dribbe

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??)

DavidR

My Feedback: (1)

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.

Doug Cronkhite

My Feedback: (34)

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

rcjetsaok

My Feedback: (7)

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


ttfn

Danno

hornet driver

My Feedback: (7)

danno

you be ...tore up from the floor up

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

hanna

My Feedback: (3)

Does the diamond shaped airfoil used have anything to do with this? Also what is the radius of the leading edge? What is called out for trailing edge thickness? Is there a profile drawing in plans for the finished stab/elevator. Mike Krizan

bcovish

My Feedback: (8)

Mike, call me 281-5818. I'll be up for awhile

David Gladwin

Since some of my previous posts were trashed by individuals who did not understand what they were talking about I have abstained from involvement in RCU. However, this BobCat elevator problem is too good to miss. Not ONE of the posters on this thread, or BVM on the website, has touched on one of the most fundemantal aspects of control surface design which may well be, probably is, the root cause of the problem . With data recently derived from flight tests with my Avonds F16 with JetCat 160 power equipped with a GPS system I will explain it in practical and theoretical terms in the next RCJI.
Regards,
David Gladwin

grbaker

My Feedback: (29)

First, let me say that I have not been following this thread very closely since I do not own a Bobcat.

I have been flying a P120 powered Roo, the old kit with single elevon on each side. I've been flying with the speed limiter set at 170 mph. The CG was set at the manufacturers spec. I have seen several threads that suggested moving the CG forward (actually way forward) would eliminate some possible problems, such as not being able to get out of a flat spin after a chicken flop. So I moved my CG approximately 3" forward. I was able to do this by shifting batteries around in the fuse and removing some lead weight from the tail. Now bear with me as here comes the point.

I lost the Roo on the first flight with the new CG (with speed limiter set at 160mph). I lost elevon control at the bottom half of a loop. The elevons were powered by an 8411 on each one. I was running a single 1400mah 4 cell RX pack. My linkage was not set up for optimal power from the servo as I had long servo horns installed so that the linkage would clear the bottom of the wing. I sure am glad I had just received my Indian fire pack as the fire was pretty bad.

The lesson I learned is that a CG change can be the difference between having enough elevator power and having a yard dart. From now on I will be using 5cell packs (with or without regulators) and I will make sure to optimize the servos power with proper linkage setup.

I hope this helps someone out there, because it is an awful expensive lesson, if I'm the only one that benefits from it.

Doug Cronkhite

My Feedback: (34)

How about you share it with those of us who don't get RCJI. I realize you want to sell magazines David, but this is a safety issue and should be discussed IMO.

-Doug

Rojeck

My Feedback: (22)

David

I too would appreciate it if you could shed some light on the Bobcat elevator problem. I just started on a new Bobcat project and the next issue of RCJI will probably be out well after I'm done so, any light you can shed now would be great.

Sung

DavidR

My Feedback: (1)

David,

It was very unfortunate, and uncalled for the way you were trashed in the other thread. I would really like to hear your viewpoint on this. Although I have not had problems the idea of a servo with a metal gear train is becoming a little more attractive to me.

DR

ghost_rider

My Feedback: (20)

David

Let me be the first to tell you that your opinion and comments are highly valued.

I’ve learned a lot from you starting from Compuerserve (not a lot of people that lurks around here had access to computer then).

In consideration with the foregoing, I would like to ask you to forget about the past and continue to provide your opinions here. Any light you could shade on this problem might save other few $$$$ and potential for an accident.

Regards

Ben

sfaust

My Feedback: (11)

Compuserve...what memories.. I remember lurking there as well.

How many remember when the superhighway was only a dirt road called ARPANET and BBN, and an e-mail took hours or days to be delivered, not seconds or minutes. A smiley face would be a feat in ascii only. Green screens and text only....those were the days NOT!

Anyway, lets not digress. Every extra 8bits wasted was important back then.

