How do you calculate aerodynamic forces on control surfaces? Will the ability to calculate the load help in servo selection? Do any websites have this information? If it involves calculus, tell me now and I will never ask again! I flunked calculus twice.

I will tell you my approach to it with models, no calculus, I passed but did not enjoy the trip through it, lots of pain involved - I look at what other guys have used and copy a successful setup.

Back in the good old days (may they forever be gone) we used to fly with very inadequate pulsing servos. The torque out of them was so low that the centering rubber band or spring would stall out at the largest throw - there was no force available at 45 degrees of throw. However we flew the airplanes of the time which had smaller surfaces and they worked OK. Even the smallest HS-50 has more torque than we used in those old pulse rigs.

The standard $12 modern servo by Futaba, JR, Hitec, etc. will work fine in normal large airplanes. I had them in a 100 inch span G38 gas powered Robin Hood - a high wing Curtis Robin semi scale model. They worked fine. I also use them in a Great Planes Ryan STA-M with 80 inch wing span.

However when it comes to the fire belching 3d monsters those guide lines are no longer valid. At that point I would look again to what the top guys are flying and then back off a little in cost.

http://www.coloradogliders.com/servo...calculator.htm

What a fantastic tool.

A 24 inch by 2 inch surface going 100mph needs only 26 oz-in.

The $9.95 Futaba is 44 oz-in with 4 volts. The super tiny HS-50 is 8 oz-in.

It kinda puts it all into perspective.

Thanks for the calculator.

Here is a bit more input--
the ailerons need little force -but because the lazy approach is individual (unlinked) surface control--- you need strong holding control
ailerons unload rapidly once the roll starts
On our foamies we have gone to one servo -linking both surfaces - even the tinist servo works well using 40 degrees up/down throws.
The elevators -- need most control and throw - pitch is the most demanding control----
The rudder on the crummy dsigns that abound require huge power inputs simply because most of these turkeys are way over stabilized in yaw.
On our 33% CAP- we have 4, (60 in ounce) on the ailerons (2 per side )
Elevators 1 8411SA per side (130 ea) - high power and fine resolution and zero slop
On the rudder - two 8411 metal geared (130 ea) - tied using a doubletree arrangement (ask an old farmer for explanation )
On the 50 % Pitss 8 aileron servos etc..
It is all on a case by case basis and no amount of calculus or trig or quantum theory - will do it .

Former Spad-

What type of kit are you buying servos for? The majority of "typical glow-fuel kits/ARFS" on the market will work just fine with "standard" servos. Most mfc's. are pretty good about suggesting alternate size servos if necessary.

The dynamic pressure model used on one the websites mentioned in one of the other posts works fine if you want to estimate servo torque. I wrote an article for RCM Magazine in 1998 based on dynamic pressure. Despite having a NASA engineer review my article before submitting it to the magazine, I received numerous letters from folks, without credentials, telling me I was wrong. The handfull of appreciation letters I received were from licensed Professional Engineers.

I guess I am a little surprised there are not a swarm of replies to this topic with differing viewpoints. Historically this has been one of those hot debate topics.

Good luck with your project!

I'm one of those Licensed professional engineers mentioned above. But I'm not about to dust off some old text and work a solution for something I have not looked at since college. The handy servo torque calculator is just the tool. But don't forget about friction. If you have pushrods that don't move freely, that's additive drag. I know that's not supposed to be, but I've had some that were pretty stiff when I made them go whee I needed them.

What a Tool to use!!! Thanks to who ever made it available for all the use!

Ben - a 24x2" surface going 100 mph needs 26 in ozs?
I maintain that unless you know how the attached panel can react, this info is of NO value .
Note my earlier input.
Was this "data" based on an unbalanced (dynamically or statically ) surface?
Just looking at blowback force?
Or is it an evaluation of force required to move a given panel size -on a NON reacting flight surface?
How would YOU use the info ?

Consider what the calculator was written for - high performance gliders. It wouldn't take into any consideration of unloading due to pitch or roll rates. So the final answer is non reacting surface but due to steady stream straight into the wing/tail part.

Note they have the following assumptions and notes

The angle of incidence of the wing, stab, or fuse is zero (relative to the airflow).
2. Angular velocity and acceleration of the aircraft is zero.
3. Air flow may be modelled using Bernoulli's equation for dynamic pressure.
4. Conditions are: sea level, zero humidity, moderate (~55 F) temperature.
5. Control linkages have zero offset at hingeline and are perpendicular to horns at neutral.
6. Control mechanisms are frictionless and surfaces are mass-balanced.
7. The wing, stab, fuse, and control surfaces are thin, flat slabs.
8. No aerodynamic counterbalances are used. (Account for these manually, if desired.)
9. The pushrods are significantly longer than the servo and control horns.
* The calculations are completely theoretical. No empirical "tweaking" has been done.
* The assumptions (except #6) should generally yield conservative (high) predicted torques.
* Extreme control throws are probably not practical at high speeds.
* This model is best used for comparisons. No guarantees are made of its validity.
* Maximum required servo torque may occur at LESS than maximum throw.

