Given a particular CG, Will a jet perform the same (controllable) regardless of it's armament? If your recommended CG is say, 150mm, Is it the same if you are going to carry a full ordnance? Forgive me if this is a dumb question but I was considering the possibility that wing (and fuse) mounted devices may distribute lift a bit differently, Affecting the recommended CG location.

Thanks! Pete

Pete,

The basic answer to your question, is no, they do not fly the same.

In full-scale aircraft, the basic armament configurations are tested on the aircraft in the design phase - either in numerical simulation or in the wind tunnel, or both before being flown on the prototype. Once an aircraft is in production, different weapons configurations that might be used are first be tested by a flight test center before being allowed to be loaded on an aircraft in the field.

The same holds true for models. I haven't flown a whole bunch of different configurations, but the ones I have flown did make a difference in how the aircraft flys, although it wasn't really that much.

I would start at the same CG location with the armament, but you might find that the aircraft is more, or less, stable with the armament and a CG shift might help that - which you can do by adding the weight to the armament so that the CG shifts automatically when adding it or taking it off.

Bob

Thanks for the fast response Bob, Let me also ask, Based on what you just said, Would it be safe to fly an airplane using the recommended CG for no armament, With a light armament? Or is that pretty much at the pilots discretion?

Pete

Pete,

I concur with what Bob has said. I would also add that except for some fuselage mounted missles, most armament hard points/mounts are generally located close to the cg. Typically, bombs, fuel tanks, missles etc.. on our models are fairly light and mounted close to the cg and do not change it much. Of greater concern is the additional drag and potential pitching moment that the aerodynamics of armament may cause. I would start with a couple of missiles and gradually add more armament and see how it flies.

FYI, "Parent" mounted armament (i.e. one bomb, tank, missle etc per weapons pylon), produces less drag than multiple stores on a single plyon via multiple ejector racks (MER's), which can create signigicant "interference drag" between the stores. A jet loaded with several loaded MER's can noticebly degrade flying characteristics.

Craig

If you change the configuration after the airplane is balanced i.e. armament or fuel tanks you in fact change the cg. The plane must be rebalanced to be safe. In the real world we had charts to show the cg with different weapons/tanks loads as they had been flight tested. The old AF F-4C/D was a good example. A slick jet (no external stores of any type) had a cg that was at the back limit. We normally flew with empty pylons just to get the cg farther forward and make the airplane more friendly. If you hang stuff on your model you need to balance it.

Which devices tend to alter the pitch moment most significantly? Are there any devices/armament configurations known to alter/degrade flight characteristics the most?

Pete

Now Craig, how would you know that?


Bob

Pete,

I agree with what both Craig and Tom said. Start slowly and add just some armament at a time, and check the CG each time as the armament itself can change the CG. Most manufacturers don't "balance" their armament, so you may have to add weight to it just to get the CG to stay the same with it on. If you can, I prefer to add the weight to the armament itself so that when you get it all figured out, you can change the configuration without having to add/remove weight from the plane to get the CG right - but that's not always easy to do if the armament is already built and finish painted, etc.

How you build up to a full load is probably specific to an aircraft and what you want to fly with in the end. Missiles are usually small players are far as drag and weight go, so they are generally easy - although some guys have said that tip missiles have changed the flight characteristics of their airplanes significantly. Fuel tanks and bombs have more potential to change things - especially if they are on the wings, I would think. Of course it also depends on the aircraft and its characteristics, wing loading, power, etc. Craig's big Mibo A10 could probably carry the kitchen sink and still fly OK, but a 1/8th F-16 may really struggle with a couple of tanks hung under the wing...

Only one way to find out... Just treat each new configuration that you fly as a "test flight" and be conservative until you know how it flys. Seeing what other guys are flying on their specifi models of the kind you have would be good to.

Bob

Well, here are 4 examples;

• The big fuel tanks on my Rafale initilly caused a significant nose up pitch, requiring 5 or 6 clicks of down elevator to compensate. Through experimentation I have shimmed the front of the pylons down about 1/8" and reduced the pitching effect. Probably need another 1/8" to eliminate it alltogether. Overall drag from the tanks is noticeable but not dramatic.

• Fuel tanks on my F-4 actually smooth out some of the natural Dutch Roll tendencies and I prefer to fly with them on. The other missiles have no noticable effect.

• Multiple parent mounted stores on my A-10 have negligible effect.

• Fuel tanks and munitions on this scale A-7 made it heavy and sluggish but it still handled well.

Craig

It always amazed me how much this airplane could handle hanging off the wing! Is that a picture of you in yours?

Some guys put all of the pylons and some wing tanks on their turbine JHH A-7 ARFs, but I was afraid to on my EDF version. Did you try that on your A-7 EDF Craig?

Bob

Guys, here is one for you...........If you look at the missile rails on the A-7, you will see that they are angled down about 5 degrees and they are AHEAD of the CG. One would think that putting a sidewinder on them would cause a pitch DOWN moment....Nada.........The first time I flew my A-7 with them, it rotated on it's own!!

Another thing.....My A-7 flies faster with four 1000 pounders on the pylons....no trim change. It seems to land slower though. The bombs are very light and they definitely add more lift than weight to the airframe.

Tailwinds,

John

This is a very timely thread. I will be flying my F-16 with a full load for the first time this weekend. I plan on checking the balance since I am adding some additional electronics in addition to the stores.

Yes that's me or at least what I used to look like.

I never got around to mounting the pylons on my JHH EDF A-7. It was pretty heavy as it was and I didn't want to further load it up.

Good looking F-16. I think it will fly pretty clean with that armament selection.

Craig

Craig,
You know how much I love the F-16 1/6 scale! I was considering adding a Gyro since I was getting some pretty good tail wagging. But decided to wait till after I added the rails and missiles...glad I did. It "evened" the plane out completely, actually flew more stable and better with the full missile load (not the center tank or large stores).

I did check the CG after installing the ordinance and it was no difference at all.

Rex

This is a 1/8 scale or 1/6 scale?
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I have flown a few jets with and without missiles and external tanks. In my personal experience there where no big changes on how the jet handled and there was no need for CG adjustmets.
I have flown the F-18F and the F-16 1/6 from Skymaster with full missiles and external tanks. In both cases besides extra drag and becoming a bit more unstable under windy conditions no trim changes where needed.
The F-18F became more sensitive with the elevator and I lowered a bit the total throws, for the F-16 no changes where made.
I also flew the Skymaster MB-339 with the underwing tanks with no changes in the flying performance, again with exception of the extra drag.
I guess it will depend on the jet model, I am sure in the Rafale for example those big tanks most probably will change the way the air moves around the wing quite a bit.

F-16: http://www.youtube.com/watch?v=-ux_tnSdMgU

F-18F: http://www.jetsrc.com/videos/F-18F_JR_2.wmv


Michel