RCU Forums - Bobcat elevator servos

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bobcat elevator servos

This is too funny. You guys jump all over David Galdwin because he thinks there is a problem with the elevator on the bobcats. I could have sworn in this country a man could pretty much say what he wants (barring threats against others) without fear of persecution. Let him say his peace without insulting his intelligence or integrity. I don't know the man but he has a right to say what he wants.

Patrick.

Bobcat elevator servos

I just dont get it? What the heck is all the fuss about. What is the big deal if David does some research and found data supporting the theory that the BC elevator servos are marginal (meaning if rigged exactly per manual their fine, move out one hole and you could be in trouble) and expresses his OPINION of this data he collected?? Had this data been posted by say....Bob Parks, would this information scrutinized so deeply?
In my opinion David's theory is correct, rigged perfectly the servos are adequate. For someone like me, where just adequate dosen't make me feel all warm an tingly, simply upgrade to higher torque servo!
I can tell you my BC was rigged exactly as per manual and I still had times where my elevator faded, not a pleasant feeling! I have since installed 2 DS8411's in my elevator and I have not had the elevator fade since, even though I have upped the motor from P-80 to P-120 and in much higher temps than when the fade was experienced. Does this support Davids theory? YES! Can I explain why many others have not experienced elevator fade using the 3301's, NO! But... I do know that my BC feels better and more responsive than before..... I also feel better mentally knowing I am over-kill rather than adequate....
I love BV product, he gets plenty of my hard earned money but I just dont get you guys..... he's just a manufacturer, no one is going to come and remove your right testicle should you decide to change a servo! Lighten up and take the data for what it is......OPINION :confused:

Todd

Bobcat elevator servos

In addition to the above..... I will say that even though I believe David is correct in his opinion, perhaps publishing this information in RCJI without factual data to support it was not the best thing to do..... BUT, I took this article as HIS OPINION and nothing more..... And....Judging by the # of servo covers I sold, there are plenty who would feel better about their investment by upgrading ;)
Todd

Here is an idea: could it be that a PERFECTLY rigged stab linkage is OK with the recommended servos, but if the linkage is a bit out of rig, then you need a bigger servo? Seems to me that it would be good insurance to put in an 8411 or similar just for the peace of mind.

It also seems to me that Mr. Gladwin is very much worth listening to. He has tons of real world experience, and whether he is right or wrong. deserves respect.

Bobcat elevator servos

No one is being disrespectfull of David Gladwin, some of us just happen to have a whole lot of real world expereince with the Bobcats and have proven time and time again that properly rigged per the instructions the design is sound and the servos are up to the job.

Load Analysis Tools

I didn't see specific reference to any one particular "tool" but here's a link to a VERY useful product <http://members.cox.net/evdesign/page...ge_design.html>. $10US for the electronic version but you gotta have Microsoft Excel.

You can plug/chug all the variouus parameters (e.g., control surface area, linkage properties, air speed, etc.) and see the resulting servo torque reqts. and all that as a function of control surface deflection and air speed. I use this program all the time to satisfy my own curiosity and to make sure servos I'm considering are up to the task imposed on them.

Mike

Bobcat elevator servos

I thought the purpose of this forum was to prevent and solve problems people are having. To convey ideas and prevent crashes if possible.
David Gladwin noticed a possible problem that people were having with the BVM Bobcat and to suggest a possible couple of modifications.
Several very accomplished pilots have had miss-haps with this airframe.
Then The BVM police come out and try to crucify him.
I get sick of this. If anyone posts something remotely wrong with a BVM product then all of the BVM groupies take turns dumping on him.

If you are flying a plane that was designed for a smaller thrust motor or faster than the design was intended for then you have to address additional stresses.
I can't think of an industry that hasn't had to issue a bulletin for modifications or things to look out for.
GROW UP !
If you are ONE HOLE on the control horn from crashing then you could have a potential problem.

