I agree with Matt, This is what I believed caused my crash. The plans show the linkage connected to the second hole of the servo horn, The instruction book (page 34) has a picture with the linkage connected to the last hole on the servo arm. That is how I set mine up.
I was using a 2400mah 4.8 battery pack and it was the first flight of the day and had a full charge. I used all 22 AWG wires with the gold connectors(the Bobcat wiring kit from Promark).
I have another one on the table and plan on using a 6volt system with the 5.2 regulator. I do believe that the linkage was my problem, But I plan on using the 8411's in the elevators any way. I just don't want to lose this one. It flies to well and is expensive to build.

I would be interested in hearing from other people who have lost a Bobcat and how they had theirs set up.
Did anyone find out why Vernon's Bobcat crashed the day after mine did? I was not there and I don't know how fast he was going but I was told he was coming out of a loop also.

David

*sigh*

This thing is never gonna get done. I have been finishing (paint) my BC for 3 months. I was going to use a high voltage system (either 5 cell nicad or lithium packs) and HD extensions. I was also gonna have a BVM rep check the whole thing (including linkages) out. Now this? I have already modified it for 9411's in the rudders. Now I don't know what to do. David R., Tony F. and the BVM crew have been flying the **** out of the stock elevator setup. Where does the cautiously over-engineered mentality stop?

One point that Matt brought up that seemed to get glossed over....the front cover of my Bobcat XL instruction manual says to limit maximum speed to 175 mph. I'm not sure why people are talking about this plane flying at 200+.

I'm not challenging anybody's methods or motivation. This is my first turbine and I don't want to do anything stupid but I don't want to be re-inventing the wheel forever either.

Bob

Bob
The new Manuals say, "limit top speed to 200mph"
Scott

OK, thanks for the info.

Bob

I fought the web site for an hour or two yesterday trying to figure out what was causing the issues with the photos. They were all there, but when I moved the site from one ISP to another, there was a problem. Part of it was that one site supported spaces in directory names, and the new one doesn't. I sorted that, and most photos load now. However, there are some that don't, and there is no space for videos. Once I get my domain and server backup up, all these problems will go away.

In the mean time, the switch photos should load now.

I would stay away from the lithium batteries for this application. they can be very touchy. I they are of the smart pack design they can turn them self off to prevent damage, charge them wrong and they may catch on fire, go below 3.3 volts per cell and they may be damaged, and you can not fast charge them, one hour is about as good as it gets. Nicads may not be as modern, but they work, and take a lot of abuse, and in a 18 - 20 lb model how much weight are you really saving. Lithiums are great for park fliers and the like where energy density is everything, but you will not see any advantage in you Bobcat.

Steven

Another advantage that I have always found in NiCads is this: I understand them pretty well. I have 30 years experience with them and I trust them. And as Steve said, how much weight are you going to save, or to put it better, what percentage of total aircraft weight is it? The warm feeling in my gut that I get from NiCads (or is that after Taco Bell?) is worth a small weight penalty to me.

Not quite Steve.. Non of the current brands use the circuit protection in the pack itself. They will both happily allow you to discharge them to the destruction point. The voltage protection is available.. but not used by default (I'd never use it).

Fast charging isn't an issue because of the sheer number of flights possible with a given pack (unless you're Jason Somes.. and fly 15 flights a day). The current ECU packs being tested will give you 10 flights without charging. Kent Nogy has been using them for evaluation and has shown that performance including using the ECU pack for the cooldown cycle in the P-120.

The other issues are real but are of little consequence if you pay attention to the details like we all should be with jet models.

Doug Cronkhite
Team JR, Duralite Plus

I was 2 short of 400 flights w/my BC before the pilot error crash. Lithium batteries in it from day one. You can hang onto old technology if it makes you feel good, but the current technology works.

Doug,

I have to agree with Steven about the Nicads. My brain is programmed to fly - charge -fly -charge- fly charge..... etc.... all day long. I often fly 15+ flights in a day at a jet event as well so charge time does become an issue. Weight savings don't mean that much in a jet after all look at all the horspower we are pumping out the back anyway.

I think you CAN fast charge the Duralite Plus stuff. Emory claims you can push 3 amps into them.. Not sure I would do it.. but he says you can and he's the man behind the product.

Does your brain being programmed to do a certain thing mean you're unable to change?

Oh.. and weight should ALWAYS be a concern. Lighter flies better mmm'kay. Repeat after me.. lighter flies better.

-Doug

mmmmm'kay but heavier handles the wind better...mmmmm'kay....

You pinoneer type dudes get the bugs worked out and I'll catch up later.

Every jet I own needed a little weight so I have 6v 3000mah NICAD and I can fly about 5 to 6 times before recharge. Seems easy enough and I dont need any special high dollar charger and I can quick charge at the field.
Scott

I was given two lithiom batteries and the charger from Powerflite. Have around 45 flights on my bobcat. Love them. I did go back to Nicads in my new Mig, however.

