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  1. #1

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    Nitro Planes Bobcat

    I am writing this in hope someone has an answer. I have a Nitro model Bobcat which is electrified. A friend had one with glow and it went in. It went the entire length of our field and went straight vertical and was uncontrollable. Has anyone experienced this and do you have an answer to the cause? We feel the problem might be the CG but can't get a definite answer from Nitro. No one there seems to have any technical answers. The mains were too far back and they were moved forward closer to the approximate CG. Also the horizontal stab was changed out to a flat stab since an aeronautical engineer in our club felt a lifting stab would keep the nose from rotation.
    I was told by John of Nitro that a user of username OPJOSE is an expert on this plane. Hopefully I will get a response from same. E-mail me at jyalove@verizon.net

    Thank you RCPULLER

  2. #2
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    RE: Nitro Planes Bobcat

    I e-mailed you are replied to you in the other thread as well...

    ----

    It sounds like you had the Nitro Models Bobcat .50. Is that correct? Did you get the one that is retract ready?

    As posted here, the C.G. should be set to approximately 10.3"-10.5" from the leading edge of the wing at the wing root.

    The plane will fly with the gear as it comes, but it is important to make sure that the attitude of the plane at rest on the ground has a positive incidence relative to the ground. If you don't do this the plane will not lift off the runway at run-up.

    There was no need to change out the stab as you've done. Many here have flown it as is.

    ----

    As everyone in the other thread has suggested, it sounds like you may have set the CG wrong initially which caused the problem.

    ---


    Everyone there seems to be having a lot of good luck with this plane going as far as installing turbines ( !!! ) in it.



    There is an art . . . to flying. The knack lies in learning how to throw yourself at the ground and miss.

  3. #3

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    RE: Nitro Planes Bobcat

    OPJOSE, I have a question please. I'm up in Alaska, my dad's in TN and doesn't have a PC, so I'm asking this for him. He's got a Bobcat Jet 50 (like you've been discussing), and it won't lift. Here's the scoop: He's got a K&B 61 engine, and has tried 11x8, 12x6, and 13x6 props. He's got a big wheel up front (fixed gear) to have a positive incidence relative to the ground. The CG is correct based on your advice to the other gentleman's posting. He's got a 560 foot runway and when attempting to take off (yes- into the wind) it doesn't lift. It does make a really nice nitro r/c car. My dad's been building planes of one sort or another for over 50 years so he's hasn't made some novice error (like I make : ). Thank you, Gary gone2dog@mtaonline.net

  4. #4
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    RE: Nitro Planes Bobcat

    There has to be something keeping the nose down, forcing it not to lift.

    Did you retain the engine's slight up pitch when you installed it?

    Could the aileron's not be centered, but are actually slightly deflected down, causing inverse reflex. This would also cause the plane not to lift. Try making the aileron centering such that there is a degree of upward incidence of the ailerons at stick center.

    Are you sure the wing has 2-3 degrees of positive incidence.

    A "big wheel" may not do the trick if the plane pushes the nose down at runup. Particularlly with soft wheels.

    You maybe loosing all positive incidence as the plane runs up to speed.

    There is an art . . . to flying. The knack lies in learning how to throw yourself at the ground and miss.

  5. #5

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    RE: Nitro Planes Bobcat

    I also lost one that was uncontrollable immediately after take off. My mistake was that I forgot a basic rule of balancing. If the fuel tank is in front of the CG, balance with the tank empty. If the fuel tank is behind the CG as on the Bobcat, balance with the tank full. I CG'ed with an empty tank. When I fueled it, I had put about eleven ounces behind the CG making it extremely tail heavy. You know the rule, a nose heavy plane fly's poorly but a tail heavy plane flys only once.
    Al Burman

  6. #6
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    RE: Nitro Planes Bobcat

    There are a few areas that you have to be careful about with this plane, especially if you have not flown a Bobcat type plane before...


    - When setting the neutral point of the elevator control surface, align the trailing edge of the elevator with the back TOP edge of the stabilizer.

    Initially I did what I would do in any normal plane. I used a straight edge along the bottom ( flat part ) of the stab, and aligned the elevator to this, assuming that this was the ideal "neutral"... boy was I wrong!

    The plane would NOT lift until it was far down our paved field, then it JUMPED into the air almost straight up.