Marty O

Bobcat elevator servos

Well guys, at the risk of being beat up, I'll offer my FACTUAL experiences with my ex-BobCat and my OPINION of what I think happened in my particular instance. I built a BC XL exactly per instructions except for adding flaps/ailerons per John Redman's recommendations, and powered it with a P120. I was very careful to follow BVM's instructions EXACTLY for all the building and control set ups. I had around 40 or so flights (great flying machine, BTW), but I experienced elevator mushiness on occasion, (not on loops as I always reduced the power on the backside of the loop) but on normal 90 degree bank fast pitchouts which acted like I had 1/2 or less normal throw (pitch only). It ONLY happened after a fast pass with a level 90 degree bank pull out. Reducing throttle (and speed) during the sluggish pull out allowed the BC to come on around with restored elevator authority as the speed bled off. This was a random happening, however, and always scared the p*** out of me when it happened, because it wasn't consistent, and I wasn't prepared for the lowering of the elevator authority when it did happen every time. I checked everything over many times, but couldn't duplicate the condition consistently. It was a random happening, so as a last resort, I changed the 3301's out for 9411's on the elevator and made new servo covers (which protruded into the slip stream with NO adverse effect). On the very next flight with the new servos, about 5 minutes into a normal 8 minute flight, as I was doing a 1/2 throttle low pass, the radio went into failsafe as I asked for elevator to stop the descent. The BC impacted the runway directly in front of me at about a 15 degree descent angle, wings level. The engine got it, the gear was retracted and survived, the radio and tanks were OK, but the airframe was destroyed.

I sent the 955 receiver, 10X transmitter, receiver battery pack (1800 mah, 4 cell 4.8v) and the 9411 servos to JR for a complete analysis. The only thing JR found was a kink (break of most wires) in the RX antenna about 3" from the case and had to replace the gears in both 9411's. It is unknown if the kink/break happened prior to, or during the crash. Not all the wires were cut, so JR felt that the radio probably hadn't gone into failsafe from the kink since it happened at a very short distance from the transmitter (but it WAS possible). I (and my other jet buddies here) suspect that since I had used standard servo extension wires (not heavy duty), that the 9411's may have lowered the voltage which caused the RX to go into failsafe. As everyone knows, the elevator servos are a LONG way from the RX and battery pack. The 3301's may have not had enough voltage to work against the speed either, but since it wasn't consistent, there's no way to really know for sure. If heavy duty wires were required in the 3301 set up, I would expect that fact to be in the instructions issued by BVM. Maybe it's just good practice to always do that, but I didn't know at the time and my other modeling activities didn't require heavy duty leads. In any event, I'll sure use heavy duty in all my new installations, just to be sure. In my opinion, the best way to determine if the recommended 3301's are adequate for the task, is to do a wind tunnel analysis as suggested by David G. In the meantime, as also suggested by David G. (and others), just change the servos to larger ones, and make sure to use heavy duty wiring if there is doubt. After I made the change to larger servos, my BC had been flying with no problems during the ill-fated flight until the lockout (cause unknown). Unfortunately, at this point, everything is pure speculation as to the exact cause of the crash and RX entry into failsafe. I DO know that during the flight I had NO elevator mushiness occur with the 9411's, and I SPECIFICALLY, DELIBERATELY tried to induce the condition with no luck, so I suspect the 3301's were not up to the task demanded of them - it may be the small gauge servo extensions I used, but the 9411's worked fine until the RX went into failsafe. Everything was checked out, and there was no smoking gun that we could find. In any event I have since changed to large gauge extensions (heavy duty gold connectors), and am using a 5 cell AT LEAST 1800 mah RX battery pack.

I am currently flying a EuroSport hard with 8411's on the rudder/canards, and on the elevons with NO problems - and am very happy with my new bird. So far, I haven't experienced any control problems with my new set up in the EuroSport.

Regards, Les

Bobcat elevator servos

Quote:

Originally posted by jetjockey
I (and my other jet buddies here) suspect that since I had used standard servo extension wires (not heavy duty), that the 9411's may have lowered the voltage which caused the RX to go into failsafe.

Les, I don't know how much this might help, but its worth verifying if it helps with the diagnosis. From what I understand with JR receivers in a low voltage condition, is that it will start 'clipping' under a high load condition, and not go into failsafe. Basically, my understanding is this. If a servo load pulls down the voltage, the receiver basically turns off when it reaches its threshold level. Once it turns off, the servo load ceases, and the voltage then rises above the threshold. Once it rises above the threshold, the receiver comes back on line and starts applying load on the servos as it would normally, and the process repeats itself. Thus, one would feel sluggishness in the controls, but it doesn't go into failsafe. With Futaba, the receiver has a battery failsafe which effects the throttle, but I am not sure what happens it if drops below the battery failsafe threshold, then further fast the receiver threshold.

