Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

Estimate of Sig Kobra Speed

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Old 12-26-2007, 04:28 PM
  #1  
frenchie79
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Default Estimate of Sig Kobra Speed

OK Speed boys, I have a Kobra with a os 46Ax and a 9x7.5 prop and turns it right at about 16K on the ground, the Plane weighs 3.5 lbs. We will never have a radar gun so can someone with knowledge of this plane tell me what the level speed could be-I know it is a guess or an calculated answer.
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Old 12-26-2007, 05:33 PM
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Mike Connor
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Default RE: Estimate of Sig Kobra Speed

You may be on the high side of 100 mph. Why not get an audio of a high speed pass and we can check it using Doppler.
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Old 12-26-2007, 05:40 PM
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gsoav8r
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Default RE: Estimate of Sig Kobra Speed

My 2 cents even thou Ive never owned a Sig Kobra.
Just based on RPM vs. pitch the numbers say 113 mph.
Initial guess based on drag and having fixed gear one might say 105-ish.
However thou, the thrust numbers for your engine/prop combo suggest your making 5.5 - 6 lbs.
This makes me believe that your Kobra may see 115-ish on a good cool day.

Cheers.

(Ive got my fire extinguisher handy. lol)
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Old 12-26-2007, 07:00 PM
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BillS
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Default RE: Estimate of Sig Kobra Speed

Simply measure a distance at the field, 200 or 300 feet. Then put a stop watch on the time to travel the distance for three passes (need to average the timing error). Someone can then calculate the speed for you.

Bill
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Old 12-26-2007, 10:11 PM
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Flyboy Dave
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Default RE: Estimate of Sig Kobra Speed

It should go about 110, no problem.

FBD.
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Old 12-26-2007, 11:11 PM
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frenchie79
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Default RE: Estimate of Sig Kobra Speed

Thanks guys-I will do both this spring. Audio? Use a video camera or just record sound? I don't understand the doppler-small club!
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Old 12-26-2007, 11:26 PM
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Mike Connor
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Default RE: Estimate of Sig Kobra Speed


ORIGINAL: frenchie79

Thanks guys-I will do both this spring. Audio? Use a video camera or just record sound? I don't understand the doppler-small club!
I usually extract a wav file from a video but all I need is the sound of a high speed pass as it it coming at you and going away form you. A program takes care of the rest.
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Old 12-28-2007, 12:03 PM
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Default RE: Estimate of Sig Kobra Speed

what program are you talking about mike ??????
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Old 12-28-2007, 12:14 PM
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daven
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Default RE: Estimate of Sig Kobra Speed

you may want to try and find an APC 9 1/2" x 8 N prop, seems to work pretty well with that motor. I'm guessing your a little over a 100 mph, with potential closer to 120 mph with the right prop combo.
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Old 12-28-2007, 12:57 PM
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Mike Connor
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Default RE: Estimate of Sig Kobra Speed


ORIGINAL: johnfly

what program are you talking about mike ??????
http://www.sprut.de/electronic/soft/scope.htm
It is in German but I let Google tranlate the page.
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Old 02-07-2008, 09:52 PM
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Default RE: Estimate of Sig Kobra Speed

does the sig kobra requieres any mods?
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Old 02-08-2008, 08:20 PM
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Default RE: Estimate of Sig Kobra Speed

Sweet little project!

I leave the speed estimates to these guys, but no matter what the speed that is an awesome little ship!

I has that same airplane in the 1980's and used an OS-28F engine and it was fast then!

Cheers
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Old 11-02-2008, 08:15 PM
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wpvrtis
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Default RE: Estimate of Sig Kobra Speed

I've got a Kobra with a K&B .48. I put my Garmin GPS in it and clocked it at 125 with a 10x8 prop. I had a 10x7 3-blade prop in it and clocked it at 121.
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Old 11-02-2008, 09:31 PM
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Default RE: Estimate of Sig Kobra Speed

I look at your plane and 115 would be my guess. that MA prop is closer to a 5.5 pitch not 7.5 compare it to an apc and you will see.

a APC10x7 would be my choice or maybe a 10x8 trimmed down some. you have another 10-15 mph left in that plane though.