    After landing the plane one of our resident "jet guys" looked it over and pointed out that I had it "forcing the nose DOWN"...

    He was right... I popped out the control horns and moved them over one tick on the servo, so that the trailing edge was level with the back of the upper part of the stab...

    Now the plane flew perfectly.


    - You can set your radio up with FLAPERON's to help in landings... e.g. right aileron on channel 2 and left aileron on channel 6, and use the radio mixing for flaperons.

    If you do this, unlike conventional planes, you want the both ailerons to come - UP - 1/4" or so when you hit the flap switch.

    This will cause the plane to balloon up in flight a bit, but it will land with a beautiful nose high attitude.


    - DO NOT break in a new engine on this plane! Because it is a pusher prop plane, the engine will tend to overheat on the ground.

    Get your engine broken in and tuned up elsewhere, then move it to the Bobcat when it is ready.

    I used a GMS .76 Ringed engine with an APC 11x7P pusher prop. This turns out to be a very nice combination. Some people are also using Graupner 11x8P props with success.


    - If you purchased the retract ready version, first attempt to loosen the mounting blocks from the underlying wood support, using a screwdriver. On one wheel one was not properly glued.

    I would recommend popping them up ( or off ) if possible and liberally applying epoxy to the whole area.

    If they are somewhat secure, expoxy all the exposed wood areas around the gear mounts. You'll be glad you did.

    Also see if you can insert a block of wood on both sides of the gear mounts, under the existing wood. Epoxy this in place.

    Choose screws that will go through the existing mounts into this new piece of wood for a very secure and sturdy gear mount.


    - I set up my front stearable gear using a Pull-Pull system. This worked great but I initially had problems with the wire tangling when the gear was retracted.

    I solved this by letting the wire run UNDER a small metal bar that I placed at the point on the wheel well where the circular area of the wheel meets the longer area of the strut... ( pictures later ).

    This prevents the wire from looping around the gear or kinking.

    The pull-pull wire is strung THROUGH the plastic wheel well using a bit of tubing at the junctures.


    ( Edit: I was STILL not happy with the results... see subsequent posts for a much better solution! )



    - I left about one degree of "reflex" ( postive or "up" aileron movement )when the ailerons are neutral.

    This helped keep the nose from dropping, for hands free flight. If you use Flaperons, try it.


    - As per another suggestion here, I ran my exhaust THROUGH the wing so it comes out under the plane by using a Dubro Silicon Exhaust Extender.

    A great idea, now the plane stays clean.

    I used grommet washer's I had on hand to strengthen the two holes I made for this... I also wicked CA onto the underlying Balsa after cutting the holes to both hold the covering and strengthen the area.

    There is an art . . . to flying. The knack lies in learning how to throw yourself at the ground and miss.

  7. #7

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    RE: Nitro Planes Bobcat

    I purchased mine without the retract option. Does it come with retracts if I went with the option? I went with the non retract option because my CMP P40's retracts were junk so I didn't want to mess with retracts again for a while. But if anyone has any easy success stories, I might change my order. Pics would help!

  8. #8
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    RE: Nitro Planes Bobcat

    I think you misunderstood the web site.

    There are two versions of the Bobcat in the .52 and 1.20 size Falcon.

    One which is retract-ready, the other which is not.

    The "retract-ready" is configured to accept second party retracts, such as the Robart retracts.

    The plane does NOT come with pre-installed retracts.


    The non-retract version would require extensive modification to retro-fit retracts.

    I've posted pictures of the retract openings on other threads.

    There is an art . . . to flying. The knack lies in learning how to throw yourself at the ground and miss.

  9. #9

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    RE: Nitro Planes Bobcat

    Yeah I figured for an extra $5.00 it wouldn't come with retracts. If I am happy with the plane then I will order another and get the retract option on it. This is a cool airplne for the price! I am actually thinking about trying the next one with a EDF unit. That will be my first shot at electric. I will watch this forum for anyone who does a good job with an EDF.

  10. #10
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    RE: Nitro Planes Bobcat

    The plane flies great, but you would be advised to allow a longer than normal runway length especially for your first flights.

    DO re-inforce and epoxy the main gear area as much as possible. Especially at the beginning, this area takes a lot of of abuse... you WILL bounce the plane on your first flights which is not good for it.

    It lands hot... see above... for the flaps suggestion...


    IMHO it is very different than a standard tail dragger.

