Bobcat elevator servos
 

Anyone switched to a higher torque servo in the elevator on the Bobcat? Just bought two 8411's. They are too tall(wide when installed) to fit between the horn and one of the ribs in the stab. Anyone else run into this? I guess I need a shorter servo or have to put a kink in the control rods.

Mike Jensen

Bobcat elevator servos
 

Mike,

I have been considering the 9411's for mine. I think you would be defeating the purpose of what you are trying ot accomplish by putting a bend in the pushrod. I am still undecided if I am going to change from what I have in there. I have over 300 flights on mine with plenty of high speed split S's and high G turns and loops. Mine has never felt mushy or lacking in elevator authority.

David Reid

Bobcat elevator servos
 

On the very first flight with my BobCat with my son Paul flying, the elevator felt mushy or very soft in steeply banked turns, even though I had all the travel you could put in the elevator with the CG at the recommended point. The plane handled beautifully in every respect except for this. We went on to put 30 normal flights on it with only an occasional hint of the softness exhibited. On that last flight, it simply would not pull out of a split S. Even with full up elevator from a very high entry, it would not recover. It just made a big, gentle arc into the ground.

If I ever build another one, or anything with leads that long, I am going to install something comparable to the 8411 AND check the voltage/current at the servo while under load.

My 2 cents worth.

Dennis Lott

Bobcat elevator servos
 

I too have felt the elevators to be a bit mushy coming out of a split S or during a full throttle steeply banked turn. On my next one, I will be looking for a different servos and running a 5 cell not 4 cell packs RX packs.

Sung

Bobcat elevator servos
 

You guys having problems need to make sure that you have the linkage set properly. If you are too far out on the servo arm and have the electronics dialed down in the TX to reduce the throw, you are giving up a lot of leverage. Move the pushrod in on the elevator servo arm and increase the throws in the tx. You will now have a lot more power to the control surface.

Bobcat elevator servos
 

tony is exactly right..............by putting the linkage out further on the servo arm you change the geometry ( aka leverage ) servos get their inch oz rating by being "linked up " at a set distance from the axias..........once you go to a hole further out on the arm you loose leverage and inch ozs


with leverage us 250lb linebackers can stand those 350 lb linemen up like tent poles............without it all you get is a bus ticket with a box lunch

Bobcat elevator servos
 

I understand the leverage problem. I used the servo arm BV recommended in the plans. Too late now, the servo mounts are out. Going to put the bigger ones in. Dave, Is the 9411 smaller?

BTW, mine never felt "mushy", guess I got spooked watching two "go in" a few weeks ago due to apparent elev. problems.

Mike

Bobcat elevator servos
 

Well, IMHO, the servo arm shown on the plans is too long. If you use that you will still have to dial down the throws too much. Either move in on the arm or install a longer control horn into the elevator.

BTW, the 9411 is a great servo. It is smaller then an 8411. I'm not sure if it will fit in the stab, but it is a bit narrower then the 8411. A pair of those on the elevators should work fine.

Bobcat elevator servos
 

I had the linkage attached to the outermost hole on the servo arm. Probably could have tried to move it closer to the center, but really wanted to go to a metal gear servo.

I need about a 1/4" shorter servo to align with the existing horn. Both servos will probably stick out the bottom of the stab, but I figured I could make some kind of cover for it to smooth the airflow.

Mike

Bobcat elevator servos
 

The servo arms I used were short. The pushrod clevis pin was inside the surface of the stab.

Dennis Lott

Bobcat elevator servos
 

I'm gluing both halves together on mine and installing a single 8411. Never felt comfortable with the 3301's since it's maiden. Bobcat has been grounded since my gyro failure, waiting till I find the time to do the servo swap before it flies again.

Todd

Bobcat elevator servos
 

Dennis,

The real question is what were the TX settings to get the throw on the elevators? If they were not set to allow 80% to 90% of the elevator channels maximum throw, you were not getting the most out of the servo. You can see this pretty well on the Servo Monitor screen, #75. If when you move the elevator stick the bug on the elevator display doesn't at least move to the lines on the top and bottom, you need to increase the elevators throw in the TX and change the mechanical linkage.

Bobcat elevator servos
 

Tony,
I'm at 96% ATV and still get that mushy feeling during hard Split-s or pulls from a fast dive. This is with P-80 for power....

Todd

Bobcat elevator servos
 

Where are the DR settings and the Trace Rate? In the 10X, the Trace Rate default is 100%, but it can go to 150%. Increasing that will give you a lot more electronic throw.

