BOBCAT servo's question?
 

This is what I am using in mine:
Elevators - JR 3301's
Rudders - JR 8411's
Ailerons - JR 8411's
Nose Gear - JR 4721
JR 460T Gyro,

Todd

BobCat
 

Sirrom,
Thanks for the explanation. I 'm sorry that you lost your BobCat, but know that if you only had one servo that stripped it's gears, it still would have crashed. That's why the metal geartrain is a good idea. Perhaps an 8411 or equivalent would be in order if you wish to go with the single elevator in your next BobCat.
We can all profit from this exchange. Using equipment that is stout enough to handle the loads should be foremost in our minds when we gear up.
When we go to meets, and place our ships on the runway for the noontime spectator lineup, we need to be aware that many spectators, especially the younger ones, don't have the knowledge or appreciation of the complexity and the fragility of our creations. How many control surfaces have you seen bumped by a spectator? I've seen several geartrains stripped this way.
This can and has cause the loss of some good airplanes. I won't use any plastic geared servo on an elevator or aileron again.


Harley Condra
BVM REP

BOBCAT servo's question?
 

Hmmm... lots of Bobcat owners are running each elevator half on a separate channel... so it seems to me that if someone wanted to prove that a single elevator-half working is sufficient to have a fair amount of control of the aircraft, it would not be all that hard to set up a swicth-activated mix such that one elevator half could be temporarily 'turned off' for testing purposes - then turned back on if necessary to save the model... ? This would leave the inactive servo in a fixed position, rather than flapping around as might be more realistic - but might nevertheless provide some level of feedback about the effectiveness of a single elevator half.

Just a thought.

Gordon

bobcat
 

Harley,
Fortunately this crash did not happen at a show. This was at my local flying field when this happened and it was the second flight of the day and about the 30th flight of the aircraft.

I am really considering going with single elevator with dual servos. I could set it up and test what would happen if I lost a servo to see if the other servo could out drive the damaged servo.


Gordon,
I wish somebody had thought of your idea before saying that dual elevators is for redundancy. I think that they would have found out if you lose one half of the elevator (especially if it is flapping in the breeze) your control of the aircraft is severly diminished.

patrick.

BOBCAT servo's question?
 

Did BVM design the dual elev servos for redudancy? If so, why did he not have dual servos on the ail(each side)? It would have been extremly easy to do so and hence have a more robust and redudndant system. I think JR9411 are the best choice for the BObcat all around including dual 9411 on the ail!!

On my XL, I am planning on 9411 on rudd and elev and 8411 singles on ail (since it is already built by a master scottsman!!!) otherwise I'd go with dual ail each side.

BOBCAT servo's question?
 

Because the fact that there are two ailerons means there is already redundancy. If one aileron comes off or the servo fails etc, you can usually still control the aircaft with the remaining aileron.

Just ask Reggie.... weren't you there the day he landed his massive C-47 with one aileron working while the other one trailed behind the aircraft, attached only by the 4-40 pushrod ? (hinges all pulled out; he didn't build it BTW)

That's Scotsman ya daft git ! Capital 'S' to show some respect, and only 1 't'. ;)

Gordon

BOBCAT servo's question?
 

It should also be noted that Patrick's Bobcat is the RAM500/Std. version. Those flying the heavier and more powerful XL versions should exercise even more caution. Patrick's elevator failure occurred about 5 minutes into the flight and up until then he seemed to have perfect control. It was also a very well built aircraft.

Patrick, hope to see you soon:)

bobcat
 

George,
Thanks for the clarification. I did forget to mention that mine is the 500 powered standard version.

Also, hopefully flying next tuesday. This past tuesday was clear and sunny but a 20 mph wind straight out of the north.

Patrick

BOBCAT servo's question?
 

Ok ok ok Gordon, so I was feeling generous and added a "T" and forgot the big s!!!

"Because the fact that there are two ailerons means there is already redundancy. If one aileron comes off or the servo fails etc, you can usually still control the aircaft with the remaining aileron. "

Not true in our jets. I have seen a Maverick lose one aileron and it flew straight till a turn was commanded and then... poof! spin into the ground, Adverse yaw effect came in. Heavy wing loading, slight wind and other factors caused that crash. You try your bobcat with one aileron ( via mixing like you suggested) if you truely believe you have redudancy!

Also, a C47 is designed to be flown with rudders and aileron, so there....it does not apply here!!

BOBCAT servo's question?
 

