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Old Oct 25, 2007, 10:56 AM
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kevin's Avatar
ohio
Joined Aug 1999
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true.

I have been watching this thread somewhat closely. I think you might be putting too much emphasis on thrust. Think about many of the edf's out there. They are fast but don't necessarily have a thrust to weight greater than 1. My HET f-16 doesn't (its close), but if i pull up vertical after a high speed pass it will climb out till i cant see it anymore. the combination of pitch speed and thrust is important but you don't need all that thrust unless you want to take off vertically from the ground, although you might want that
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Old Oct 25, 2007, 11:08 AM
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New Jersey
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True, thrust is not everything but you have to have enough of it. 200mph speed without adequate thrust behind it is just a prop spinning its wheels and a plane hardly moving. If you have a 5 lbs plane with 2.5lbs of thrust and if you have 150mph pitch speed, it is going to suck when it comes to acceleration and at top speed it will be limited because that amount of thrust can only push you so much. The thrust of a plane drops off as the speed increases If you have a 150mph pitch speed and 4-5lbs of thrust, now you're talking!

EDFs don;t use pitch speed, they use efflux velocity. if you have two planes with the same efflux velocity but one has more thrust, the one that has more thrust will go faster. Your F-16 goes good because of the combo you have...I ahve had severeal HET planes. If you lost thrust but kept the same efflux velocity, your plane would not be nearly as fast.

The way my plane is flying now, it goes half way decent at full throttle one the sped is built up, but stalls in a shallow climb and takes forever to take off. That is not fun to me. I want speed and thrust.

Quote:
Originally Posted by kevin
true.

I have been watching this thread somewhat closely. I think you might be putting too much emphasis on thrust. Think about many of the edf's out there. They are fast but don't necessarily have a thrust to weight greater than 1. My HET f-16 doesn't (its close), but if i pull up vertical after a high speed pass it will climb out till i cant see it anymore. the combination of pitch speed and thrust is important but you don't need all that thrust unless you want to take off vertically from the ground, although you might want that
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Old Oct 25, 2007, 01:03 PM
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United States, MI, Detroit
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hmm i would say try actually flying with different sized props.. land put a new prop on, land put a new prop on.. until you get what you're looking for. If you're looking for speed, due to the fixed prop, acceleration will be slower..

Forget the computer, i've found the programs to be only a suggestion. Gotta try it out in real life. Get like a dozen props and try them all until you find the sweet spot, then tell the rest of us
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Old Oct 25, 2007, 01:48 PM
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Thats why I like turbines...the only thing you have to wory about is thrust. you know the efflux veclocity will be good, so just pick your thrust level and be done with it! For the cost of these high powered electrics, it almost makes more sense to just buy a turbine. I have been looking at a JetJoe 12lb thrust engine for the Bobcat .46. Then engine is $1100 with manual start. It would cost that for a high powered electric setup.

Brian

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Originally Posted by ashdec87
hmm i would say try actually flying with different sized props.. land put a new prop on, land put a new prop on.. until you get what you're looking for. If you're looking for speed, due to the fixed prop, acceleration will be slower..

Forget the computer, i've found the programs to be only a suggestion. Gotta try it out in real life. Get like a dozen props and try them all until you find the sweet spot, then tell the rest of us
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Old Oct 25, 2007, 02:10 PM
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you could get a smaller turbine for the .25.. from what i've heard, JJ engines are slightly unreliable and that Wren is the way to go, even tho its more expensive. But ya i completely understand what you're saying. There's no way an electric setup will compete with a turbine. Best advantage is no fuel and you can fly pretty much anywhere...
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Old Oct 25, 2007, 02:16 PM
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New Jersey
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JetJoe made a lot of changes to their engines 6 months ago to improve reliability, starting, and longevity. The Wren 44 is $2500. Way too much to puc on a $100 plane IMHO.

Yeah I hear you on the electric advantage. I would never put glow on it, especially as a pusher....ugghh the mess!!!