Mike J

For those interested in engineering I now have available a seven page mathematical analysis of aerodynamic loads likely to be experienced on a BobCat elevator at 200 fps in an air density of 0.002378 slugs/ cu. foot. This was written independently by a retired aerodynamicist who worked on the Concorde project and latterly on satellite design ( he is also a structures expert).

As my honest and genuine attempts to contribute towards finding an answer to this elevator problem have met with hostility in some quarters, the question is what should I do with this paper, the maths . of which go way over my head ?

Mr Faulks who wrote the paper has come up with much the same answer as reached by Bob Parks in his initial assesment of the loads.
David Gladwin

David,

Valid information like that is always useful I'd think. Simple load information isn't the entire equation of course.. the control geometry matters a great deal as well.

David:

I would appreciate a copy of the paper...thanks in advance..

Tom

Hi David G,

I would like to see the analysis. I guess the issue some folks have is that some reports such as yours do not research or make clear that MANY (Practically everyone) of us are running the BVM reccomended setup with perfect results.
(The few that have been lost (that I know of) have NOT been confirmed to be within MFG reccomendations (that I am aware of.)

I have heard seemingly reliable reports that some were flying well beyond V NE, or did not have the described linkage, atv, or voltage)


I will not participate in the nasty comments, but a lot of us including myself (with my P120 powered Bobcat) are flying the MFG's setup and it works perfectly. (I completed my Bobcat after these reports came out based on having flown Bob's airplane, and talking to other experienced Bobcat pilots.)

I have flown BV's Bobcat with a P120, and my own, and they has NEVER felt at all sluggish in pitch. I have let mine go quite fast, and the response is brisk. It will carve a mean corner in every plane.

If a person is going to fly the plane faster than the V NE listed, or using a different setup, then fine, change the servo. (But this should be clearly described as a mod outside the MFG flight envelope, not presented (or insinuated) as a fix to a defect.)

I think several of these incidents were pilots just trying to pull out of a vertical dive more sharply than the weight, balance, lift, and air density will allow. (Controlled flight into terrain.)

My only cause for aggravation is that these real life facts are not more clearly represented in your coverage.
Hope to fly jets with you guys some time soon!

Would you care to summarize the result again here? At what (calculated) speed is the servo nearly stalled (at say reccomended deflection)?

Best Regards,
David Ribbe

If the analysis is confined to the forces on the elevator and the moments at the elevator hinge then noone can complain about not following the manufacturer's setup instructions.

Readers can apply some simple trig to calculate the torque at the servo with whatever linkage they desire.

And on it goes.......


Mr. Gladwin,

You were not at the Heart of Ohio Jet Rally last weekend. I put 44 flights on my P-160 powered Bobcat at the event. This pushes the total number of flights on this airframe up close to 500 flights total on the airframe. I think that I can safely comment about the airframe based on MY experience, I would also bet that I am pushing my airplane as hard if not harder than most. It has been flown to the extremes many times over. Bottom line is the stock servo setup for the elevators has performed flawlessly for each and every one of those flights. You can engineer, cypher, calulate and scratch your @$$ all you like but you lack the real world expereince with the Bobcat. If you want to change the elevator servos in your Bobcat change the damn things but don't keep insinuating, and claiming that there is a flaw in the design when you DON't have the real world experience to support it.

BTW of two Bobcats that I KNOW had elevator problems, both of them were rigged incorrectly, I helped a buddy last weekend change his rigging to the correct one. Give it a rest!

David Reid

--------------------------------------------------------------
RE: The instruction book (page 34) has a picture with the linkage connected to the last hole on the servo arm. That is how I set mine up.
--------------------------------------------------------------------

As I am intersted in this topic, I looked this up. The picture on P. 34 is from a perpendicular angle, and the hole cannot be seen.
The 4 arm cross servo arms included with my servos only have two holes per side, and this would seem to be the 'last hole' that is illustrated in the photo.

The Illustration on the plans that the builder is referred to clearly shows the linkage hooked to the second (and apparent last hole of the arm selected). It is to scale.

The step referring reads:
"Make linkage and cut servo control horn as shown on the plans."

My point is (for the record) that the instructions are clear and complete on this matter.

My plans are dated 8 August 2001.
The latest date I see in my manual is 15 November 2001.
(Mine listed maximum speed at 175 mph).

Just thought I would provide some facts as clarification.

Best Regards,
David

These two bobcats are insainly fast with no problems. Those that saw them at Jets Over Whidbey will know how much we work our birds. Correct rigging lets you look like champ instead of a chump.

Justin Sands

OK.. so now tell me what the effective torque applied to the surface is with the pushrod at this distance from center, coupled to a control horn at its distance from the hinge line (it's lever point).

My guess is off the top of my head that you'll find this servo delivering over 120 oz" of effective leverage to the surface. There is a fair amount of mechanical advantage in the Bobcat elevator linkage geometry.

