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Old Jan 19, 2011, 10:22 AM
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Joined May 2003
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Need Help with correct CG Location on my New EDF Jet

[IMG][/IMG] Hi Guys:
I am told that there are experts available on this site who might be able to help me. i just built a scratch jet (that is, from plans) jet called a Force 10, designed by a fellow named Dan Marlowe. I am unable to locate Mr Marlowe for his assistance in this matter, so i am turning to you guys ( and/or gals). I was told that you can locate the proper CG from the photo of the plan view, so I took the picture today and here it is:



Wingspan 55.5 inches
distance from LE wing to LE stab= 25 inches
wing root chord = 17.75 inches
wing tip chord = 6.5 inches
Wing sweep distance = 8.25 inches
Stab root chord = 9.25 inches
Stab tip chord = 3 inches
Stab sweep distance = 5.25 inches
Stab span = 22.25 inches

If it matters, AUW is 14.5 pounds.
I have inputted all this data into the CG calculators available online. My primary issue is that these plans are not very accurate in other areas and I am terrified to trust what is shown on the plans for CG.The blue tape shows the plan's suggestion for CG location. If what you guys say coincides with what I calculate, then the plans CG is too far aft. I will have a much higher level of confidence if your suggestions are in accord with the calculations I have done.
Thanks for any help. I do appreciate it. As far as I have been able to determine in my emails with Bob Ruff, this may be the only one of this particular design ever built. Obviously if that is true, none have ever flown, so I need all the input I can find prior to attempting the first flight.
I was quite surprised when Bob indicated that there are folks who can simply use the planform view and determine the correct CG for most planes. COOL!!! I anxiuosly await your responses.
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Old Jan 19, 2011, 10:48 AM
High Altitude Specialist
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go here: http://www.geistware.com/rcmodeling/cg_calc.htm
it explains what dimensions to plug in and then where your CG should be. I usually choose 30% of the mean aerodynamic chord (MAC) for my CG, but 1/4-1/3 back works.
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Old Jan 19, 2011, 11:29 AM
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Thanks, snivilous. I appreciate your help. Actually I have already used the available online calculators, at those I could find. I am hoping that someone or ones will have an alternate method of calculation in order to confirm waht these calculators say.
Thanks again!
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Old Jan 19, 2011, 11:32 AM
EDF all the way!
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Gilbert, Az
Joined Jun 1999
7,958 Posts
Greetings, one of the guys will give it a shot. Jet looks good!
Bob
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Old Jan 19, 2011, 11:33 AM
High Altitude Specialist
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Quote:
Originally Posted by bonefishfool View Post
Thanks, snivilous. I appreciate your help. Actually I have already used the available online calculators, at those I could find. I am hoping that someone or ones will have an alternate method of calculation in order to confirm waht these calculators say.
Thanks again!
yeah, I saw what you said above and how to verify the results. You could do a long hand method, but it should yield the same thing as your doing the same calculations. Or if you just want to be quick you could measure 35% back from your root chord and that would probably give you a rather generic ball-park figure as well.
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Old Jan 19, 2011, 11:35 AM
HOW HIGH DOES IT GO???
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Jacksonville, Florida
Joined Nov 2004
2,153 Posts
Not an expert but I would want to error on the side of caution. Im thinking that 1" in front of the pic cg may be a good place to start.

Great looking jet by the way...


Ben
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Old Jan 19, 2011, 12:08 PM
"...certainty is absurd."
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Old Jan 19, 2011, 12:23 PM
life is an oyster. shuck it!
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bennington, VT
Joined Mar 2008
3,175 Posts
wow, that came out nice!
looks like a sniper/f20 hybrid. very sweet.
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Old Jan 19, 2011, 12:46 PM
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Thanks everyone! i appreciate your input. kcaldwel, my calculations put it about the same- 7 inches back from wing root LE. that makes me feel more confident. Hopefully others will confirm as well.
The jet did come out nice. Most of the credit should go to my brother. He did the hard stuff. It looks really nice. i decided to defer the detailing until after the maiden. It has the new XPS fan and I am trying the new Eflite large retracts. It was a very tough build- most everything required complete re-engineering, which is why I am suspicious of the plans recommended CG
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Old Jan 19, 2011, 01:01 PM
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Guys:
Herer is another photo of the jet in my brother's shop. Still considering details such as decals, art, etc. Better wait to see how the first flight turns out. i love the air intake he put in the forward part of the stab base fairing.