David Gladwin

OK guys, safety comes first so here goes:
In my RCJI review of the BobCat I questioned the servo specification on rudders and elevators, see page 54 of the April/May 2002 issue. The BVM spec. of the 3421 mini servo on the rudder I believed to be quite inadequate for a gyro controlled rudder and I recommended this servo be upgraded to a 9411 as was fitted to my review BobCat. Time was to prove that assessment was correct, the 3421s could not cope and the 9411 is now recommended by BVM and JR recommends that the 3421 is not used for a gyro application.

I also mentioned in that article that the 3301 servo for elevators was only adequate with 500 sized engines and that my next BobCat with a bigger engine (now almost complete) will have more powerful servos (at least 9411s) for a greater RESERVE of power. The reason is this: The tailplane and elevators of the BobCat are, like most model aircraft control surfaces, basically crude lacking both aerodynamic and mass balancing. The BobCat elevators are heavy, having ply skins and mine weigh a little over 2 ounces. The center of mass of these elevators is about 1 3/4 inches fom the hinge line so that the torque resisting the servo power is 2x 1 3/4 or 3.5 ounce inches when the model is static.

I know from flight tests with my JetCat GPS equipped F15 that the F15 pulls a peak of 10.9 g in moderate aerobatics and upto 12.9 G in hard turns. ( I spoke to Horst Lennerz of JetTronics about this at the JWM in South Africa and he agrees that these values are realistic). I am sure that the big, thick wing of the BobCat can easily attain load factors of 10 and in a 10 g manoevre the torque generated by the mass of the unbalanced stab. is now 35 oz/ounces, so more than half of the servo power is used just to hold the elevator against G forces. The BobCat elevators have considerable travel so we must add blowback forces to the torque required to hold the elevator position and my guess is that the servo is having a very hard time indeed and in some instances is inadequate to provide the required pitch response.

It should not be forgotten that in a high G situation the heavy ailerons will also cause the aileron servos to pull a high current to just hold position which will do nothing to help battery voltage. If the long leads to the elevator servos are not heavy duty and a 4.8 volt battery is in use, the actual voltage at the elevator stabs. may be insufficient for the servo to deliver its rated 60oz/inches further reducing pitch control.

The solution is easy, (certainly easier than mass balancing the elevator as is done on full size machines) just use a bigger servo, at least a 9411 or a 8411. Sure, the servo will protrude slightly below the stab. skin. and when I emailed BVM recently to order some BobCat parts I mentioned I was using bigger servos and suggested BVM make some little ABS fairings as they do for the wheels on the top skin, no reply to that issue ! Many fullsize aircraft with thin wings have their servos (PCUs) mounted externally in fairings and there are no flutter or buffet problems even on Concorde at Mach 2. Make sure ther is NO play in the servo mounting, there is little or no vibration protection requiredand I screw mine down HARD , again see the picture on page 54 of RCJI.

The slightly heavier servos may require some noseweight, no problem use a bigger battery and if its a nicad. make sure it is an SCR type which can deliver high currents without significant voltage drop.

I use twin Duralite batteries with 5.2 volt regulators with heavy duty leads THROUGHOUT the BobCat and have experienced no problems whatever.

The BobCat is a delight to fly. (I fly mine with the excellent PST 600 R which someone in the US malicioulsy tried to discredit by writing to the AMA claiming he had seen one explode) and my XL will soon be airborne with a JetCat 160, Jetcat GPS, JetCat speed limited and Data link system to obtain in flight performance data, but with JR 8411 servos for pitch control.

There may be other contribitory factors to pitch control degradation such as flow seperation on the diamond shaped airfoil (was it tested at all angles of incidence in a wind tunnel ?) but my guess is that a decently powerful servo with a reserve of strength will see the problem disappear. If that does not fix the problem then it has to be aerodynamic, something I very much doubt.

