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Old Jan 31, 2002, 11:53 AM
That should fly!
Admiral_Red's Avatar
San Antonio, Texas
Joined Oct 2001
2,406 Posts
Daren, and other CG maestros "F-14"

I want to double check Matt's and my CG on the F-14 at 20 degree minimum sweep. Since there is such a large amount of flying surface generated by the fuse, this one could be tricky. As for the CG at higher sweeps, if the real one does not shift fluids or weight, then I don't think more than reflex mixed in to the stabilators is necessary.

James
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Old Jan 31, 2002, 12:13 PM
That should fly!
Admiral_Red's Avatar
San Antonio, Texas
Joined Oct 2001
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Here is a top view to work from.
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Old Jan 31, 2002, 02:05 PM
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Los Alamos, NM, USofA
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Actually, Dan's the GC Meister. Perhaps he'll grace you with his knowledge.

Daren
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Old Jan 31, 2002, 04:03 PM
That should fly!
Admiral_Red's Avatar
San Antonio, Texas
Joined Oct 2001
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Oop's, wrong Savage brother Thanks!

James
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Old Jan 31, 2002, 07:35 PM
Scott Black, Montreal
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montreal quebec Canada
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It is a common misconception that conventional fuselages have a large effect on stability. While they do produce lift, their lift curve slope, i.e. change in lift per change in AOA, is very low. That is predominantly a function of aspect ratio, and fuselages have an aspect ratio well below 1. They are not efficient lifting surfaces. They will shift the cg 2 or 3% at most. I don't have any direct experience or data on fighter fuselages but I suspect that at the AOAs where we fly that the story is much the same.

That lift curve slope is what determines the effect on stability, which is defined is pitching moment change per unit AOA change. Same with LEXs and such - sure they create vortex lift at high AOA, but below that they don't do much and we can't get to 20 deg AOA sustained (without an afterburner!) as the drag is so high. If we got a model up to that AOA we would fall out of the sky in no time flat.

I would extend the wing into the centerline and do a classic MAC calc. You can do a sanity check by looking at a side view of the full scale bird with the gear down. Those things have to rotate in a hurry to get off a carrier deck. The wheels can't be too far behind the cg. Probably a line drawn 10-20 deg forward of vertical starting from the wheel contact point will go through the cg. That will give you a good indication.

I would not be surprised (though I don't know) if the wings on that airplane are wet, so that when it sweeps back its cg moves back too. Tom Hunt would probably know. Bob Kress sure would as he was one of the Senior Engineers on the program I think.
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Old Jan 31, 2002, 08:18 PM
Purple power
gregg f's Avatar
shadow hills,ca
Joined Apr 2000
6,593 Posts
Scott wrote:
That lift curve slope is what determines the effect on stability, which is defined is pitching moment change per unit AOA change. Same with LEXs and such - sure they create vortex lift at high AOA, but below that they don't do much and we can't get to 20 deg AOA sustained (without an afterburner!) as the drag is so high. If we got a model up to that AOA we would fall out of the sky in no time flat.
----------------------------------------------
Scott: i'm not an engineer and won't pretend to be one.
but from my experience with a k.c.f18 the lex has a profound effect on lift at slower speeds. landing this plane at a 20deg aoa is not hard to do.
as far as sustained flight at that aoa, again i think it's easily doable at least while the pack is still producing well.
my plane is at 150w @ lb, and a t/w.7

perhaps i don't fully understand your post?if so, please enlighten me. .........gregg
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Old Jan 31, 2002, 10:42 PM
Scott Black, Montreal
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montreal quebec Canada
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How would you know unless you had flown it with and without the LEX. I doubt very much that you could maintain level flight beyond 20 deg nose-up. I have seen Kevin fly his and it is a good flyer, but I never saw him fly nose-high in level flight. Ask Chris Golds about trying to climb out with his Concorde. He couldn't do it, or if he could just barely. If you get to the point where you have vortex lift you had better have an after-burner.

I think what you are seeing is the effect of a low aspect ratio wing - much lower than most sport models. The lower the aspect ratio, the lower the lift curve slope and the higher the stall AOA - the obvious example of this is a delta wing. YOu can get the nose way up in the flare.

Another thing, I don't know the Tomcat that well, but the F14 doesn't really have LEXs does it? It is more of a glove that fits over the pivot and it is rounded on the LE is it not? LEXs must be sharp to develope vortex lift. A rounded surface won't do it.

The Delta fins on the aft fuselage of a Learjet are very similar in shape to a LEX and are sharp. Also, they are much further away from the cg so you would expect them to impact stability much more. Having wind tunnel tested them, I can tell you that at cruise and maneuvering AOA, they don't do much at all. They have to get to high AOA to function.

The LEXs "kick-in" so to speak, at high AOA. At the AOAs where we are flying at high power they are not doing much. I don't expect them to have any effect on pitch stability and hence cg location
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Old Jan 31, 2002, 11:31 PM
Purple power
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shadow hills,ca
Joined Apr 2000
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Scott: thanks for the explanation......gregg
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Old Feb 01, 2002, 01:44 AM
Lithium Member
Herb's Avatar
Irvine, Calif USA
Joined Feb 1999
12,176 Posts
F-14 CG

For the first flight I would place it slightly ahead of my calculated red dot, for safety.
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Old Feb 01, 2002, 01:54 AM
It wasn't me...
DanSavage's Avatar
Trabuco Canyon, CA
Joined Nov 2000
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Re: F-14 CG

Quote:
Originally posted by Herb
For the first flight I would place it slightly ahead of my calculated red dot, for safety.
That's about the same spot I came up with.

Dan
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Old Feb 01, 2002, 09:06 AM
That should fly!
Admiral_Red's Avatar
San Antonio, Texas
Joined Oct 2001
2,406 Posts
Thanks guys. The plans show it just a bit foward of Herb's spot. I figured it was in about the right spot, I just wanted to double check it. BTW the beta cut went to the laser cutter yesterday. You guys should see some updates soon.

Scott, the F-14 has no LEX really, just the wing glove as you mentioned. There is however a ton of surface to the fuse. The wing gloves also form a large symmetrical airfoil shape, be a low aspect ratio, it's still there. There is a bit of reflex action in the fuse called the beaver tail, right between the two exhaust cones.
To my eyes, this bird is gorgeous.

James
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Old Feb 01, 2002, 05:35 PM
Scott Black, Montreal
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montreal quebec Canada
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Herb, can you post the side view with the gear down?
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Old Feb 01, 2002, 06:46 PM
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Louisville,KY
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side view
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Old Feb 01, 2002, 06:48 PM
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Louisville,KY
Joined Jan 2001
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Sorry about the size.
Ken
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Old Feb 02, 2002, 01:09 AM
Registered User
Lakewood, Ca.
Joined Sep 2001
34 Posts
Quote:
Originally posted by Admiral_Red
.
To my eyes, this bird is gorgeous.

James
Not only your eyes James but mine too. This bird is awesome looking, it has been my favorite jet eversince I was a kid. I ordered the plans two days ago and just waiting for it to arrive.

One question, guys. You think it would be possible to turn this baby into twin electric pusher type aircraft? I need suggestions. I don't really have the fundings to power it with ducted fans (due to wife!) too costly. I'm thinking of powering it with glow power before, but I got into electric flight too deep that I'm shifting my flying to electrics (except to heli's).

Harold
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