look HERE Chris...may help with this - and future projects.

[chris henton]

Hi Tim,

 

I did come across this before and is very handy to givean idea of the general operating perfomance/specification of the servo.

But how do i work out what torque i require from the servo for the particualr model in question?

I suppose space is a major influncing factor?

 

Regards

 

Chris

[Tim Mackey]

Chris have you checked THIS - I have just added the link a while back to the above thread.

[chris henton]

Hi Tim,

 

Thanks again, not being a mathamtical genius i may struggle to work out contol surface chord and surface angle etc. I will try and give it a go, never say never!

If all else fails i will hold out until somebody else who has built the B-17 replies .........hopefully.

 

Cheers

 

Chris

[andy watson]

Now this might be a stupid way of doing it.........but.

 

Compare the size of the control surfaces with your normal hack. Look at the torque ratings you have there. Then if the surfaces on the B17 are double the hack- double the torque on the B17 surfaces.

 

Simples.

 

I would suggest this will probably give you miles over specced servoes for the following resons:

1) I believe most hacks are over specced anyway

2) Most hacks are flown far harder than a B17.

 

I would be confident using this method for one of my scale builds.

[chris henton]

Hi Andy,

 

Now that seams like a good idea and even my maths can manage basic multiplication to work that out. I think i will see what servos are in my PZ Wildcat and see what the outcome is.

I might drop Tony an email asking what servo's he recommends just to confirm my findings..

 

Cheers

[Tim Mackey]

I never bother messing with the surface angles and the like - just leave them at the default setting

I just enter proposed max speed in MPH, the chord, and the length of each control surface, and hit the calculate button.

[Tim Mackey]

Heres the results I got when i was planning the servos for my PC9.

The model weighed over 12lbs, and had a span of 70 inches.

Expected speed = 60MPH

Average length and chord of...

Ailerons = 38 X 5 servo torque required = 12oz-in

Elevators = 60 X 8 = 48 .5

Rudder = 29 X 11 = 36.2

 

Several mini servos give this easily - and more, for instance

Hitec HS82 @ 39 oz-in .....or the HS85 @ 49 oz-in

[chris henton]

Good afternoon,

 

Tim i will try and get all measured up over the weekend and use the calcualator.

 

I will post up the results when i have them.

 

cheers

[chris henton]

Howdy,
I sent mr Nijhuis a mail asking the question and got a very speedy reply of 1.5 - 2.0kg servos.
So next question of which brand?
Tim I'm guessing you favour Hitec and the price does look to be good?

Cheers Chris

[Tim Mackey]

HS65 would fit the bill then @ 27oz-in ( approx 1.9kg-cm )

if you run 6V then this increases to 31oz.

Yup, always liked HiTec servos but that's not to say plenty of other good stuff around these days.

I just think they are decent performance for reasonable cost.

[chris henton]

Cheers Tim, ill get searching for a deal on a set.
Chris