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All Forums >> RC Airplanes >> RC Jets >> RE: Bob Cat Copy Falcon 120 Jet
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RE: Bob Cat Copy Falcon 120 Jet - 6/22/2007 9:07 AM   
i3dm


 

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From: ReutIsrael, ISRAEL
Status: offline 		On that subject, why does the BVM bobcat use so much down thrust angle on its turbine ?

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RE: Bob Cat Copy Falcon 120 Jet - 6/22/2007 3:14 PM   
Woketman



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Status: offline 		I have heard that it was due to the higher velocity air going under the horizontal stab due to the turbine exhaust. Said higher velocity air causes higher downward stab lift and, therefore, a nose-up pitching moment. The exhaust downward turbine incidence compensates. I do not know the angle (and Bobcat guys here? Can you add the details?). But I agree: you should tilt the turbine nose-up a few degrees.

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RE: Bob Cat Copy Falcon 120 Jet - 6/22/2007 4:16 PM   
TCHIEF


 

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From: , FL, AFGHANISTAN
Status: offline 		does anyone think with the flat bottom stab that it may make a diifference ,im not sure but i would think with that airfoil on it it would produce positive lift on the tail and give a tendancy to push the nose down
could explain the take off jumping (high speed,lift on the tail keeps the nose planted) and when the throttle was cut it dove toward the ground again lifting tail

not sure how that works

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RE: Bob Cat Copy Falcon 120 Jet - 6/22/2007 4:22 PM   
bruce88123


 

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quote:

ORIGINAL: TCHIEF

question for the moderater why does it say FL, afghanistan when im in jacksonville fl.


Go to the top of the page where it says "my account" and see if you can fix it.


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RE: Bob Cat Copy Falcon 120 Jet - 6/23/2007 7:30 AM   
rocketman-RCU


 

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Status: online 		Angel, Where did you buy the OS 45 degree exhaust adapter and the 45 degree curved header for the OS 91? I don't see it in Tower Hobbies. Regards, Gary

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RE: Bob Cat Copy Falcon 120 Jet - 6/23/2007 7:20 PM   
Woketman



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Status: offline 		"take off jumping (high speed,lift on the tail keeps the nose planted) and when the throttle was cut it dove toward the ground again lifting tail".

That could also be cured by tilting the turbine ala Bobcat.

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RE: Bob Cat Copy Falcon 120 Jet - 6/23/2007 7:48 PM   
rcjets12



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From: Palm Bay , FL, USA
Status: offline 		I am no expert but to me any time that you have down or up thrust on a turbine or engine you are compensating for some thing in the air frame. I have seen the so called original bobcat fly when it was called the RAMBAT only one made that I know off and the turbine is mounted on straight and it flys very well no up or down tendencies. Also why most of all the other twin booms design don't have the problem of the bobcat. By the way yes I do have a BVM Bobcat and my friend has NP bobcat 50 with no down thrust at 105mph and no pitch up problems like my BVM Bobcat, So be carfull of how much down thrust you put on the NP Falcon you might not need as much if any at all.


Jose


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RE: Bob Cat Copy Falcon 120 Jet - 6/23/2007 8:03 PM   
SinCityJets


 

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Status: offline 		Just an FYI, the King Cat also has built in down thrust.

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RE: Bob Cat Copy Falcon 120 Jet - 6/23/2007 8:10 PM   
Kevin Greene



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Status: offline 		With a flat stab airfoil I would think that the problem with the jet exhaust efflux causing a lifting moment of the stab would be magnified...

Kevin

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RE: Bob Cat Copy Falcon 120 Jet - 6/23/2007 8:14 PM   
Kevin Greene



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Status: offline 		Perhaps the prop versions of this plane will not exhibit the pitching problem due to the way the air is distibuted over and under the stab from the prop and not just totally under the stab as does jet exhaust...At a VERY high velocity I will add...

Kevin

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RE: Bob Cat Copy Falcon 120 Jet - 6/24/2007 4:13 AM   
Woketman



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Status: offline 		If the flat side is on the bottom, that would tend to make the stab lift more creating a nose-down pitching moment. The high speed jet eflux would do the opposite. So they should tend to work against one another.

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RE: Bob Cat Copy Falcon 120 Jet - 6/24/2007 8:21 AM   
swat1


 

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RE: Bob Cat Copy Falcon 120 Jet - 6/24/2007 9:31 AM   
Kevin Greene



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quote:

ORIGINAL: Woketman

If the flat side is on the bottom, that would tend to make the stab lift more creating a nose-down pitching moment. The high speed jet eflux would do the opposite. So they should tend to work against one another.


Yep, you're right--Basic aerodynamics 101..Now I know why you work for NASA!!!

