RE: Nitro models pusher Bobcat?

RE: Nitro models pusher Bobcat?
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RE: Nitro models pusher Bobcat? - 10/7/2008 7:06:33 PM

BillS

Posts: 3077
Joined: 9/6/2004
From: Greensboro, NC, USA
Status: online

Below are my findings after a few flights and a several repairs.

Tried two different .45 engines and they will barely fly the airplane. With inadequate power the initial leap off ground will be much worse.

OS .50 engine
All up weight 8 pounds.
11 x 6 p 12,500 rpm – very marginal on take off
10 x 7 p 13,000 minimum rpm for take off
10 x 6 p 13,800 minimum rpm – most reserve and best take off power

Ground incidence 1/4 to 1/2 degree positive – sticks to ground well on landing
Weight on stab to rotate – 18 1/4 oz.
CG – 10” to 10 1/4”

Ran out of fuel Sunday and surprisingly the airplane handles and glides well without power.

The top of the elevator is flat with the top of the stab, which seems wrong. Other experience indicates the airplane should fly faster with elevator centered on the stab resulting in cleaner air. Will experiment with shimming the booms up.

I have flown with aileron reflex (3/16) and the change in climb is reasonably small. Flight change with airbrake (5/32 and 10% down elevator) is also reasonably small. Flight with canard seems to add drag during take off. Lots of possibilities for more experimentation.

Bill

(in reply to TUMBLER)

Post #: 3501

RE: Nitro models pusher Bobcat? - 10/7/2008 10:35:49 PM

opjose

Posts: 6145
Joined: 11/26/2005
From: Poolesville, MD, USA
Status: offline

quote:

ORIGINAL: TUMBLER

I just carved one out of foam and glassed it and then hollowed out the foam to make it a little lighter. This is the .25 size though. Same thing though....they all come broken.

Tumbler

Hah thanks, that's another logical way of doing it, ( which I overlooked. ).

Looks nice.

Did you do anything to the gear on the Bobcat .25 to strengthen it?

Thanks.

_____________________________

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Build em'', fly em'', crash em'' and build more.

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Post #: 3502

RE: Nitro models pusher Bobcat? - 10/8/2008 1:15:19 AM

TUMBLER

Posts: 462
Joined: 6/24/2002
From: Voorhees, NJ, USA
Status: offline

I re-covered the entire plane with Ultracote because it had to tear up the fuse and wing to modify it for retracts....so yeah I did haha.

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Post #: 3503

RE: Nitro models pusher Bobcat? - 10/9/2008 6:10:32 PM

BillS

Posts: 3077
Joined: 9/6/2004
From: Greensboro, NC, USA
Status: online

quote:

The top of the elevator is flat with the top of the stab, which seems wrong. Other experience indicates the airplane should fly faster with elevator centered on the stab resulting in cleaner air. Will experiment with shimming the booms up.

I shimmed the rear of the booms up 1/16” and then 1/8”. Shimming seemed to introduce minor twitches (mostly aileron) in the air, which were not noticed previously. Don’t know why but it’s possible linkage play is starting to show up. Will probably remove the shims.

Bill

(in reply to BillS)

Post #: 3504

RE: Nitro models pusher Bobcat? - 10/10/2008 5:54:27 PM

BillS

Posts: 3077
Joined: 9/6/2004
From: Greensboro, NC, USA
Status: online

quote:

I can't seem to keep the Bobcat from ballooning up on touchdown when I give it a little elevator to soften the landing. Four landings, four bad balloon-stall-crash events.

After a landing bounce I wondered if the airbrake switch could be used for both spoilerons and for flaperons. Discovered today that ‘airbrake’ switch can be used to control spoilerons or reflex the ailerons up and to control flaperons or reflex the ailerons down.

Airbrake is a three-position switch. One position was set for spoilerons for take off and landing. One position was set for normal flight. One position was set for flaperons with a little down elevator. Hopefully flipping the switch to flaperons after ground contact will reduce the landing roll and tendency to bounce back into the air. Hope to test tomorrow. Might end up with one spoileron setting for take off and another spoileron setting for landing.

Bill

(in reply to BillS)

Post #: 3505

RE: Nitro models pusher Bobcat? - 10/10/2008 10:12:31 PM

sbrackets

Posts: 110
Joined: 3/28/2007
From: Simpsonville, SC, USA
Status: offline

Fixed gear or Retracts! That is the question.

Hey Guys, Well I'm getting ready to start the build on my Bobcat and just wondering if it's worth installing retracts, or just go with the fixed gear that comes with the kit.

Is the wood mounting in the wing strong enough to support retracts? what about the extra weight with the retracts? etc,etc,etc.

I am going to use my "NEW" Evolution .52 NX in this bird. Just wondering about the power to weight ratio. Then if I go with the fixed gear I have the drag to worry about. Let's not forget about the added cost of the retracts also.

Your thoughts? What would you do?

