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.

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

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

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

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!

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

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

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

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

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.

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

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.

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.

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.

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

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!

I've put the last ten flights on my buddy's BobCat. I haven't noticed the elevator being mushy during normal flying. Since it isn't mine I've been flying it pretty conservatively. However, we did get the sh*t scared out of us twice when the plane would not come out of a loop easily when my buddy was flying it. Luckily, there was enough altitude to recover.

I'm wondering if two things could be going on here....the servo geometry is wrong as per David Reid's comments, and/or, (I'm pulling this out of left field here) as the BobCat is pulled from the loop-if the airplane is pulled hard at the bottom-I'm wondering if the plane is "pushing" at the bottom of the loop allowing the model to get at such an attitude that the wing starts to blank out or cause turbulence to the high mounted horizontal stab??? This is just a thought.....Tell me what you guys think about it.

Kevin

That's originally what I thought was happening with mine Kevin but then I paid real close attention to the amount put back into my battery after flights where the elevator got mushy. Every flight that this occured I had drawn an extra 75 to 100 mah off the pack compared to flights that I did not feel any elevator loss. This would indicate stalled servos....at least on my BC
Todd


BTW....Anyone ever find out why Terry Nitch's Bobcat went in.....Remember seeing the video, back side of a loop I believe and never pulled out.... I think the 3301's just dont quite have enough umph during certain manuvers....

Hi all, reading all the posts and not one person mentioned about moving the CG a little further back, works for me.
Rcpete

Kevin,

I don't believe it is an aerodynamic issue. My Bobcat has been in every attitude imaginable and a high and low speeds. I fly the snot out of it and have never felt any sign of mushiness on any of the surfaces. I had several gyros puke over time but that's another issue. There is no doubt if I was building another Bobcat I attempt to cram 8411's in the stab but I doubt that I will be hacking mine apart after this many flights.

I do fly 6 volt pack and I use a 5.2 Volt jaccio regulator.

I have been watching this thread, and I have 3 questions, and a comment:


1. Do all the BobCats that are experiencing "mushiness" of the elevator have the recommended 1 3/4" travel ?

Everyone is talking about ATV, but I have seen nothing about actual measured travel, aside from Tony F. who gave his measurements in degrees.

2. Dennis said "I had so much elevator throw I thought it should snap really well if full stick was given." (post #30)

Do you recall if you had a measured 1 3/4" of travel, or did you set the travel to a lower "more conservative" amount?

3. Todd said "My linkage is exactly like Davids above, as always...I try to get throw required at 100% atv." (post #31)

Did you have to deviate from the BVM suggested set up to accomplish this? It has been my experience that when the suggested servo arm is installed, 140 - 150% ATV is necessary to produce the required elevator travel.

4. Did the BobCats that crashed have any problems pulling up after a high speed pass?

David said "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" (post #26)

If the servo in Vernon's BobCat wasn't powerful enough it should have never pulled into a loop to begin with. However coming down the backside of a loop it would not be difficult to over-speed the Elevator and reduce it's effectiveness.

If someone can give me the chord and length of the elevator (the moving part) and the length of the horn (with respect to the hinge center line) and the length of the servo arm, I can calculate the servo loads at any combination of surface throw and airspeed.

This program will do a lot more than that, including design studies of rotary drive linkages (the totally concealed type used in sailplanes primarily)

CompGeek,
Mine was one of the two lost in Austin. The answers to your questions in my case are:

1.I had the recommended 1 3/4 travel. I set it using a surface deflections indicator

2. I though it seemed like a lot of throw to me, I even called BVM while setting it up, But it was fine when using 30% expo.

3. I had my linkage set up like the picture in the instruction book. The clevis was in the last hole in the servo arm as pictured in the book. (After talking to some of the other pilots there, I think this is where part of my problem was). The control horn angle in the elevator is set with a gauge supplied with the kit.

4. The first time I experienced mushiness coming out of a loop. The second time it was during a high speed pass, I pulled up and it was slow to respond.

While building my Bobcat I had concerns about using the 3301's in the elevator 1. because of the nylon gears(all the other servos were metal geared) and 2. because of the torque (56oz).

Before my crash, I called and emailed a few other Bobcat owners to see what their experiences were and they too had mushiness at one time or another and or had lost an aircraft.

I should have replaced the servos when I experienced this problem but I wanted to fly it at the Austin fly-in and it cost me a nice jet and a lot of money. I was going to replace them when I returned home the following week.

My crash happened when I did a high speed pass, I throttled back to half throttle at the end of the runway and pulled up and climbed out, I then rolled it upside down and pulled back on the stick to come back across the runway but it did not respond, I then throttled all the way back and pulled the stick all the way back ( both people standing with me saw the stick positions) but it did not respond and it went straight in.

I don't know why some Bobcats experience mushiness and some do not.

I have another one on the table to build and I will be using 2 8411's in the elevators this time.

David

CompGeek,
I actually have 1 7/8" throw ( that's how I like it) with the atv at 96% and 35% expo, I suppose I could bump the atv up to 120% (thats as high as my radio goes) and re-adjust my linkage for the same throw but why?? I am already pushing the limits of the servo with optimized linkage setup, it is apparent to me that I just need more torque on my elevator servos to solve the problem the correct way. If the margin of linkage error is that close to stalling a servo then it is simply not enough servo for the job.

Todd

Todd, when are you doing your mod.? Will it be done before you come up?
I'm looking at my BC right now and can see where I could do some improvements from the current set-up. I don't have enough flight time to see if I am personally experiencing any mushiness, but want to correct it now before I do experience it.

George

I'll have the mod done before I come to Fentress, once I get it done I'll send you pics on how to modify it. Should not be to difficult, I will mold us some new covers too....


TTYS,
Todd