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Working in the Shop Pt.3

 

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7 Responses to “Working in the Shop Pt.3”

  1. catman says:

    I don’t want to beat a dead horse, but thinking about this whole moving CG thing is giving me a headache. Somebody help me out here: As I see it, unlike the foamie where the airframe contributes almost no weight to the model, on the big model the structure makes up a sizable chunk of the overall weight of the airplane. Moving parts around on the big model will have more impact on the CG than moving the same parts on the foamie. Let’s say there is no provision to move the CG on the big model and that it balances at the desirable 25% MAC at full flying weight with the wings extended. At that point it is configured like a conventional airplane. But when you sweep the wings back, it becomes a tailed delta and the mean aerodynamic chord moves aft as the wings sweep. Right? So does the model become nose heavy or tail heavy at this point? Logic tells me it becomes nose heavy and becomes a lawn dart because the CG moved aft along with MAC. However, with the wings swept, the mass of the structure of the wings has moved well aft of the original CG. It seems possible that the airplane could become tail heavy at this point, but with all the fixed weight so far forward, it seems to me that it would be really nose heavy because of the moment arm from the CG. Am I missing something here? Is it possible that the ballast required to fly this thing will have to move aft rather than forward?

  2. Catman, now you are starting to see some of our problems ! The one thing Bob has done that I wouldn’t have is start flying the foamy with the wings back and then try to CG it as the wings come forward. But he has got it to work. We will have it figured out before we fly the model. Don’t know what the answer will be but we will have a answer.

  3. catman says:

    Spent much of the day today driving which gave me plenty of time to think. The nose moment arm from the pivot point of the wings forward is pretty long. Assuming the pivot point and MAC are about where it will balance with the wings forward and that MAC will move aft a good bit when the wings are swept, then the already long nose moment will become even longer. If it has to balance at about MAC to make it flyable, it is likely that the trim ballast will have to move aft as well (maybe WAY aft)to make up for the increased weight and moment arm of the nose of the airplane. On the other hand, maybe just the shifting of the weight of the wings when they are swept so far aft will be enough to trim the airplane to neutral. Now wouldn’t that be a happy accident? Additionally, would not adjusting the thrust line of the engines to affect trim apply only when they are producing thrust?

  4. Bill W says:

    Hi Bob!
    Been watching your project for some time now. I thought it to be a good time to chime in with some thoughts on the project. Before I do that, let me just say I work as a professional aero engineer and have been into RC models since Jr high school, or since 1975 or so. My specialty is subsonic wing design and airfoil analysis, I’m also a professional pilot (B777 for United).

    You are understandably concerned about center of gravity management with wing sweep on this airplane, and this is a necessity. However, some of the comments I’ve seen have been incorrect.

    To cut to the chase, CG does need to be managed on a variable geometry airplane. As the wings move fore/aft, they also move the aerodynamic center of the airplane fore/aft. In describing what’s important in stability terms, a parameter called the “neutral point (NP)” is what changes with wing sweep. The NP can be thought of as the aerodynamic center of the *whole airplane* (not just the wing). Every airplane has a NP; yours is variable.

    Its the combined wing/tail/fuselage aero center, and it will move aft with aft sweep and forward with forward sweep on the wing.

    For an airplane to be stable – and this is the really important thing to understand – the center of gravity MUST remain ahead of the NP.
    This distance, expressed as a fraction of the mean aerodynamic chord (MAC, and we use the MAC only as a fixed reference dimension), is also referred to as the “static margin”. On most aircraft that are designed to have docile flight characteristics, the static margin will be in the 20% to 30% range. On aerobatic aircraft, or aircraft that are designed for high maneuverability without augmented stability systems, the static margin will be as little as 5%. On aircraft with augmented or artificial stability, static margins can run in negative territory.
    When I worked as an engineer on the F-16, I know the static margin was around -30%.

    So, with the wings forward, the CG also needs to be in the forward end of its range. As the wings move aft, the CG needs to move aft also, but it cannot be allowed to move aft too quickly or it could get behind the NP – and you will become unstable. With the wings fully aft, the CG needs to move back also, but only to a point where the static margin is still in a good position.

    Somebody mentioned weighted wingtips. Not a bad idea; variable weighting would allow you to tailor the CG shift with wing sweep. Too much weight (or too heavy wings) could result in an unstable airplane (tail heavy with wings aft).

    For your model, I strongly recommend starting a bit nose heavy, with a static margin of 30% as a target. In order to ascertain the neutral point, I would recommend starting with a hand analysis and follow up with a wind tunnel test, since model aerodynamics are notoriously hard to analyze due to low reynolds numbers. Any good university with a tunnel and a graduate student looking for a project could do this for you. It would be cheap insurance for that first flight.

    Best wishes,

    Bill

  5. Jim J says:

    Thanks for the info Bill W! I like aerodynamic engineers who explain acronyms instead of assuming we all know what NP (neutral point) means etc. Reminds me of the time I had FUBAR explained. LOL (Laugh Out Loud).
    A wind tunnel test would be very sweet. The data supplied would be priceless, maybe even the USAF would be interested since this B1 model is so true to scale.

  6. catman says:

    Yea, Bill!!! Thankfully somebody has finally explained this stuff in a way I can understand it.

  7. Bill W, I can give you all the dim. if you want to calculate the CG. Butch rssickels@sbcglobal.net

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