Jim

(in reply to BillS)

Post #: 3506

RE: Nitro models pusher Bobcat? - 10/11/2008 12:21:49 AM

HeliTB

Posts: 1203
Joined: 9/14/2006
From: Fort Washington, MD, USA
Status: offline

Once you do fixed it will be a bear to later install rectracts. Not saying is can't done but difficult. The plane was put together using hot glue not epoxy, so it's everyone's advise to put epoxy on everything that connects to the landing gear blocks. Trust me I know. A soft landing can knock the landing gear out.

It's a great plane to build and bear to land.

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Raptor v2 OS 37/Bobcat Jet OS .55 AX
Extra 330L 25% SV26cc/SIG Kougar OS .46 AX

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Post #: 3507

RE: Nitro models pusher Bobcat? - 10/11/2008 1:39:28 AM

cyborg1

Posts: 61
Joined: 11/24/2005
From: Bangkok, THAILAND
Status: offline

Retracts will definitely help with the top speed of the plane. I noticed that immediately after folding the gear up. However, then there's the complexity of actually cutting the wings up and reinforcing everything in the wheel bay and retract mounting area, plus snaking the air lines into the wing and then into the fuselage...sounds like a lot of work. On the other hand, the 52 engine is just enough for the plane to fly, depending upon your choice of prop. If you go with a 7" pitch prop, you'll probably get about 70-80 mph top speed, but takeoff may be a little hairy. Add a tower hobbies tuned muffler, and performance will be a bit better. With a 7" prop, you need to be turning about 13 - 14k to actually get decent performance, but nothing compared to all the videos posted with the .75 engines.

(in reply to HeliTB)

Post #: 3508

RE: Nitro models pusher Bobcat? - 10/11/2008 2:23:27 PM

BillS

Posts: 3077
Joined: 9/6/2004
From: Greensboro, NC, USA
Status: online

Cyborg1 is correct. A 52 is small for the Bob Cat and you will need rpm on take off. More power is your friend during take off. The airplane will probably weigh 8 pounds w/o retracts.

For speed the retract money could go towards a larger engine. For the additional work and money retracts might be considered for the second Bob Cat.

Bill

(in reply to cyborg1)

Post #: 3509

RE: Nitro models pusher Bobcat? - 10/13/2008 5:20:54 PM

BillS

Posts: 3077
Joined: 9/6/2004
From: Greensboro, NC, USA
Status: online

The giant and dangerous big bounce damaged the Bob Cat over the weekend. However the wind was about 12 mph and got under the wing during the bounce. I am about to conclude that when the take off is smooth (not leaping into the air) the sink rate during landing is not high enough. As each boom shim was remover the take off continued to get better. The landings continued to get worse.

Bill

(in reply to cyborg1)

Post #: 3510

RE: Nitro models pusher Bobcat? - 10/14/2008 2:20:45 AM

kurten

Posts: 1
Joined: 10/13/2008
From: AAlesund, NORWAY
Status: offline

Hi, what do you think of this modification of the Bobcat 50.
Picturelink
Upsidedown view
Interior
This is actually a video project, but as I have this itch to modify everything I could not help to try a forward elevator prinsip and removed the elevator and tails all to gether, fittet rudder on tip wings instead.
There's also a tuned pipe installed inside the fuselage with firewall offcourse, done so for optimal aerodynamic performance for propeller and general.
The plane was ordered with a fixed landing gear, but i managed to fitt medium sized spring retract and cramed the air bottle far in the nose.

spec:
Bobcat 50
Engine, Picco 60
Remote video system.
The bullet camera has a home made tilt and swing system that can be controlled by the radio.
Weight: abt 4,2 kg with full tank
A home made scope underneeth with the video transmitter inside, possitioned so for best cooling.

Have to edmit that the plane has yet been in the air, suspect it be a bit heavy.
By this time the season for model flyers in Norway is over and the flying fields has to high gras so i might have to wait for spring to try it out.

< Message edited by kurten -- 10/14/2008 8:41:36 PM >

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Post #: 3511

RE: Nitro models pusher Bobcat? - 10/26/2008 1:16:00 PM

Xairflyer

Posts: 313
Joined: 12/17/2002
From: Co. Donegal, IRELAND
Status: offline

quote:

ORIGINAL: sbrackets

Fixed gear or Retracts! That is the question.

Hey Guys, Well I'm getting ready to start the build on my Bobcat and just wondering if it's worth installing retracts, or just go with the fixed gear that comes with the kit.

Is the wood mounting in the wing strong enough to support retracts? what about the extra weight with the retracts? etc,etc,etc.

I am going to use my "NEW" Evolution .52 NX in this bird. Just wondering about the power to weight ratio. Then if I go with the fixed gear I have the drag to worry about. Let's not forget about the added cost of the retracts also.

Your thoughts? What would you do?

Jim

Have a look back at a post of mine (p130 no. 3245) regarding the undercarriage mountings, I would definatly recomend making the mod. I have fitted the fixed gear and happy with it, if you wanted to fit retracts you would be better with the retract version.
The demensions etc on my previous post may help anyway if you decide to modify it to suit.

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Post #: 3512