Kevin

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RE: Bob Cat Copy Falcon 120 Jet - 6/24/2007 9:48 AM   
TCHIEF


 

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Status: offline 		i think ill give it the nasa try if it dont work build anouther

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RE: Bob Cat Copy Falcon 120 Jet - 6/24/2007 11:01 AM   
Woketman



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Status: offline 		If you do decide to tilt the turbine nose-up a bit, ala BobCat, I would start easy. I'd measure a BobCat and use half the angle to start. In other words, if a BobCat has 3 degrees of tilt, I'd go with 1.5 degrees to start. See how it works and adjust from there. Should be easy to change by shimming with washers.

Please let us know.

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RE: Bob Cat Copy Falcon 120 Jet - 6/25/2007 5:33 AM   
angelrcdesign



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Status: offline 		Sorry

I thought it was OS But its MACs' header. They should help you.

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RE: Bob Cat Copy Falcon 120 Jet - 6/25/2007 8:23 PM   
KHORTON



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From: Raleigh, NC, USA
Status: offline 		Got mine last week. I was looking for a everyday, local field, beater I which would work with my 4 year old Wren 54 (fully manual version). Our runway is only 300 ft aspalt. The smaller Nitro Bobcat was too small however this one seems to be a perfect fit.
Quality of the ARF better than I had anticipated considering the price. A couple of the fuse formers are loose but that is a good thing, because it makes them easier to remove which is necessary to mount the 50oz fuel tank and equipment. In my opinion there are way too many formers anyway. The fiberglass fuse is pretty thick and the wing root is reinforced with a ply doubler. I am also replacing all the plastic bolts w/ metal bolts for the boom and stab attachments. I am doing away with the twin elevator servo setup in favor of a single 9411 mounted in the stab. All of the retract wells/rails are installed and precut ready to accept your retracts. Anybody have their retracts installed yet? I am thinking about Robart 630's

Here are a few pics. I think I just finished the hardest part of the build which was , How to mount the turbine?



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RE: Bob Cat Copy Falcon 120 Jet - 6/25/2007 11:02 PM   
Todd Jackson


 

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From: west monroe, LA, USA
Status: offline 		All my formers were loose also so I pull all of them out except the very front one and the two rear ones with no trouble at all..
I will reglue the remaining ones and I plan on putting in a tray front to rear to mount everything on ..I have started putting Robart 630's in mine but had to make up my own mount for the nose gear because it is a little to wide for the blocks they had and the steering arm will hit the formers that they installed plus it needed to go up inside some so the steering cables would be inside the fuse. Will go to work on the wings next putting the gear in it and overall I am imprested with the plane for what you pay...(Got to get the turbine next).. I am wanting to ad some paint detail and clear it. Anybody know what it is painted with? Will try to post pics tonight....
Todd


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RE: Bob Cat Copy Falcon 120 Jet - 6/26/2007 12:01 AM   
i3dm


 

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Status: offline 		For you turbine guys, when are you planning to get this baby air born with a turbine? should be ab wesome combo i believe..

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RE: Bob Cat Copy Falcon 120 Jet - 6/26/2007 12:20 AM   
KHORTON



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Status: offline 		Todd, How about a few pics of the retract installation. What is the lenght of the strut?

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RE: Bob Cat Copy Falcon 120 Jet - 6/26/2007 1:07 AM   
Todd Jackson


 

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Status: offline 		I will have to adjust them for this jet they were in a t-28 plus I will wait till I have the mains in to set the length because I think it needs a little nose high attitude for takeoff roll. Will post pics tonight.

Todd

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RE: Bob Cat Copy Falcon 120 Jet - 6/26/2007 3:35 AM   
DAVID WILDE


 

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From: COLCHESTER,ESSEX,CO3 4DF. U.K., UNITED KINGDOM
Status: online 		I have built and flown the smaller 46 bobcat with wren 44 gold and i have 5 degrees of down thrust which i will be doing the same to the falcon 120. I believe the bobcat has about 7 degrees of down thrust.The reason for this amount of down thrust is two fold ,one so you don't burn your lovely covering but i suggest you ali tape it and secondly the increased thrust velocity under the flat bottom tailplane if was dead straight increases pressure under the taiplane giving a tail up tendency hence the reason for the down thrust to decrease this pressure and reduce pitching down tendencies.
David Wilde COLCHESTER A1 MODELS UK SELLER OF THESE MODELS

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RE: Bob Cat Copy Falcon 120 Jet - 6/26/2007 5:16 AM   
figueroa



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Status: offline 		Nice install ,
Looks like the wren 44 is a perfect size for the airframe.

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RE: Bob Cat Copy Falcon 120 Jet - 6/26/2007 7:54 AM   
Todd Jackson


 

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Status: offline 		Here are some pics but having to do them one at a time for some reason..Todd

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< Message edited by Todd Jackson -- 6/26/2007 8:10 AM >

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RE: Bob Cat Copy Falcon 120 Jet - 6/26/2007 8:00 AM   
Todd Jackson


 

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Status: offline 		more... well 2 worked I will try for 3...

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