Also, just how much elevator are you using? If you're trying to throw it more then needed, you will reduce the power to the surface.

Bobcat elevator servos
 

I wanted to make sure I was not telling a tale so I just went and looked at my Bobcat. I used a standard servo arm (4point) and used the small arm. This would be around 1/2" max from the center of the screw. My ATV's are set at 109% up and down. I might feel a little more comfortable with more servo torque but I am thinking I have the optimal mechanical setup and with over 300 flights on the airframe, and the fact that I have never felt any mushiness I think I will be OK with the 3301's. The pair gives a combined torque of 120 in oz of torque should be more than adequate. After all the recommended elevator servo for the Phantom used to be the JR 605. (139 Inch Oz) Vernon put 400+ flights on his Phantom with that servo and it is moving a whole lot more control surface than the Bobcat elevators.

BTW the last 50 flights on my BC have been with a P-160 for power!

Bobcat elevator servos
 

I fly futaba and never fly on low rate elevator..... BTW what is trace rate???

Todd

Bobcat elevator servos
 

The elevator travel on mine was set at 79% up and 80% down. I believe that if the servo is so near its limits, then it should be upgraded to a more powerful servo. I had 22 ga. servo leads, btw.

I have personally witnessed three BobCats go in due to this problem. I have a friend who is a conservative flyer, who almost lost his twice in either a loop or split S. I think it is a miracle that David Reid hasn't lost his the way he flies it. In fact, mine was lost when David and Paul were flying together, and flying in a similar fashion. Paul was even split-Sing (is that a word?) from a higher altitude than David when ours went in. That was about 200 flights ago for David. Vernon's was lost doing a loop. He knew by the 10:00 position on the down side that he wasn't going to make it.

If I stll had mine, I would install 8411s or similar and make a blister to cover it. I would also check the current available at the servo in the tail with a load applied to the elevator. I would compare this to what is available to the servo with a 12 inch lead with a similar load applied just to make sure it is getting enough current to do it's job.

Dennis Lott

Bobcat elevator servos
 

Dennis,

Maybe it's my SUPERIOR piloting skills that keeps mine from going in! :D THat and the fact that they keep the bleachers behind me when I am flying the BC.

I haven't got the book in front of me right now (nor do I remeber my college physics) but there is a direct correlation between the length of a lever arm and the force that is required to move it, or rotate it as is the case here. Stronger servos is not necessarily the cure for a poorly setup linkage. In my opinion I have proven time and tiem again that the servos on mine with the linkage setup correctly is more than sufficient.

Here's a picture of my linkage.

Bobcat elevator servos
 

And another

Bobcat elevator servos
 

I happen to be doing an Isobar turn around down at the same end of the Lumberton runway when Dennis' Cat went in. I could see that the aircraft was in trouble with peripheral vision. I almost wrecked the Isobar 'cause I started paying too much attention to the BobCat!

Bobcat elevator servos
 

David,

You may recall that we had the discussion about servo arms and length while I was building mine, since you already had yours flying. I did set mine up like yours with the standard short servo arm. My arm lenth was the same as yours. Like you, I had to cut out the servo cover so the entire clevis would fit through it.

The reason they put the bleachers behind you is so the rest of us could hide behind them while you fly!!! :D :D :D

So far as piloting skill, it doesn't take much skill to bend the transmitter stick till it breaks, only adrenalin. :)

Dennis

Bobcat elevator servos
 

Dennis,

There is obviously something correct in David's set-up and something was incorrect in your's. I have explained that if improperly set-up you will not be getting the correct hinge moment to the control surface. Not much more I can say. I hope I've explained this as clearly as possible. But I will repeat, the Travel Adjust feature is not the only indication of the travel set in the TX. The Trace Rate menu and the DR menu's adjust the throw also. But if bigger servos will make you more comfortable, then by all means install bigger servos.

I just helped a pattern buddy with his models this weekend. We spent 6 hours optimizing the set-up as I've described and the results were much "tighter" feeling models and less battery drain. We could fly more flights before needing to charge. This was a direct indication of the improvement.

Todd,

Trace Rate is similar to the AFR in Futaba radios. It allows adjustment of the ailerons, elevators and rudder throws. It is often overlooked by the owners when setting up a model.

Bobcat elevator servos
 

David,

What is your trace rate set at? Mine was set at 100%. If yours is 100%, then ours were set up identically. I set mine up that way because yours was working well. I did not want to use the long servo arms because I did not want the reduced leverage inherent with that set up. The elevator horn was installed per the plans with the clevis hole at the hinge line and the recommended distance from the surface of the elevator. I was very careful about the setup of the elevator servos.


