I'm working on a jet design with a long nose and low aspect ratio wing. Think F-104. I'd like to approximate the effects of the long nose on the neutral point. Can anyone suggest a resource? I've built a small cg glider and get a NP of about 21% mac. I've attempted to perform traditional np calculations and incorporate estimates of fuselage effects, but my makeshift calculations yield an np much further forward than than I'm seeing on the glider.

Thanks for any assistance.

Bob

Try this link, it seems to be a good caculator. http://www.geistware.com/rcmodeling/cg_super_calc.htm The length of the nose though doesn't seem to come into play as far as this caculator is concerened.

Another good site http://www.ae.uiuc.edu/m-selig/ Work your way around this site, there is more info here than you can imagine.

Don

Bob,

There is an "If it flies" article in one of the 2009 Model Aviation magazine that describes the calculation for a similar model (Sorry, I don't remember the month).

This on-line article explains how these things work for non-conventional models:

http://www.geistware.com/rcmodeling/...calc/index.htm

Regards!

This isn't a direct answer to your question, but since you said, "Think F-104", maybe it will give you some insight. A lot of years ago, I worked for a flight test organization which had, among other planes, 5 F-104's. Part of what I did was perform weight and balance calculations whenever we did modifications to these airplanes. I don't recall the exact numbers, but it seems like the acceptable CG range was from -7% to about +12%. MINUS 7%? Yep, that's right. The CG was ahead of the leading edge of the wing. From that you might surmise that the fuselage (and intakes) had a huge affect of the airplane's aerodynamic center.

Dick

Thanks for the replies.

lnewqban, I used essentially the same method as described in the "If It Flies" column in MA. I broke the fuse into multiple sections and then did a weighted sum of all the components including the wing and stab (stab effective area was reduced for downwash). I think the disagreement with the cg model occurs because my calculations assume that the fuse sections have the same lift characteristics (for a given area) as the wing. I would think the lift slope for a fuse section would be much flatter than for a wing section. Seems to me that the a fuse section contribution needs to be adjusted downward to account for a much flatter lift slope. But what adjustment?

Hard to say, Bob; that is the reason of wind tunnels.

In your case, generous forward CG initial location and test flights will tell the last word.

I am sorry I don't know more to help you better.

Not a problem, I appreciate your input. The answer is probably buried inside a 10 lb aerodynamics textbook.

Or in otrcman's reply above..... I'm not sure a model needs to be as far forward as to be in front of the leading edge but it certainly points out that the fuselage has a major effect.

My 80" A-10 has " air dams or fences " on the bottom sides of the fuselage. The bottom of the fuselage is as flat as can be. No real difference in COG or flying qualities from other planes.

Rich

Ah, but Rich, your A-10 is much more a "classic" planform. The size of the wing makes any effect from those items minor in comparison.

But consider what the same things would do on a design like the F104 where the top view of the fuselage is at least as big as the wing area. Or the fact that on many modern "wide body" fighters like the F111, F14 and F15 that the engine pods, the tunnel between them and the LEX strakes are generating somewhere around half the lift of the design. One such design, I don't remember which, said it got something like 60% of the lift from the inner body and the remaining 40% from the actual wings that were outboard of the engine pods.

I think they just ran the history of the 104 and it DID suffer from loads of yaw instability. Kept making the vertical stabilizer bigger.
The landing parachute was also used when the plane ran out of runway on Stinky Takeoffs. Too bad so many first test pilots died debugging it...[&o]

Rich

The downward ejection seat didn't help any either. Too bad for those souls that they didn't have today's zero-zero seats.

PS: An old flying buddy flew 104's in the Canadian Forces out of Germany during the cold war. He said it was a superb and forgiving plane when kept in the envelope. But one step over the line and things got "interesting" in a big hurry.

What CPA ordered DOWNWARD seats ?? He should have had his first ride in one !! ...Stupid

That is right. We ran short of ejection powder.

Rich

So easy to fight a war from a stateside desk.....

The first or first few models of 104 had downward seats. And possibly a few other craft at the time. The why for it is because they couldn't ensure a safe punchout at supersonic speeds over the high T tail of the 104. But it obviously meant that the pilot needed quite a distance under the keel before ejection was safe. Rolling to the side was an option of course but that's hardly a good thing to do during takeoff and the initial portion of the climb when speed is low. It was the best they could do at the time.

I love to see the posts from old engineers who declare that the wind tunnel testing is what produces a truly correct aircraft.
It helps - but the real reason the planes work is from the field work .
The flying darts from the fifties n sixties were attempts a for "speed-at any price."
I have a small Convair DART model made by one of the wind tunnel guys there - .
The long noses acted just like our pattern models with excessive forward side area -

why is it NO ONE can ever credit the WW II German engineers for safe ejection seats ???????

THEY DID HAVE THEM.... For the pusher twin tails with the prop behind the pilots head.

Speaking of the F-104 and the downward ejection seat...

I was watching Hogan's Heroes...and it reminded me that the radio op. character Sgt. Kinchloe or "Kinch" was named after Iven C. Kinchloe A.F. pilot/Korean War Vet. and Astronaut...

http://en.wikipedia.org/wiki/Iven_Carl_Kincheloe,_Jr.

He was killed in an F-104 take off accident...engine flamed out and he rolled inverted so he could punch out, but didn't make it.

Mr. Matthews is quite correct in his reasoning on the why of the F-104 down seat. I worked on both the F-104 and on ejection seats as an engineer and perhaps can add some additional insight.

Early ejection seats used a ballistic cartridge (think, "cannon shell") for propulsion. Basically, the seat and it's occupant got a gigantic bang to separate them from the airplane. As you might imagine, there is a limit to how much of a jolt the human body can withstand. Later in the evolution of ejection seats, rocket propulsion came along. The rocket motor imparted a lower force on the pilot over a longer period of time, thus getting him higher and farther from the airplane while hurting him less.

The F-104 can be truly said to be an airplane that was ahead of its time. Many of the internal systems and components were not up to the performance potential of the airframe. With only ballistic propulsion being available for the ejection seat, Lockheed did the only thing that could be done they made the seat go down rather than up.

When you examine the overall performance envelope of an airplane such as the F-104, there wasn't much difference between a ballistic seat going up versus one going down. The upward or downward velocity component of the airplane's flight path is a giant consideration on a safe ejection. If the airplane is descending rapidly, it doesn't make much difference whether the seat goes up or down you will still hit the ground before your parachute opens. Likewise for the case where the airplane is climbing. It you have a large upward speed component when you eject, either the up or the down seat will do the job.

Is an upward seat better? You bet. Once rocket propulsion became available, changing to the up seat was a no brainer. The up seat extended the safe ejection envelope to lower altitudes in combination with higher sink rates. It also gave the ability to eject at ground level as long as there was no sink rate involved.

Today we have ejection seats in which a crew member can escape at zero altitude and zero airspeed and expect no back injuries. But things were not always that sweet. The F-104 came along slightly before the big seat improvements were available. We also retrofitted the T-33 trainer with a rocket seat. The F-80/T-33 first appeared with no ejection seat at all, so it went from a "climb over the side" airplane, to a "ballistic seat" airplane and ultimately to a "rocket seat" airplane.

Dick Fischer

Very interesting explanation, Mr. Fischer.

Thank you!

Bob, I grabbed a RCMOdel World off the shelf to post some info on another thread. Low and behold there is an article in the
aerodynamic Forum in the July 2009 issue just what your question is about. Not a lot of detail, but it may get you what you want.

I checked their web site, and they don't have online archives.

Don

Thanks Don, I'll check it out.