In the meantime I must get back to writing the report on the JWM in South Africa where I, and the other competitors, received the warmest of welcomes and was loaned a model with which to compete in the open class. ( Thanks James and Mark). and where the South Africans proved they have fixed the Fiber Clkassics Mig 29 hydraulic problems, even offering me the chance to fly it for them after the contest.

I hope this helps to prevent any further uneccessary loss of any more BobCats.

Regards,
David Gladwin

lov2flyrc

My Feedback: (24)

David,
Thank you kindly for your valuable insight. You brought up points I had not even considered! After reading your comments I have been reassured the data I have collected to this point is valid.

Thanks again!
BRG,
Todd

S_Ellzey

Hi David,

Interesting analysis, however I feel that you may have a few errors. I just took some measurements from my set of drawings and I can not get to the 1.75“ moment arm you speak of for the center of gravity. The geometric center is only 1.4” from the hinge line, and since there is more structure forward of this point than aft, the CG should be forward as well. Could you have meant 1.25”? If so, for your 10 g case this gets the hinge moment due to inertia down to 25 ounce inches. Now that load must be transferred to the servo shaft. To do this the moment is multiplied by the ratio of the arms (servo arm divided by the control arm). For the as drawn configuration this ratio is 0.5625 (.45”/.8”), also note that since this ratio is less than one it is the maximum that will exist. As the servo moves its effective arm reduces faster than the control arm on the surface so it gains more advantage the higher the deflection. Multiplying 25 ounce inches by the transfer ratio of 0.5625 gives us the torque the servo must produce to hold the surface, 14.06 ounce inches. Again, this the maximum, if the model is pulling 10 g’s the elevator is deflected a fair amount and the servo gains advantage thus lowering the torque a bit.

Respectfully,

Steven Ellzey

DavidR

My Feedback: (1)

Another factor that no one has mentioned is the holding power of digital servos. JR does not publish this data but the holding power of digital servos is somewhere around 3 times the torque rating of the servo. (Compare on Mutliplex's website)

lov2flyrc

My Feedback: (24)

True David....
But you have to deflect the surface before you hold it there. If you cannot deflect the surface to pull out of a dive what good is the holding power?

Todd

DavidR

My Feedback: (1)

Todd,

If you can deflect that surface flying at 200 mph straight and level it can deflect it in a 200 mph dive. The holding power would apply during the high G manouvers that David G was refering to. Also for what it is worth I have been running the JetCat GPS in my Bobcat for the last 40 flights and have had one flight where it was over about 7 G's. It did hit 9 G's in one flight but I was also snapping the hell out of the airplane and really racking it around, but at lower throttle settings. One flight I tried to pull the airplane as hard as I could to see just how many G's I was pulling and it was still around 7. During that flight I was doing full throttle passes followed by square 90 degree pullups, and full throttle 360 degree high G turns. If you rig the airplane correctly the 3301's are up to the task.

DR

David Gladwin

Gentlemen,
Your mathematical analysis may well be better than mine, I gave ball park figures but the bottom line is this: A number of BobCats have been lost due to diminished pitch response and the power of the servos used is now in question. The fix is easy, install much more powerful servos and that possibilty is eliminated.

I doubt if the problem is aerodynamic. The high tail of the BC MAY be a problem but I doubt it. The behaviour of the BC at the stall is absolutely fine and although the spin is oscillatory (perhaps due to the gyro on the rudders) recovery is almost instantaneous on releasing the controls.

I trained in the RAF on the Vampire T11, which had a configuration similar to the BC, tailplane blanking or diminished pitch response was never a problem throughout the flight envelope including stalling and spinning, although I should add that control response of the manual ailerons and elevators in roll and pitch was dreadful as Mach crit. was approached. However, I have many years of flying a T tail airliner, the VC10, and after the loss of Tridents and a BAC 111 in deep stall accidents caused by tailplane blanking both in flight test and in service (Trident at Heathrow) we were very careful with the VC10 during stall training because of that high tail, never taking it beyond the stick pusher.