Dennis

Bobcat elevator servos
 

Ummmm... this is in no way meant as criticism - just curiosity - if the problem was known that early, was a roll out, or a down-elevator recovery to level attempted, or did he stick with the loop all the way ? If the latter - did the radius stay constant or increase or decay from that point, or was the elevator so ineffective that a straight line was drawn from the 10 o'clock position to the ground ?

What kind of speed did the aircraft have at the time ? i.e was the elevator mushiness definitely a servo issue, rather than there just not being enough air over them for them to be effective ?
What was the status of the receiver, servos, linkages & control horns after the crash ?

Again - no crtiticism of any of the pilots is implied - I just wanna understand what was tried, what worked to some extent, what didn't work at all, etc., so that I am somewhat forwarned if I should ever be unlucky enough to face that problem on my Bobcat. Fortunately, the only softness on my controls (thusfar !) is the expo.

Gordon

Bobcat elevator servos
 

One other point, anyone experiencing any mushiness of any control, if it doesn't crash, you should ground the model and check it out. Make improvements to the system until any experience of mushiness is eliminated. A loss of control like this indicates a problem. If the servo is getting blown back the current drain will go sky high and burning up the servo is a distinct possibility. That may have what ultimately caused the loss of these models.

So please, if you're having these problems DO NOT continue to fly the model without fixing it. Even if that means putting in bigger servos.

Bobcat elevator servos
 

Dennis,

I will have to go back and look at the trace rate but I doubt I changed it. I also have the elevator set up with NO dual rates. If ours were set up the same why does mine have ATV's of 109% and yours was around 80%? Not trying to kick a dead horse here but I have had ZERO problems and a bunch of yankin' and bankin' flights. I have never once felt any lack of elevator authority. I may just have to keep mine set up like it is to prove a point!

Gordon,

Vernon and I were flying formation at the time we both went into the loop at a relatively high speed, roughly 180-200 mph. We both throttled back as we approached the top of the loop. We were both at about 400'-500' altitude. As Vernon came over the top he said "Oh no" I could see in my peripheral vision his loop was just getting larger. He impacted at an angle just did not have enough to pull out. We were less than 2 minutes into the flight and had a full fuel load so that probably added to the situation.

Bobcat elevator servos
 

Guys..

Tony has a LOT of flights on Bobcats as do many others who've contributed here. I've flown quite a few myself, from Tony's RAM 500 powered, to several P-120 powered versions and never felt the slightest bit of softness in the elevator. All with the 3301's on the elevator.

It makes me wonder where the differences lay? Battery voltage? Linkage setup? It just doesn't add up.

Tony makes a HUGE point here too that I think has been missed. If ANYTHING doesn't feel right, do not continue to fly. FIX IT. To do anything less is simply irresponsible.

-Doug

mushy
 

I have noticed this mushy before only full of fuel. I always burned a little before doing any manuevers.
Scott

Bobcat elevator servos
 

Gordon,

I only saw the smokin' hole. I did not see it go in. I heard Vernon's comments about it. Think about it, how long would you have to react between 10:00 and 3:00 at 150 to 160 mph? Vernon impacted at about a 30 deg. angle off vertical. If he had rolled and pulled, it would have been between vertical and 30 deg. the other way. It is a no win situation. Vernon did have the recommend servo arms, so his were longer than mine or David's. He also flies a bit more conservatively than we do. He was flying the same way he has for nearly 300 flights.

I suggest all BobCat pilots take a close look at their control setup. I recommend going with the standard short servo arm. The only difference between David's setup and mine is that I have always used a 4.8 volt battery pack. David and Vernon use a 6 volt pack with a regulator. All of us in the Miss. Gang charge rec. and ECU packs after two flights. Vernon had charged his for the fatal flight. He was near 300 BobCat flights when his went in.

Why some crash and others don't I don't know. I do know I now like overkill when it comes to servo power.

Dennis

Bobcat elevator servos
 

I didn't see David's post until after posting my last reply. I don't have an answer as to why there is a difference in atv between our setups. Obviously I can't go check mine now, but I had so much elevator throw I thought it should snap really well if full stick was given.

I also just got off the phone with Vernon. Vernon said he was definitely going less than 180 mph. Maybe this explains why David is always zooming around. He has done it so long that anything under 200 seems like 100 or so. You know, Eintein's theory of releltivity. As speed increases, time slows down. May David's eyeballs are in warp drive! :D

In any case, it is very good advice that if you feel any softness in elevator athourity with your BobCat, ground it until you figure out why.