A buddy of mine was flying his BobCat when one of the elevator servos stripped in flight . He did not notice it until he pulled a loop and on the downhill leg the model barely pulled out...SEVERE pucker factor!!! The point is that he still pulled out of the loop and had plenty of elevator left to land easily. I would leave the split elevator system as is. Seeing is believing! Most everyone that I've talked to has gone to a stronger servo, both in gear train and torque than using the JR wing servos. I used the new Hitec digital wing servos in my HotSpot on the rudders with excellent results. All metal gear train with hardly any slop, metal arm, only 10 mm (3/8th's inch) thick, and they should drop right in to the BobCat with very minimal modifying. I think these have somewhere around 150 inch ounces of holding power.

Kevin

BOBCAT servo's question?
 

Kevin; are you talking about the hitec digi wing servo(# ends with 125)? If so, Are you sure the holding power is 150 oz.inch?

I am using hitec's now, and so far they have been relaible. I did notice that the end point travel changes one to two clicks on the subtrim with temp changes (75 to 60 F).

That's good to know that in his case the bobcat was landed with one elev...

BOBCAT servo's question?
 

Ehab,

I could be very wrong on the holding power of the Hitec---I thought that I saw in another thread the rep from Hitec stating that the holding power was about three times the output torque with their digitals. He was stating that the output torque was 50 inch ounces but the force to forcibly move the servo from a still position was about three times that amount. We're talking from memory here----I just turned 40 and it's been all down hill every since!!! The eyes went first...

Kevin

BOBCAT servo's question?
 

Kevin, You may be correct! because the moving torque is around 46 oz.inches at 4.8V, so " IF " the holding torque is three times, then you are not THAT old.....Yet!!!

BTW, I was telling my jet flying buddies that anything can go away as I get older as long as my sight and hands are still there for me to do the 200+ mph :stupid:

BOBCAT servo's question?
 

Ehab,

You are right---a big "IF".....

I've got a set of spring guages here at work that I might just try on these digitals. These spring guages are the type used to set the amount of spring tension on a roller. You just pull the guage until the roller (or in this case---the servo) moves and then look at the scale. The tricky part is where to measure on the servo arm. I would be willing to bet that the manufacturers use the inner most hole on the servo arm/wheel to get the best published torque figures. It sure would be nice to have a set standard---say 1/2" or 3/4" out from the center of the servo arm/wheel. That way we could we could have a set standard for us to go by. The spring guage technique is probably not all that scientific but if you test every servo the same way you will at least get an idea of how one servo rates to another regarding equal published torque figures. I will also need a constant voltage DC power supply to ensure that every servo is tested at the same voltage and a dial indicator to measure when the servo deflects. Anyone want to give me a grant to perform this study?!? :D

BOBCAT servo's question?
 

The hole you use does not matter, all of the specs are torque values (ounce-inches)


Mark

BOBCAT servo's question?
 

Meanmf,

I disagree---The further out on the servo arm you go the more leverage there is to move against the servo. This is why everyone highly recommends that you always use the inner most hole on the servo that you can and adjust your ATV for at least 125% for the best mechanical advantage.

Kevin

Torque
 

Torque is torque, it does not matter about the length.


If a servo produces say 100 ounce-inches at stall, then on the end of a 1 inch arm, it can produce a 100 ounce pull.

100 ounces*1 inch moment arm = 100 ounce-inches


With a 2 inch arm, it can produce a 50 ounce pull:

50 ounces*2 inch moment arm = 100 ounce-inches


with 1/2 inch arm, it can produce a 200 ounce pull:

200 ounce*.5 inch arm = 100 ounce-inches


The issue of arm length and ATV is resolution, you want to have the servo moving over as much of its travel as possible to minimize slop and increase resolution.

Mark

BOBCAT servo's question?
 

Mark,
Your chart makes sense when I read it, and your obviously more versed on this subject than I. But take a car for example;
engine has 500 ft lbs of torque and run it through a torque converter which multplies it by 2.5 resulting in 1250 ft lbs of torque. Then through the transmission with a ratio of 1.90 and now your up to 2375 ft lbs of torque. Now run it through the rear end with a ratio of say 4.88 and your torque is now 11,590 ft lbs of torque at the tires. Your initial 500 lbs of torque becomes almost 12000. Your right that torque is torque, but the book I am reading says that it can be multiplied.
I know a jet doesnt have transmissions and gear ratios and all that cool stuff, but isnt using different arms to achieve the best mechanical advantage increasing the torque at the flight control?
Sorry about the car analogy, but I am a knuckle dragger and that is all I know.

BOBCAT servo's question?
 