What do you guys think??

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Old Jan 19, 2011, 04:29 PM
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Kevin
I measured the jet. The actual numbres are-
nose to wing leading edge at the centerline of the aircraft = 25 inches
fuselage measures 6.5 inches wide, but flares as at the inlets to 9 inches. I am not certain if ths makes any difference. I am trying to plug these numbers into the spreadsheet you sent but am trouble understanding exactly how to do it.
Thanks again for your assistance.
James
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Old Jan 19, 2011, 05:47 PM
"...certainty is absurd."
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Old Jan 19, 2011, 06:32 PM
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Thanks, Kevin!!! You are the man. i truly appreciate it. I feel so much better about giving it a go. Forward bests aft every time, but I will need to have an effective elevator to rotate.
I'll keep working with the spreadsheet. i did actually read the accompanying instructions, but math is not my strong suit.
It is nice to have folks like you available to help the rest of us.
James
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Old Jan 20, 2011, 03:36 AM
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Quote:
Originally Posted by bonefishfool View Post
[IMG][/IMG] Hi Guys:
I am told that there are experts available on this site who might be able to help me. i just built a scratch jet (that is, from plans) jet called a Force 10, designed by a fellow named Dan Marlowe. I am unable to locate Mr Marlowe for his assistance in this matter, so i am turning to you guys ( and/or gals). I was told that you can locate the proper CG from the photo of the plan view, so I took the picture today and here it is:



Wingspan 55.5 inches
distance from LE wing to LE stab= 25 inches
wing root chord = 17.75 inches
wing tip chord = 6.5 inches
Wing sweep distance = 8.25 inches
Stab root chord = 9.25 inches
Stab tip chord = 3 inches
Stab sweep distance = 5.25 inches
Stab span = 22.25 inches

If it matters, AUW is 14.5 pounds.
I have inputted all this data into the CG calculators available online. My primary issue is that these plans are not very accurate in other areas and I am terrified to trust what is shown on the plans for CG.The blue tape shows the plan's suggestion for CG location. If what you guys say coincides with what I calculate, then the plans CG is too far aft. I will have a much higher level of confidence if your suggestions are in accord with the calculations I have done.
Thanks for any help. I do appreciate it. As far as I have been able to determine in my emails with Bob Ruff, this may be the only one of this particular design ever built. Obviously if that is true, none have ever flown, so I need all the input I can find prior to attempting the first flight.
I was quite surprised when Bob indicated that there are folks who can simply use the planform view and determine the correct CG for most planes. COOL!!! I anxiuosly await your responses.
I would say 1cm in front of your marker will be spot on, good luck with maiden, Joe.
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Old Jan 20, 2011, 06:08 AM
Da' Cajun
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Lake Charles, Louisiana
Joined Jun 2009
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Wow! nice lookin' bird.

I've scratch built a couple jets and nothing cause me more sleepless nights than the CG question.
After doing all the CG calcs out there's nothing like the tried & true foam glider. Cut out a to scale glider and start throwing it around in the backyard.
I usually start balancing the glider tail heavy, toss it up in a steep climb and watch how it reacts to the stall.

If it floats too much or does the falling leaf stall = bad.
I look for a definite nose drop and steep glide with little pull out in the after stall glide (really rough on the nose). Transfer that CG spot, as a % of the root, to the model and it's worked every time.
It will be slightly nose heavy and as you fly the model more and more you can creep the CG back to suit your flying style.


Julian
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