Building Tips and technical articles. > ARF'S

T/F Score Scratch and Dent another one 1/21/07

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Mike Scholtes:
Hi Bob:

I'm curious how you checked the vertical CG. I don't get how it could be evaluated by hanging the model from the leadouts. I personally think this is more of a theoretical abstract to be debated on forums like this than a real factor in trimming a model, except maybe at the extremes.

Changing the thrust line is another matter altogether. I would really advise the new Score buyer against monkeying with it for aesthetic "improvement." I think it looks a little odd too but that is mostly because we have been raised to think a "real" model has to look like a Nobler or a P-51 with the high prop hub. The Score has a thrust line close to the wing centerline and the location of the stab is presumably calculated to be in balance with the thrust and drag forces of the design as a whole. Plus, moving the prop hub location means a new cowl and a lot of other changes, so why get an ARF in the first place? Change out the control system and leave it at that, I would (and do) say. This is a very capable design that has a lot of potential as is, aerodynamically at least.

Bob Reeves:
Well Mike everyone is entitled to an opinion, I somewhat agree that ARF's are what they are. I've put together 4, all for a specific purpose centering around either engine testing or trying something that wasn't worth the time to build and finish a kit. Funny part is they all flew great once I had them flight trimmed, most even better than some of the many airplanes I have built from scratch or kits.

I do have a different opinion on the importance of the vertical CG, the leadouts need to be located at the vertical CG or the airplane will not fly with the wings level, again assuming the wings are straight. Most designers take this into consideration as I am sure the designer of the Score did.

In my case I rotated the engine moving the weight of everything above the engines centerline up. This obviously changed the vertical CG which my leadout check showed. If you can't accept the fact that hanging an airplane from the leadouts is a viable and obvious way to check the vertical CG I'm not going to try to change your thinking.

Mike Scholtes:
Not trying to start an argument here Bob; I have no doubt that side-mounting your big Saito changed the vertical CG from what the designer anticipated, as would deleting the largish wheel pants. Question now is, how and to what degree has it changed and what effect on the flight characteristics can be expected? I asked about your method of determining the VCG because I cannot think of a way to suspend the model that will allow it to be located reliably.

I too think of ARFs as test beds and learning tools. Trouble is the darned airplane only "knows" forces applied around moments and centers of lift, drag and gravity and, not knowing it is only an ARF, proceeds to fly just fine. Plus, they are a Godsend for not-yet-retired guys with jobs and families.

Mark Scarborough:
I would like to start my comments with a disclaimer. I don't have a well known reputation in the stunt community i.e. no name flier however I talk to a great many people and absorb information well. I spoke on this subject to a great many people not the least of whom was Pat Johnston who has designed and built many planes. This topic was of great importance to me because I am in the middle of designing and building a semi scale stunter based on the bf 109. The low wing placement on the prototype leads to problems with the vertical cg in flight. I have decided to add dihedral to the wings in order to deal correctly with this problem. The real issue with the vertical cg placement is that as stated before, if the weight mass is above the leadout position, the airplane will fly wing low while upright, suggestions indicate that by using a tab or adjustable "aileron" you can correct this. This is true for upright flight. In a perfect flight where there is NO speed variation this may work. however this tab adjustment will be by design speed sensitive and therefore introduce other trim issues as our planes do speed up and slow down throughout the pattern. The correct way to offset this is to get the leadout position in the correct relationship to the vertical cg, or for understanding, the center of mass. To check for this relationship in an assembled airframe that is ready to fly, hang the airplane from the leadouts in a door jamb, orIEnt the airframe so that it is "flying" through the door, ie from one room to the other. Assuming that your door jamb is truly plumb the distance from the wingtip to the jamb ideally should be the same on both wintips. To understand this phenomenon and how it affects flight, imagine a bucket of water on the end of your lines. If you hold it by the handle it will swing straight out in line with your lines or arm. If you hold it by the edge of the bucket, it will instead find its own center so that the mass is evenly divided above and below the lines. This is the problem with vertical cg issues. As has been discussed before, the bellcrank does not care where it is, its all about where the leadouts are. Most of these discussions center around fore and aft issues. However the same is true in relationship to the vertical cg. If you have ever seen a "spirit of St. Louis" model fly control line, you will note that there is a hanger off the wingtip to locate the lines below the wing. This is a dramatic illustration of how to correct for vertical cg location. In my case, and Al Rabe, and Pat Johnston the solution is to raise the wingtip up to more closely approximate the center of mass in flight. Hence by using dihedral we move the wintip and more importantly the leadouts up into a more correct location. Al Rabe could more correctly describe it than I have perhaps but that is my understanding. By moving the engine vertically away from the designed and flight tested location, we have altered the vertical cg location and must somehow accomidate for this.The same holds true for rotating and engine that was designed inverted into an upright position or outward position ie as in a profile. Understandably there are more than one way to skin a cat but this is my take on it. I don't like using aerodynamic bandaids to fix a mechanical problem. If you would like to read more, there is some additional  posts in the ARF section under the two posts regarding the Brodak P-40 and some things people have observed and dealt with regarding it. It was designed as a low wing with dihedral, however in production it became a low wing without dihedral and people are moving the wing, or adding dihedral to correct this issue.

I am open to any and all comments on my stance please feel free!

Steve Helmick:
It might not be all that difficult to raise the LO guide in the tip enough to fix the VCG problem, if you find one. FWIW, the first PW-51 Mustang had an adjustable LO guide that was adjustable vertically as well as fore/aft. Wish I had a picture of how that was done. I suspect the normal slider plate was mounted on vertical slider pieces. The exit slot might need some fixin', but it was there, if needed. 
 :!  Steve

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