Mark,

I see what you mean but let me give you something to ponder....Let's say that JR rates a servo at 100 ounce inches of torque measured at 1" away from the center of the arm. Let's say that servo made by XYZ company also lists their servo at 100 ounce inches of torque but the measurement was taken at .5" away from the center of the arm. According to your chart wouldn't the JR servo actually be TWICE as powerful than brand XYZ? If you moved the measured position on the JR servo to the .5" position, like brand XYZ, would it not read 200 ounce inches? This is what I want to know---Where on the output arm do the servo manufacturers measure torque from?

Yes, adjusting the ATV does increase resolution. I was thinking about analogue servos when keeping the servos at 100% of their throw or greater. Digitals produce almost max power even right off of center where analogue servos don't make max torque until they are near max throw. However, linkages also play an important role in "torque multiplication" as Ultra Viper put it. To maintain max leverage on a control surface it is best to put the control rod on the inner most hole on the servo and on the outer most hole on the control surface. (While still maintaining enough control surface throw) Setting the ATV to 125% increases resolution AND allows the control rods to be positioned for the best mechanical advantage.

Kevin

BOBCAT servo's question?
 

Kevin,

Torque is a measure of rotational force and is the product of a force and a distance (ounce inches) . The amount of force (ounces) you get out of your servo is dependent on the length of the lever arm but the torque doesn't change. Nothing comes for free though. If you use an inner hole you get more force but over less linear distance.

You should try to use the longest servo arm you can with an appropriately sized horn at the control surface to deliver the required amount of throw while using 100% of your servo throw. Less slop and better resolution.

Hello UltraViper
 

Hello,

In this case the torque is multiplied, but the horsepower is not, right?

Its just like gearing up a little winch on the front of your truck. If you gear it up enough you can pull a house off of its foundation, but you have to wait.

Remember the power did not change, just the torque. There are a lot of powerful servos that are very slow, because of the gearing.

Mark

Torque
 

It does not matter what the servo arm distance was when measured.

If one company measures 100 ounces at 1 inch, that is the same as measuring 50 ounces at 2 inches or 200 ounces at .5 inches.


If a servo made by XYZ lists there servo at 100 ounce-inches, but made the measurement with a .5 inches lever arm, then they measured 200 ounces FORCE at the .5 inch lever arm, which is still 100 ounce inches of TORQUE.

Mark

This could take awhile!!
 

Here is a pic I drew. It will take a while to describe it so I may come back to it tomorow.

remember

Moment = radius * Applied Force

Or

"Torque (Oz-In) = Arm length (In.) * Force on the surface"

In our linkage systems we have two different moments we need to consider.
1. The moment applied on the servo
2. The moment applied on the control surface

The torque of a servo is given in (Oz.-in.) nomatter how far away from the center you read. It in the end relates to (Oz. - In).

The length of your control arm on the surface and the size of the control surface determine how much load is place on the push rod either in compression or tension.

The ratio of the lengths between the servo arm and the control horn determine how the load gets multiplied to the servo. (Can be seen in sample 3,4)

You may notice that the longer both the control horn and servo arm are the less load is applied in the push rod. This is not necessarily the moment (torque) applied to the servo.
Sample 1,2)


As you can see the worst set up you could have would be case 4. It could be prone to flutter. The slop in the linkages are multiplied to the control surface. The servo would likely have to be run in the 50-60% ATV range

The best system would be #3. This in moest planes would require the servo to be operated in the 100-125% ATV


I will stop here for tonight. If I am way off let me know. Im not saying this is the gospel, you don't have to beleive me. But the theory is right out of my Dynamics book.


Session entry #2

As you can see. If you have a really big control surface, and a short control horn you create large stress on your push rod.

In the Giant scale planes, I beleive they use really long control horns and servo arms. Thus reducing the load on the push rod. (Less bending, Less Chance of failure do to fatigue)

I feel the longer you can make your control horns and servo arms while mantaining a proper control deflection/servo movement ratio, the better off you are.
Now we have to be realistic about this too! We don't want or need 2.5 inch control horns on our jets!

This will make for good chatting!
Later
Dustin

BOBCAT servo's question?
 

The book I am reading says that

Force x Moment Arm=Torque.

I think you guys are saying Torque X Moment Arm= Force and therefore Torque doesnt change, only force changes. This seems to contradict the book definition of torque.

Hello
 

I looked back at what I wrote:

100 ounces*1 inch moment arm = 100 ounce-inches

That is:

Force X Moment = Torque

or:

Force = Torque/Moment


Mark