Dennis

Bobcat elevator servos
 

Amen Dennis..... As I said.....Mine is grounded till I have a chance to upgrade the servos.

Tony,
My linkage is exactly like Davids above, as always...I try to get throw required at 100% atv. Also running 5 cell packs with a MH regulator. I have noticed on flights that the elevator got mushy I also used 75 to 100 mah more than normal. I just assumed I was stalling the servos and decided to replace them with a single 8411. Not familiar with trace rate or AFR? Not an option on my radios (9C) must be on the 9z??? Can you be a little more specific as to its operation? Is it the same as ATV ( servo travel volume)??

Regards,
Todd

Bobcat elevator servos
 

Doug,

Do you have any experience with the new JR 8311 servo. I know this is the replacement for the 8411 in the helicopters. I was told by Horizon that it has 125 oz./in. torque at 6 volts and that it has nylon gears. Shouldn't this be a very good servo for use in jets? I would like the really tight gear train.

Dennis

Bobcat elevator servos
 

It should be very good Dennis.. but for me personally.. if I can get an 8411 somewhere, I'd go with that over any of the others of that size. The metal geartrain lash can be mitigated by a good mechanical setup. The 'slop' you'll see at the surface will be minimal with a good mechanical setup and I'd challenge you to tell me you could feel it.

Just remember this simple rule..

Longest control horn coupled with the shortest servo arm & max ATV.

Always setup your surfaces with the maximum resolution you can get. You'll find you have a more precise and 'tighter' feeling aircraft.

Doug
Team JR

Bobcat elevator servos
 

I do like the 8411 a lot. I have a sack full of them and have never had one problem. I read about the new 8311 recently, and was just wondering about it.

Bobcat elevator servos
 

FWIW and PMFJI, when I started flying my Roo with a RAM 1000, at full throttle and making a turn, the airplane will become mushy and seem not to respond to the elevator input. Same thing happens when coming out of a loop. It scared the living daylights out of me and I decided to make a switch. I went with multiplex servos and the problems went away. I never experienced that type of problem when I flew the Roo with a 750.


Regards

Ben

Bobcat elevator servos
 

AFR is in the 9Z's. I'll try to explain both Trace Rate and AFR this way. Travel Adjust w/JR or ATV w/Futaba adjusts the throw of each individual channel. Trace Rate w/JR and AFR w/Futaba adjusts the throw of the stick. Example; you are using two channels on the ailerons. You use Travel Adjust to get each aileron throwing equally and you use Trace Rate to change the amount of roll throw you want.

With a JR the DR maxes out at 100%, but the Trace rate can go to 150%. I'm not familiar with the Futaba 9C, but I'll bet the DR will go to 140%. Just keep increasing it until you find the point where the servo quits moving before the stick is at max. Back the throw down just a bit from that point. Then adjust the mechanical linkage to give you the control surface throw you want.

I would not live with any mushiness regardless of the fuel levels. It's an indication that the loads are too high on the servo. Either get the linkage set-up correct or overkill it with a bigger servo. But whatever you do make sure the linkage set-up is correct.

Putting in a lot of elevator throw is not the answer, either. This just makes this leverage issue worse. Please check my BobCat set-up article on BVM's website for a good elevator set-up. I used 20 degrees of up and 24 of down. My BC would snap fine with this amount.

Bobcat elevator servos
 

seanreit,

You're first assumption is incorrect. Rotary output servos are rated in torque, which is a combination of force over distance. It's inch/ounces. If a servo is rated at 110 in/oz, that means at a distance of one inch from the center of the output shaft the servo can exert a 110 ounce force. If you hook up closer to center the servo can exert a higher force. Move out, and it's a smaller force.

There are programs available to calculate what is called the hinge moment, or the conrol power needed on a specific shape and size surface at a specific speed.

Bobcat elevator servos
 

TonyF Thank you for the explanation. Does anyone know what airspeed was used when the bobcat elevator surface was entered to get the servo torque suggestion program? Also, does anyone know where I can see a running version of a program that will calculate torque requirements? I think even spending a couple of hundred on a program like this would save 10 times that in servo's on future planes.

Bobcat elevator servos
 

Well, my Bobcat manual says that the maximum speed should be limited to 175 mph ... they no doubt added a saftey factor, but who knows how much.

Gordon

Bobcat elevator servos
 

Multiplex has a simple little program to run on their website, for estimating servo loads. Sorry to hear about the recent Bobcat problems. Maybe you guys are pushing 'em too hard. Then again, I've seen David Reid fly his - I'm surprised his ain't broke! :)