Bobcat elevator servos
 

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

Bobcat elevator servos
 

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....

Bobcat elevator servos
 

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

Bobcat elevator servos
 

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.

Bobcat elevator servos
 

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.

linkage
 

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)

Elevators
 

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

Bobcat elevator servos
 

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

Mod.
 

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

Bobcat elevator servos
 

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

Bobcat elevator servos
 

George,

Do the mods, because the first time you experience it might be the last, if not it will scare the heck out of you. I know and have been there and when I build the new one, I will definitely do the mods and upgrade the servos. Never hurts to have a little extra under the hood.

Sung

Bobcat elevator servos
 

Thanks for your input to my question guys. Do you think that two 9411's will be enough to fix the problem??? I know that 8411's will fix the problem but I'm concerned with the amount of modifications that are necessary to install them.

Kevin

Bobcat elevator servos
 

Has anybody talked to BV or Tom about this. I wonder what they have to say. Do they think there is a probem and what is it and what can be done to fix it. Shuleman I know you are reading this.


Doug A

df
 

Doug
Myself and two others up here in MN are just about done with Bobcats. This will be my second. I have not called Bob on this issue but the other two have. They were told that the 3301's were fine and that they have over 300 flights with no problems. However I have talked to a BV rep. and he recommended the 9411 conversion. My first Bobcat had a Mercury only pushing about 14-15lb. of thrust. My new one will have a JC p-80 plus at about 20lb. of thrust, so this thread is making me a little nervous. I'd really prefer not to have a servo hanging out of the stab. But it seems better than no plane.
Good Luck!
Scott

Bobcat elevator servos
 

Take a look at BV's website or CLICK HERE. It just got updated.

Bobcat elevator servos
 

I was looking at the BobCat Stab plan last night, and was wondering if there is too much room for interpretation on the elevator travel.

It reads
"Elevator Travel:
1 3/4" up and down."

I was wondering if some people were thinking that this meant that the elevator was to have 1 3/4" total travel. Instead of 1 3/4" up and 1 3/4" down.

Looking at everyone's ATV settings was what lead me to think this.

I did a little drawing and would like to know if this is what everyone's elevator looks like at full deflection. And as I said before it has been my experience that 140-150% ATV is necessary to achieve what is shown in my drawing.

Bobcat elevator servos
 

Mine was set up the same way you show in the picture. Just bought two 9411's. Going to install them just to give a little extra safety margin.

Mike Jensen

Bobcat elevator servos
 

Mike,

sent you a pm

failure
 

David,

I was not there friday in Austin when you lost your bobcat, I showed up 2 hours later, but 2 guys I work with were there and they were watching your flight very closely. They said indeed you low pass was very fast and the pull to vertical was pretty agressive. As the plane went vertical they both said they saw something fly off the airplane. Never really looked for the piece due to the aftermath of the crash, everyone was busy.
Don't know if this info will help or add more confusion.

Regards,

Dan M.

I ran the numbers
 

but they will only add confusion on here.

If you want them PM me.

As for the technical issues, I think BV has hit it on the head, namely the power supply to the servos. Imagine if you are using a 4 cell pack, and it is somewhat run down, and you have small gauge extensions (or too many extensions), slightly corroded connectors, etc. You could have 20% less torque than other Bobcats with 5 cell packs, regulators, 22 gauge gold plated extensions, hi quality extensions etc.

Also BV mentioned 200 mph, if you enter a dive, split S or enter the backside of a loop already at 200 mph you are going to go way over 200 on the downside if you do not get out of the throttle fast.

215 mph requires almost 15% more servo torque than 200 mph

So like most things it adds up. 4 cell pack, somewhat depleted, numerous small gauge extensions, corroded connectors, 215 MPH vs 200 and now you might have a problem

Just my thoughts

PS I just noticed the Vne for the Bobcat is 175, so the torque required for full deflection at 215 mph is 34% greater than 175 mph!

Bobcat elevator servos
 

I had an Isobar Rudder servo start to fail on me, where
it would worked fine but it only had ~30 oz of torque...

I changed the servo to a brand new one with over 100 oz
and still very week.. say ~30 oz of torque..

I traced the problem back to the Y harness used for the
steering servo and rudder..

You would never know it because everything worked fine
on the ground just no power...

My point is, my Y harness acted like it had a resistor in
it, limiting the current and the output torque.

Any wiring / battery problem will not show up until
its too late..

I think bob cat owners should just measure there elev
torque by hanging weights on the end of them and
compare theirs to all the other bob cat at the field that day...
This could tell a lot and save a plane... !!!

I can bring my scale to Miss Afterburner... and set it up..
Won't take but a sec... If any one will use it...

Eddie Weeks

Bobcat elevator servos
 

Just install 8411(s) check your leads and never think twice about it ;)
I have the Utmost respect for BV but I dont think a servo who's torque rating is that close to max flight load values should be the "Recommended" servo. Sure, the 3301's are probably sufficient for a BC on 500 power but I dont trust them on 20+ lbs. I prefer to have twice what I calculate the surface would need and know I'm never coming close to stalling a servo....
Todd

Bobcat elevator servos
 

Well....