Those assumptions (which are pretty reasonable for this kind of work) would make it OK for everything except the 3D type of airplane. If you want the 3D type to always move the control surface regardless of what is happening, like falling backwards with rudder maxed out on one side and wanting to go to the other side it probably will under estimate for that. That is the reason I always give my advice to look around at other designs and copy if reasonable.

Run the case of your big rudders (not your's personally but the ones on your airplane -- sorry couldn't help it) and see how it compares to what you use.

OK -I will buy it --If that is the criteria -
But ---------
Even a conservative pattern model reacts quickly to inputs - so they unload .
If you are trying to hold a knife edge flight with NO assists - I would buy the info offered .
I get caught up in models whch are quite active - that is - they react instantly to inputs.
I once was flying a sim for a 727 ( the one use by FedEx )
I quickly caught on to the lead /lag of the plane -except for landing - I really could not get in sync with that .
On that type craft -with huge inertia involved - I understand that the force calcs would easily apply.
Now let's take a full scale EXTRA .
This plane at 200 mph can be rolled -instantly -using two fingers applying pressure .
The elevator also is very light but progressive - etc.
I have to take the pilot's word for it -cause I missed my moment of opportunity -and the plane was sold - rats!
So that is why I said what I did about having to know the criteria for the surfaces needing servo power .
the differences in aircraft is just too disparate.
Iwasn't just stirring the pot.

Sureeeeee you weren't. Yes the differences like the extra with balancing, aero balancing, etc make a difference.

Flying one of the really big beasts takes a heck of a lot of lead built into the pilot for sure.

I am hurt ---

Thanks for your responses. Appropriate servo selection is a question much easier asked than answered. I am flying 46 size 3D planes but I want to move up to 8 - 12 Lb 3D and IMAC style airplanes. From what I have figured out so far is that servo torque is only one of several factors to be considered. A trainer and 3D plane of same weight have very different servo needs. The trainer can get by with a slow servo with just enough torque. A 3D plane needs more torque, more speed, and better control resolution for more precise control and feel. With costs of servos ranging from $15 to $90 (reflecting improvements in technology), this is getting to be very difficult to figure out. Part of the fun of this hobby for me is to figure out the technical aspects, but going to the field and asking the guy next to you will probably always be the safe way. I found another servo calculator at R/C Aircraft Proving Grounds, but have not compared the two. Does any have any experience with Vexa Control ServoXciter? I have one on order.

If possible - switch to all digital servos -
don't worry about the big buck one$ but get 50-+ ozs torque ones .
These servos hold position far better than equal size or even much higher torque rated ones
They do eat more batt power but the holding power has to come from some scource!
On the size model you mentioned - JR 811's on 6v are just fine.
Be careful in selection of kits or arfs tho - -in this size model ,there are a lot of grossly overweight offerings -which look great but are real slugs.
As a rough giuideline - try to come up with a 1000 sq in model which weighs UNDER 10 lbs. or 1100 sq in under 13 lbs -or 800 sq in model under 7 lbs.
These parameters will allow for ligher wing loadings - which require LOWER servo forces to do maneuvering.
Ignore the looks of the model and favor the wing loading -whenever possible .

Use this and you will be done.

http://members.cox.net/evdesign/page...ge_design.html

More questions:

JLarson - are copies of your article still available?

In reference to balanced control surfaces - Does a small part of control surface in front of hinge line aeodynamically reduce torque needed to move control surface? Does the amount of force reduction correspond to the ratio of the areas/moments? Does mass balance refer to placing a weight on the small part in front of hinge line?

Dick Hanson - False marketing information is why I have started to look more deeply into this subject and why I want to start making my own measurements to verify. I have heard that using 6 volts requires use of a voltage regulator because the fully charged battery may be over 7 volts. What is risk of burning out servos/receivers with 6 volts? I enjoyed your comments last year about CG on weightless/overweight planes. I constantly have the same problem in my professional field because I get better results when I ignore convention. That drives them crazy! Are there situations where analog servos will work as well as digital, to save some money?

I will check out the calculators later, but for now off to work to pay for digital servos.

JLarson - are copies of your article still available?

Former Spad-

You will have to back order it from R/C Modeler or ask a buddy if he has the July '98 issue (I think that's the right year.) The calculator websites mentioned earlier are basically doing the same thing I talked about in my article and will give reasonable estimates.

You mentioned 8-12 lb aerobatic models, .60 to .90 size right? Like a Goldberg Extra or Suhkoi? Sure, digitals are going to feel tighter, the torque comes up right away. However, I think for your current projects you might be making this a little too cumbersome. Do you need them for that type of plane; that's personal prefrence. I've got a Midwest Extra on the bench and Carden Cuda in the box, they'll both get digitals. My other .90 size and smaller models all have standard BB servos.

It's not much different than the fact I would "like" to drive a Corvette to work, but reality is my Toyota is all I "need" and costs a fraction of a vette. When you get into the bigger size/$$ planes, you'll know you need/want more servo. Until then, as long as your having fun, I wouldn't worry about it.

Mr Matt,
What a great program. I am going to order Linkage Design. I will have a lot of fun with it. The more I study, the less I fly. The less I fly, the less I crash. The less I crash, the more money I can save for the next set of killer servos.