There is another full size precedent, too. The early RAF Hunters suffered from jack stalling on the powered flying controls (mainly ailerons , I believe) at high indicated airspeeds. which frightened quite a few pilots in the '50s. That was fixed with better and more powerful PCUs.

On balance I think servo power is the problem so, if there is even a hint of doubt about servo power, why not just install more powerful servos and eliminate the possibilty that the servo is the problem and perhaps save the aircraft, certainly that's my plan with my XL.

No more ideas, nothing thing more to add, I'm afraid so I'll see how I get on with my XL with 8411s on the elevators. In the meantime if any more BCs are lost because of this problem perhaps owners would keep this list up to date and advise of the servo specification.

Regards,
David Gladwin.

PS If one is running out of pitch control rememeber that extending the gear produces a marked nose up pitch at high speed, that MAY just save the day

Tom Antlfinger

My Feedback: (24)

I did notice one of lost planes was a Roo with 8411's with a 4 cell, 1400 pack. My buddy and I have hundreds of flights on Roo's and HS's doing all kinds of snaps, spins, chicken flops, etc. all with the C.G way forward like Dennis Lott recommended. In fact, when I built the planes, I used Dennis's old Roo for comparison and the green Wilcox machine that Jason spun to set up the HS's C.G.

Even a gentle pull on the backside of a loop, at least in full scale, gives about 4 + G's, which has the effect of essentially stalling the 8411's on large surface area elevons, made worse early in the flight when still full of heavy fuel.

I can't find the printout someone posted a while back showing current draw vs. stalled 8411's at various hanging weights on the ouptut arm, especially if you are using the outermost hole on a 1" or longer arm, but the currents were very high. I remember that at modest weights, it was 3-4 amps thru each servo.

Pulling that much current thru an AA 1400maH, 4 cell pack is Russian roulette IMO. Definitely could lead to mushy response or momentary voltage drop enough to failsafe the receiver. Remember, those alkalines and Nicads were not designed for our use, but for low current, long duration uses, most notably in penlight size flashlights, which draw a couple of hundred milliamps, not AMPS.

Minimum I now use is either an 1800 or preferably 2000 or 2400maH 4 cell sub-C pack w/o regulator or a minimum 1800 maH 5 cell, sub-C pack with a regulator to 5.6 volts or lower. Lots of guys have had success with Lith's but only with high power density packs of at least 2400-3600 maH.

Also stay away from those lightweight switches that come with many new radios. Those things date back to the 4-cell, 450 maH days of 20 years ago, and many are not up to handling high current loads. Take one apart, and you will see they are junk for a 6K+ airplane.

And don't forget the long lead length in BobCats from Battery to Servo. Use 22 gauge 60 strand wire...the heavy duty stuff from JR or Andy Low are examples.

I have seen David R., Jason, Torrey, and others pound the piss out of BobCats with the recommended servos with no ill effect....so something else is most like the culprit here.....like are you getting enough juice to them under heavy load and do you have the geometry correct?


Tom

sfaust

My Feedback: (11)

And if you don't feel like taking them apart, go to my web site at http://webpages.charter.net/mediasho...cal%20main.htm and click on the switch comparison link at the bottom left. I took apart a Futaba standard switch, and a JR Charge switch, and posted photos of the designs. I won't use a standard switch from any manufacturer anymore.

Woketman

My Feedback: (6)

Stephen, I just tried your site but none of the photos loaded.

mr_matt

My Feedback: (10)

As I said I ran an analysis, and I can assure you that the available torque is HIGHLY dependent on the proper linkage geometry, in particular the length of the servo arm.

For instance, going from a .43 in. length arm to .6 in. DOUBLES the servo torque requirement.

A builder must be very careful to make accurate linkages with this design, as in any design, but more so here because of the geometry and the very high speeds people are pushing.

BTW this post is not meant to fly in the face of anyone else or any theory, I am only saying that ALL THINGS BEING EQUAL, watch the geometry!