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

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

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

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

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.

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

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

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.

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

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

Dennis Lott

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

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.

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

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.

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!

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

Todd

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

Dennis,

Maybe it's my SUPERIOR piloting skills that keeps mine from going in! 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.

And another

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!

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!!!

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

Dennis

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.

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

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

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.