I don't know the exact distance of the control horn off the hingeline, so someone else will have to do the actual math.

Well I feel compelled to throw in my two cents worth ---- fuselage shape and CG -- I believe are at the root of the mushy elevator problem with the Bob Cat. Let me explain before everyone has a great laugh and chews me up --------

My Bob Cat is more nose heavy than most, I originally installed the "Rat Trap" and in doing so could not get a perfect dry balance on the CG points! My Bob Cat is slightly nose heavy with no fuel, 1/4 ---- 3/8 inch forward. You know I've never checked the CG with full fuel, but I can feel the CG when flying and at takeoff and for about the first 5 min of flight I am very careful through high speed high G turns, split S pull outs etc. ------- CG changes toward the end of the flight, but I never get the recommended CG configuration.

Now for my really unconventional idea :stupid: ------ the fuselage shape ----- at high speeds I believe the canopy and general up sloping shape of the front of the Cat most of it being in front of the CG also contributes to the soft elevator response. I notice it especially when really smoking through a descending to level high speed round house turn -- the higher the speed the more mushy the elevator response. It feels like something is pushing the nose out and or down, something more than a slightly nose heavy CG force and different than servo stalling. (although to my knowledge I've never flown any model aircraft that the elevator deflection was stalled because of aerodynamic pressure, even during the extra hard pulls when racing)

My reasoning is if the servo stalled OK -- so be it -- but I already had enough up elevator to continue the turns diameter, but I still was getting a strange push on the nose only to be reduced by reducing the speed. I have tried increasing the throws and that helped slightly. So the geometry of the elevator linkage, travel distance, and power of the servo will help at the margin.

Now for an observation I made that helped convince me the fuselage aerodynamics also is in this mix and has an effect, and possibly at the root of this anomaly. I tried the same high speed turn similar fuel CG configuration "inverted" inverted the mush seemed to be greatly reduced fact is I didn't notice it at all, I had plenty of elevator authority! Go out an try it and see what you think, but be careful to remember down is up and up is down

OK I'm ready to get clobbered for my thoughts on fuse shape, but I couldn't see this thread wilting on the vine yet before I got to add my 2 cents ---- Note: A straight and level high speed pass shows none of this push tendency so maybe the push feeling is some kind of airflow blanking created by the nose ------

I wished we could put the Bob Cat in a good wind tunnel with full smoke indicators so we could really see whats happing aerodynamically.

The only thing I haven't tried yet is stronger servos ----- I would guess more elevator travel and more servo strength will over come almost all of the mushy "pushing" feel at high speeds -------
Lee H. DeMary
AMA 36099

PS I don't think this anomaly is a real problem, unless you've already stuck your Bob Cat in the ground because of it - ---- the Bob Cat has so many other good characteristics --- I just live with it and love it Fact is I've ordered the King Cat ---- but I am going to setup the CG more tail heavy on the King for starters -----

David Reid etc.

My final post is this. I do have two Bobcats, the first with 100 flights on it which, I think, may count as some real world experience, and I like them so much I don't want to lose them, period. Some aircraft HAVE been lost and I would like to get to the bottom of it, no more, no less. The only real answer is to put the thing in a wind tunnel and MEASURE the loads. However, someone far more qualified than I has done the calculations to see if there COULD be an elevator problem and the results of this work are available free to anyone who wants them, his employer paid him well for his work, he did this as an intellectual exercise, in the hope that he could shed some light on the problem for us and for which he asked for no return.

When a fullsize aircraft is lost the accident is investigated in very great detail, there is rarely a single cause, but usually a primary cause and contributory factors. What is not know for CERTAIN is why BCs have been lost, or if indeed there is ANY common cause. What can be easily done is to install more powerful servos and if the problem continues then we must look elsewhere but at least one obvious gap has been plugged.


There is a most interesting parrallel in the US at the moment. The Learjet 45 is currently grounded after one aircraft suffered a brittle fracture of the horizontal stab. actuator screw (HSAS) a failure which occurred on just one aircraft long after the certification process. The FAA has grounded the entire fleet at enormous cost to the operators and Bombardier, the manufacturer. The fleet remains grounded until a new spec. HSAS has been manufactured and fitted. A new actuator was manufactured after the first failure but the FAA are not entirely satisfied with its manufacture, so the Lear 45 is grounded until further notice. ( source : Flight International). I am most certainly not suggesting the Bobcat be grounded but just illustrating the way in which the FAA approaches control system problems.

Whilst BCs are not man- carrying they can, like any other model jet, cause serious injury or death if they are out of control with reduced pitch control so I think the problem should be identified if at all possible in the best interest of the manufacturer, operator and third parties.

Whatever anyone may say, I believe I have done my bit to identify the problem and, if nothing else, builders and operators may now take very close notice on installation and geometry and make sure that there are no errors there.

I have nothing more to offer.

Regards,

David Gladwin