Quote:
Originally Posted by ashdec87
you could get a smaller turbine for the .25.. from what i've heard, JJ engines are slightly unreliable and that Wren is the way to go, even tho its more expensive. But ya i completely understand what you're saying. There's no way an electric setup will compete with a turbine. Best advantage is no fuel and you can fly pretty much anywhere...
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Old Oct 25, 2007, 02:44 PM
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United States, TN, Soddy-Daisy
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Looks like for me it's going to be a compromise. I hate those looong take off runs and I can't stand to have a plane with limited vericals. My eyes and ears probably can't tell much difference between 85 and 105 unless there are 2 planes in the air. Gimme some thrust and acceleration and hold the top end please. I have the same problem with another plane - it's fast on top end with a 10x10 but won't climb but with an 11x7 it's a whole different and better plane.

'course there might be another motor besides the Mega and the Medusa that can do it all for less than $200 but it looks like slim pickings so far.
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Old Oct 25, 2007, 04:27 PM
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United States, MI, Detroit
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whoa... didnt realize wrens were so much..lol ya that is kinda silly isnt it...lol. Well if you got the money, go for it. I have to stick with what i got.

Actually a hacker b50 on 28 cells should perform quite nicely. At full power flight times are only about 2 minutes, but i dont wanna be doing 150 all the time..lol
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Old Oct 26, 2007, 11:22 PM
Foamy Season is here! MX2#1
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Elizabethtown, Kentucky
Joined Nov 2005
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got my orange and black falcon25 today have to say this is a sharp looking plane! very simple build, and low tech instrustions haha, are you guys using hs 55s for the tail??? i seen the servo holes i figured it would use 81s, but i guess thats on in the wings?? need some help..
mike
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Old Oct 27, 2007, 06:08 AM
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United States, TN, Soddy-Daisy
Joined Mar 2006
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Mike,

I got 81s in the wings and will put Futaba 3107s and 3114s in the rudders/elevators because I have them. 55s are supposed to work well though.

Go back and look how Brian installed his elevator servos. Much better way if you have the skills.

Blair
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Old Oct 27, 2007, 07:57 AM
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New Jersey
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Thanks for the compliment Blair. I wouldn;t go with anythign less than 81's on the ailerons and elevator. I have HS-55's on the rudders but will eventually swap them out because it has trouble sustaining knife edge the full length of the runway with the 55's.

Brian

Quote:
Originally Posted by Volunteer
Mike,

I got 81s in the wings and will put Futaba 3107s and 3114s in the rudders/elevators because I have them. 55s are supposed to work well though.

Go back and look how Brian installed his elevator servos. Much better way if you have the skills.

Blair
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Old Oct 27, 2007, 02:19 PM
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New Jersey
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Measured with a fish scale on the tail....

7x7 cam cut down to 6.25x7

65 amps right off the charger. 60 amps if i let the batteries sit for a while before using. So lets go with 60 amps
1300 watts
3lbs 5ozs of thrust


7x5 APC cut down to 6.5x7
1200 watts
4lbs 2ozs of thrust
(will go slower but have more vertical)


6x5.5
850 watts
2.5lbs of thrust
(this would suck)

Looks like the 6.5x5 and the 6.25x7 will be the one's to try out. Been rainign for 5 days though, so no idea when that will be!

Brian
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Old Oct 27, 2007, 11:25 PM
Foamy Season is here! MX2#1
pitviper51's Avatar
Elizabethtown, Kentucky
Joined Nov 2005
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Quote:
Originally Posted by Volunteer
Mike,

I got 81s in the wings and will put Futaba 3107s and 3114s in the rudders/elevators because I have them. 55s are supposed to work well though.

Go back and look how Brian installed his elevator servos. Much better way if you have the skills.