1. put the ant. in side the plane
2. try get you tail servos back inside the plane too.
3. get rid of the MA prop LOL not really speed props or not as good as the APC's
4. try some Jett race wheels
5. get rid of the tail wheel and use a wire skid back there if for all our speed for that airframe.

I do like the that plane and looking at yours will inspire me. I have a 60lx looking for a home.
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Old 12-22-2008, 01:30 PM
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Default RE: Estimate of Sig Kobra Speed

I built my Kobra Almost the same as wpvrtis' Kobra and used his Garmin GPS. I couldn't believe it when I got it on the ground, but I have witnesses. With a new, unbalanced MA 10x6, I got it up to 149mph!!!! Not bad for a little plane. I plan to use a better prop (APC 9x8 maybe) and see if that does me any better. Oh.. and I wasn't flying with the wind either.. I was flying cross wind, low altitude.

Mods? trike to taildragger. Moved LG blocks forward. Used hardwood where the LG blocks where and made it span the wing center (really strong). OS 46AX. twin wing servos HS 225 (stripped them with the standard plastic gears, bought metal, works great). HS 225 (with MG) for Elevator. F16 tail. Seperate servos for Rudder and tail wheel on y-harness. All servos except throttle in the tail. All internal.

I also didn't put the canopy or turtle deck on. Lastly, the landing gear wires in the pictures are different now. They were long to allow for a big prop to clear, but now I shortened them to the orginal size. Problem is I almost always smack the prop.... Gotta learn how to land this thing... its FAST!
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Old 12-23-2008, 05:23 PM
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Default RE: Estimate of Sig Kobra Speed

You could change the wheels to e-flight micro wheels in 2" size. They still have some foam and give a little prop clearance. The biggest prop i can use is a 9". The APC 9x7.5 or 9x8 work I think i even tryed a 9x8.5?. This plane is as fast as i can handle. Lands like a dream- not fast at all, just takes along time to bleed the speed.
Found the wheels-2.5" from Dubro
http://www3.towerhobbies.com/cgi-bin...6&I=LXTDS7&P=K
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Old 12-24-2008, 08:01 AM
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Default RE: Estimate of Sig Kobra Speed

I noticed you also went with the f16 tail config. How did you do the tail wheel? Mine was a pain, but only because I tried a different setup that broke right after I boxed in the fuse...
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Old 12-24-2008, 08:27 AM
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Default RE: Estimate of Sig Kobra Speed

Humm..................Not one of you guys are using .19 to .35 sized engines that’s recommended by SIG.

I must be in the right place.......

Merry Christmas Ya bunch of speed freaks !
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Old 12-24-2008, 08:36 AM
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Default RE: Estimate of Sig Kobra Speed

Does anyone use the recommended engine size?
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Old 12-24-2008, 08:55 AM
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Default RE: Estimate of Sig Kobra Speed


ORIGINAL: cptcrusty1

Does anyone use the recommended engine size?
Not since my trainer with a K&B .40
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Old 12-24-2008, 09:00 AM
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Default RE: Estimate of Sig Kobra Speed

It was a rhetorical statement.. .LOL
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Old 12-24-2008, 09:01 AM
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Default RE: Estimate of Sig Kobra Speed


ORIGINAL: cptcrusty1

Does anyone use the recommended engine size?

I put a .15 on a SIG wonder.

Correction TWO .15 on a SIG Wonder
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Old 12-26-2008, 10:14 AM
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Default RE: Estimate of Sig Kobra Speed

I used a Pull/Pull set up for the tail wheel. If i did it again i would just use a skid and forget it. Takes off in 5 to 10 feet. Sorry so long on the reply.
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Old 12-26-2008, 01:21 PM
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Default RE: Estimate of Sig Kobra Speed

Hopefullly, WPVRTIS and i are going to race them today... should be sweeeeeeeet!
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Old 12-26-2008, 05:25 PM
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Default RE: Estimate of Sig Kobra Speed

I have got to MOVE-Wisconsin sucks 6 months out of a year. Screw Ice fishing. I wanna fly!
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