Actually, you can't really 'just install 8411's' as they don't FIT, and the resulting buldge could cause a buffet or flutter. We are talking about 200+ mph here. Granted, it likely would not, but I don't really want to test that theory (and it is ugly).
I would say, that I have flown BV's personal BC with a P120 on it, and that plane gets wrung out with no issues. It never feels soft!
All of this does make me think twice as I am finishing my Bobcat... and I have always liked the simplicity of the 4.8 v system.
BV runs a regulator at 5.2v I think. Most of the '4.8' packs I have tested seem to hold 5.0-5.1v with a pretty good load.... Still, I think I may be ordering a 6v pack and regulator.
My Atv's are up arround 130% to get the 1-3/4" throw, and this is good. Seems to have a lot of torque even on 4.8v.
Anyone running 110% or lower ATV to get the throw should think hard about getting in there and changing the linkage to get a better mechanical advantage, and more torque at the elevator. (use more servo rotation) Also, because the wheel is rotating, the further you rotate it, the 'shorter' the perpendicular moment arm is getting, so the guys using higher ATV's are getting a double advantage at higher elevator deflections! This is really evident on mine.
D

Bobcat elevator servos
 

>Sadly you can't really 'just install 8411's' as they don't FIT, and >the resulting buldge could cause a buffet or flutter. We are >talking about 200+ mph here. Granted, it likely would not, but I >don't really want to test that theory (and it is ugly).

The Isobar has 6 full size servos sticking out to the mounts
and its been clock at over 210 mph for many years now.... Buffeting or flutter is not caused by a servo sticking out a plane...
It only produces parasite draw witch is very stabilizing that
far behind the CG

Eddie Weeks
http://www.corpcomp.com/weeks1/

Bobcat elevator servos
 

Ok, I give in, what is "Parasite Draw". I looked it up on yahoo and the closest match was some type of card game where if "you get a 'parasite, draw' a card..........LOL

Bobcat elevator servos
 

Parasite draw is where it attracts people that don't have their own airplane but ask to fly yours..

Hey.. wait.. that sounds like ME!!

Er.. nevermind.

I think he means
 

"Parasitic drag", which everyone knows is when someone tries to DRESS like you. It's a sad, sad psychosis...

Bobcat elevator servos
 

Sorry to make an ongoing joke, that was not my intent. I have a really high opinion of Eddie's comments, I've gone through his site extensively and my opinion is that this guy thinks outside the box better in this hobby than anyone. If he says there is "parasite draw" and it's not an official term, my bet is that it's there, I just don' t know what it is. My Isobar is about three weeks from being ready and I was just hoping to get a better understanding of this term.

Bobcat elevator servos
 

Ok Eddie is parasite draw an official aerodynamic phenomenon or is it something some of those south Louisiana women get that makes penicillin run for the hills.


Doug A

Bobcat elevator servos
 

I think its when a turbine wannabe AMA EC member dresses up as a woman!

I think I know
 

I saw him at the corner of the stage at Regina's one year, the term "parasitic draw" come to mind, I think it works this way....

The "parasitic draw" is directly proportional to the number of dollars on the rail. Is that how it works?

No disrespect intended
 

Just blowing off steam-I also have respect for the man. I do think he meant parasitic drag as it pertains to miscellaneous items hanging in the breeze off your airframe, but let's let him comment.

And as far as Louisiana women; well never mind that's another "story". Let's just say I appreciate my Texas "belle".

Bobcat elevator servos
 

Matt has it correct. I've seen it live (except it was Regina II)!

For DavidR and others.
 

David, with the 109% ATV, are you acheiving the 1 3/4" travel? I thought I had read in some very early BC post, that some people were going with a little less throw initially.

I've gone back and looked at my set-up and it was no where near correct (bought used). I have corrected the linkage and ATV settings and am very close. I'm also going back to a 5-cell pack and regulator.

Also, does anybody know what throws specified in inches converts to degrees? Is there a formula or chart.


Thanks in advance,
George

Bobcat elevator servos
 

Yeah, me three,,,,,,, I spent two weeks in there one night........ Two weeks pay that is .... Ha Ha :D

Bobcat elevator servos
 

Well, it looks like this useful thread is being turned to a waste of space.....

The bobcat servos lay on there side, so they will stick out for a pretty high percentage of the stab ahead of the elevators.... Do whatever you prefer.
I guess Eddie thinks you will have no problem.... I hope someone molds a cover to descise the ugly hole in the bottom of the plane.
D
(Any chance the moderator can clean the junk out of this thread??)

Bobcat elevator servos
 

I am pretty sure I have the 1 3/4" of travel, hell I set it up almost two years ago. I do know that I did not touch the dual rates, etc... on the elevator. My CG is a little further aft than what is speeled out on the plans. That would likely have an effect on elevator authority.

Bobcat elevator servos
 

Good point David..

The CG can affect your pitch authority a great deal. There's just way too many things that can be done wrong, go wrong, etc.. to absolutely conclude the servo isn't up to the task.. Especially when you consider that BV and MANY MANY others have literally thousands of flights on Bobcats without incident.

-Doug

SERVO PROBLEM
 

Do I need to piss on a spark plug again !...???


ttfn

Danno :confused:

Bobcat elevator servos
 

danno

you be ...tore up from the floor up

:-)................