    Your first flights should be spent trying to get it back down safely and practising for landings.

    There is an art . . . to flying. The knack lies in learning how to throw yourself at the ground and miss.

  11. #11

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    RE: Nitro Planes Bobcat

    OPJose,

    Thanks for the advice! I just got my plane in today and I didn't think the place where the front gear goes into is right. I have attched photos below to see what you guys think. It looks like the mounting blocks angle and the fiberglass on the front of the mounting blocks isn't really glued to anything. If the pics are blurry, i will try to take cleaer ones. See what you think. By the way, I ordered the plane with standard gear but got the one for retracts so now I'm tempted to get some spring airs.

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  12. #12

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    RE: Nitro Planes Bobcat

    Looking at it now, It doesn't look so bad.

    Opjose, What gear did you use for this? I don't know if I saw you post spring air 300 series or if that was someone else.

  13. #13
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    RE: Nitro Planes Bobcat

    It looks just like mine...

    I used Robart retracts all around and these work fine.

    The only problem I've had is with the steering linkage.

    I WAS using a PULL-PULL system, and I did not like the results... I had two instances where the front gear failed to lock because of tension on the Pull-Pull lines ( they may have gotten caught up... ).

    The nose gear then collapsed or rather retracted on landing causing a nose "rash".

    BTW: I'd advise putting a bit of metal on the nose underside... I just did this to mine to prevent future scrapes.


    I just saw an EASY way to do the linkages for the nose gear...

    One of those "duh I should have thought of that" moments...



    Basically put a metal "rod" perpendicular to the steering "T" that Robart ( or others ) provides.

    ( You can see it in the left most gear in the above picture... )

    On this put a linkage that will slide up and down the rod, the cap the rod by any means so that the linkage will not come off.

    You can then attach a stiffer single flexi pushrod to this.

    When the gear retracts the linkage will slide on the rod thereby taking up the slack on the line.

    When the gear goes down, the tension pulls the sliding linkage back into place.

    There is an art . . . to flying. The knack lies in learning how to throw yourself at the ground and miss.

  14. #14

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    RE: Nitro Planes Bobcat

    How does a steerable front tire retract when it has a metal wire that is used to push and pull the steering "T"? I kind of see what youre talking about but I need a better picture. Do you have a photo of your setup? I will probably use air retracts because I have had nothing but bad luck with my mechanicals. $190.00 for spring airs air kit and everything.

  15. #15
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    RE: Nitro Planes Bobcat

    In the "long" Bobcat thread, you'll see pictures where people have used guy wires for the stock Pull-Pull stearing for the "T".

    This works but my wires inevitably get caught up... so I was looking for a better solution.

    I'll try to make a diagram and post it.

    There is an art . . . to flying. The knack lies in learning how to throw yourself at the ground and miss.

  16. #16

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    RE: Nitro Planes Bobcat

    opjose, May I first say that your insight in this model will help me alot. I am about to venture in the building a Bobcat 50 and could use the advise. A few things, what servos are you using for the tail? Is a EVO .61 engine have enough power? ( I want to keep the plane lite) Also the exhaust, very interesting, how you do that? I am using the Robart retracts, and will do the mods you advised. Have attached the wing perm or can you remove the wings? Thank you

  17. #17

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    RE: Nitro Planes Bobcat

    I just finished building a Bobcat 50 with retracts (see pic). I am now setting the CG and finding it very nose heavy using the 260mm balance point. Since the fuel tank is located in the rear of this plane, I am wondering if I should balanced with the tank at least 1/2 full. I currently have the receiver battery located in the forward compartment as shown in the instructions, but will most likely need to move it back to the rear compartment. Any suggestions will be appreciated!
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  18. #18

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    RE: Nitro Planes Bobcat

    Same question, should I balance it with an empty tank or with a full tank? is 10.5" the farther back the C.G. should be?

    Which pusher prop would your recomend for an OS 55AX engine?

    Thanks.

  19. #19
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    RE: Nitro Planes Bobcat

    I have mine at 10.3" instead of 10.5". It flies very well this way.

    I had to put the battery pack in the nose to get the c.g. right with my GMS .76 2C engine.

    The full tank is located almost on the C.G. so you do not have to worry about balancing it with a full tank.

    I find spoilerons to be almost mandatory.