Blair
thanks! would it be easy to use 81s all round for flight controls?? looks like only minor mods needed? im going for a fast plane so i need good control haha
mike
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Old Oct 28, 2007, 05:00 PM
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Macungie, PA
Joined Dec 2004
194 Posts
Hi guys, I've just received my Bobcat 25 in yellow and white. Some of the covering needs some work but my iron should take care of it. What's with the undersized hatch cover and the non threaded pushrods? Oh well the airframe was cheap enough to overlook some of the misgivings. I'm still undecided on the power plant. I have a Mega 16/25/3 and with 7 cells (Lipo) it should produce 1:1 weight to thrust ratio (static) on a 5x5 prop (care of Motocalc). My total weight will be just over 4lb. I intend to hand lauch and belly land as I fly in a large area with playing fields and rough grass. I'm limited with current to 45A as I'm using a Castle Creations HV45. I could go to 9 cells but the prop size gets too small. Although the motor is good for 55,000rpm I doubt that a prop would survive this speed . I'm using HS-81's for the ailerons and HS 65 HB's for the rudders and elevator (2 each). I'm using a horizontal motor mount for 400 speed motors(HLAN 2096) that I found on the Hobby-Lobby web site- http://www.hobby-lobby.com/motormt.htm.
I have a few questions:
1. Do you think that the motor will be strong enough to hand launch - possibly with bungee?
2. Do you think that the ailerons need metal gears? I'm assuming that the karbonite gears for the rudder and elevator are OK.
3. I'm going to install different control horns and pushrods (Robart 331 Super Ball Link). What is the ideal standoff distance for the pushrod? The options are 0.5, 0.75 or 1 inch.
4. To assist in my motor/ESC/battery selection, what is the max revs for Graupner and APC props?
Thanks in advance for any assistance that you may give.
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Old Oct 28, 2007, 06:08 PM
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New Jersey
Joined Feb 2003
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I re-covered the whole thing because the covering would not shrink down and I had to make a new hatch. I used my own 4-40 pushrods and control horns. I am not sure what you mean by the standoff distance, but as long at the pivot point of is directly over the hinge line of the control surface, you will be fine. A 5" prop seems really small for this thing. Based on what I have been going through, i would run the motor first if you have it already. Caclculation software does not seem to give the most accurate info. I think your best bet is to bungy if you are not using the landing gear. You do not need metal geared servos. Your servos choices are all good. I plan to go from 55's to 65's on the rudders myself. Can't help you much on motor choice as I am having a lot of trouble finding something that will work well.

Good luck,

Brian

Quote:
Originally Posted by djcb
Hi guys, I've just received my Bobcat 25 in yellow and white. Some of the covering needs some work but my iron should take care of it. What's with the undersized hatch cover and the non threaded pushrods? Oh well the airframe was cheap enough to overlook some of the misgivings. I'm still undecided on the power plant. I have a Mega 16/25/3 and with 7 cells (Lipo) it should produce 1:1 weight to thrust ratio (static) on a 5x5 prop (care of Motocalc). My total weight will be just over 4lb. I intend to hand lauch and belly land as I fly in a large area with playing fields and rough grass. I'm limited with current to 45A as I'm using a Castle Creations HV45. I could go to 9 cells but the prop size gets too small. Although the motor is good for 55,000rpm I doubt that a prop would survive this speed . I'm using HS-81's for the ailerons and HS 65 HB's for the rudders and elevator (2 each). I'm using a horizontal motor mount for 400 speed motors(HLAN 2096) that I found on the Hobby-Lobby web site- http://www.hobby-lobby.com/motormt.htm.
I have a few questions:
1. Do you think that the motor will be strong enough to hand launch - possibly with bungee?
2. Do you think that the ailerons need metal gears? I'm assuming that the karbonite gears for the rudder and elevator are OK.
3. I'm going to install different control horns and pushrods (Robart 331 Super Ball Link). What is the ideal standoff distance for the pushrod? The options are 0.5, 0.75 or 1 inch.
4. To assist in my motor/ESC/battery selection, what is the max revs for Graupner and APC props?
Thanks in advance for any assistance that you may give.
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