I heard this description of the 'clipping' in the past, but never verified it with JR directly. It might be worth asking JR about it, as it would then possibly point you in other directions for resolution.

Regards,

Bobcat elevator servos

Assuming what Stephen just said is the way it works (the RX shuts off momentarily for JR) and the plane flying had a JetCat, would not the JetCat ECU detect the instantaneous zero volt situation at the throttle channel and shut down the turbine? I believe something like this happened to Vern's BVM F-4 at Fla Jets this year and it was due to an intermittent RX switch (receiver power would quit for an instant, too short to feel it in flight performance, but long enough for the ECU to see). The JetCat ECU detected a momentary instant of the receiver being turned off and so shut down the P-120 (I think). Les said he had a P-120, so if Stephen's theory is correct, it appears that the JetCat should have shut down as soon as the JR RX clipped.

I am not a JetCat guy, so maybe I have it all wrong. Anyone know for sure? Matt?

Bobcat elevator servos

woketman,

The JetCat did not go into failsafe on Vernon's F-4 but the engine did shutdown and I forget off the top of my head what the exact error message was. Something to the effect of power low. It was in fact a bad reciever switch.

Bobcat elevator servos

Mark,
You are correct, should the Jetcat ECU see a voltage loss at the reciever it will shut the engine down immediatly. This "Clipping" would not be possible and still have a jetcat stay running....
Todd

One thing that may be relevant is the type and quality of battery packs that we are using. After talking to Larry at SR I learned that the AA size cells may have a lot of capacity in terms of mah, but they may not provide the high current demand that we are asking for with multiple digital servos and ECU's on-line etc. After considering this I have switched to a 5-cell sub C battery pack. These packs provide a higher current than the smaller diameter cells. They also have heavier leads coming off them than the AA packs have.

I am not sure how much current the switch is able to handle as well, but these areas might need to be explored in terms of how they might be related to the potential for this clipping phenomenon to occur. We may need the expertise of an electrical engineer colleague to help out with this.

Mike Danchak

Bobcat elevator servos

That may be so, but at least in Les Morrow's example it appears we can rule out the clipping issue.

Bobcat elevator servos

I know nothing of receiver clipping but low receiver voltage can be monitored by using a MAS volt logger. This device has three functions :

1 It displays voltage in real time as sensed on the receiver power bus.

2. It records and displays the LOWEST voltage sensed during the flight which would be some help in diagnosing soft elevator response due to stalled servos with the associated voltage drop caused by stalled servos.

3. It records and displays any instances of receiver drop -out. The LED display is used to reveal the exact number of drop-outs, if any.

I have one of these units on each of my aircraft and it is an invaluable tool to ensure the proper functioning of receiver power supplies and the integrity of radio reception.

Sorry to hear that Vern had problems with his F4 due to a faulty receiver switch but, in the interest of flight safety, shouldn't all high power, high value models have TWO batteries and receiver switches ?

(according to Don Moden of Don's Hobbies the greatest cause of crashes after pilot error is 1. Batteries, 2. switches. )
David Gladwin

Bobcat elevator servos

Hey David.....
Some time ago you said that would be your final post and had "nothing else to offer".
I guess you were wrong.

Mr Taylor
If you have nothing of value to post, maybe you should refrain from posting just to dig at David Gladwin, who by the way posted some useful information even if he said he wasn't going to post anymore. what ya think ?

Jack

Bobcat elevator servos

I have nothing more to offer on the CAUSE of elevator, effectiveness, I think everything has been said, every avenue explored, but I though the information on the volt logger may be useful. Take it for what its worth, if anything.

However, since my post when I said I haven't anything more to offer, I have been asked for more information on elevator forces. These calculations are being double checked and I will post them when available.

David Gladwin

Thanks David Gladwin

I don't own a BobCat, but the information you have provided could be useful in other applications.