    I would try a Graupner 11x8P ( pusher ) prop with that engine.

    I'm running it with the more powerful GMS .76, which gives me unlimited vertical.

    With the AX your plane will unload in the air giving it very good speed, but you will not get unlimited vertical.

    The plane picks up so much speed with the higher pitch prop anyway, that if you nose straight up under full power, the plane will get very high in the sky before it's lost momentum...

    A 10x7 3 bladed pusher prop is also available from Master Airscrew too.

    There is an art . . . to flying. The knack lies in learning how to throw yourself at the ground and miss.

  20. #20

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    RE: Nitro Planes Bobcat

    Ok thank you very much UpJose.

  21. #21

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    RE: Nitro Planes Bobcat

    Hello all
    would like to get some advise on balance point for the nitro models bobcat 46 to 61 engine I have herd os many differnt figures .and would like and help with setting it up so there are no suprises on the maiden flite not any big ones thank you for any and all suggestions Carl Hanson e-mail chanson@hughes .net

  22. #22
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    RE: Nitro Planes Bobcat

    I've followed the main Bobcat thread, and I've not seen other figures quoted for the C.G.

    Everyone is in agreement on where it should be. The only variance arises when people measure the C.G. from the wrong point, or when translating english/metric and back incorrectly.

    No matter which engine you install the balance point should be the same.

    This plane is meant to fly at a good clip, so it WILL wallow around at slow speed. You need to experience it yourself to see exactly what I am talking about. The plane continues to fly just above stall, but the controls feel mush and unresponsive.

    If you are too low this way you can loose the plane.

    That means that your landings have to be made faster than what you may be used to with a tractor plane.

    In turn this made the plane hard for me to get down, as I was always coming in too fast, and the plane would just roll, and roll, and roll....

    The solution was to add the spoileron function.

    Spoileron, raises the ailerons in unison slightly ( about 1/8-1/4" ), but still maintains aileron movement.

    The raised ailerons/spoilerons, diminish lift, increasing the glide angle, making it easier to bring the plane in at the higher speed required.
    The spoilerons also diminish tip stalls to a degree.

    Once on the ground the spoilerons add a bit of drag too.

    With spoilerons turned on the plane lands with a very convincing nose high attitude which is a joy to see.


    There is an art . . . to flying. The knack lies in learning how to throw yourself at the ground and miss.

  23. #23

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    RE: Nitro Planes Bobcat

    UpJose,

    Did you programed any spoileron-elevator mixing on your radio? Do you have 1/4" or 1/8" of travel on your spoilerons?

    We have a runway lenght of aprox. 400ft at an elevation of 1600ft over sea level, do you think it will be enough for landing?

    How much positive angle of attack should it have on the ground (is it noticeable by sight)?

    Thanks.

  24. #24
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    RE: Nitro Planes Bobcat

    ORIGINAL: caiman

    UpJose,

    Did you programed any spoileron-elevator mixing on your radio? Do you have 1/4" or 1/8" of travel on your spoilerons?

    We have a runway lenght of aprox. 400ft at an elevation of 1600ft over sea level, do you think it will be enough for landing?

    How much positive angle of attack should it have on the ground (is it noticeable by sight)?

    Thanks.

    I have somewhere between 1/4" and 1/8" spoilerons, more is not needed. Each aileron is on a seperate channel on my JR.

    I also have the front gear mixed to the rudder and on a switch. When I drop the landing gear for landing, the nose servo becomes active.

    That way, I can use the rudders in flight w/o using current on the nose servo.

    I also toyed with the idea of putting each rudder on a seperate channel so I could program in an air brake.

    When I modeled this in G3, the plane became unstable, as the angle of the rudder pushes down HARD on the tail when the rudders are engaged as brakes.



    We have a paved runway length of 500ft and we are at 240ft above sea level.

    My Bobcat uses the WHOLE length of the runway to slow down w/o spoilerons, after landing just after the threashold.

    With the spoilerons it uses slightly more than half of this...

    You want about 2 degrees postitive ( nose up ) angle of attack on the wings as the plane sits on the ground. Yes it should be noticiable.

    There is an art . . . to flying. The knack lies in learning how to throw yourself at the ground and miss.

  25. #25

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    RE: Nitro Planes Bobcat

    Did you progremed any mixing with the elevators so that when you deploy the spoilerons the elevators compensate for up or down pitch?


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