It probably doesn't' make a difference but just to be fair to Windy he made this suggestion in regard to using a 90 size engine in Strega and making it a true 90 ship from a 60 size ship. I just want to be accurate in regard to what Windy said. I understand about the Flap to leave that alone as far as it current size. Just for clarity if the elevator was made 3/8" larger in its perimeter wouldn't that increase the elevator authority?
Charles
Yes, but again, that increases the control loads. Look at an Impact, or my airplane - the elevator is much smaller than the stabilizer. That's because the effectiveness of the control goes down only slowly with the "split" but the hinge moment goes down drastically. In fact, the one page of Abbot and Von Deonhoff that Big Jim and/or Windy fixated on (figure 115) shows that the lift coefficient goes up until you to about 40% "flap". That was a horrific misinterpretation for the wing since you couldn't possibly deflect a 40% flap using the existing line tension. But it works OK for the stab, and guess what, the Impact stab is 60/40. But the slope is very low around there, and 65/35 gives you almost as much lift, but much less hinge moment, which is how I did it for the Infinity.
If you make the elevator bigger, you get a little more lift at the same deflection, but much more hinge moment, so you probably can't make it deflect as far, so you will likely lose out. To get ahead, you would have to figure out some way to get more control torque, with either a larger bellcrank or more line tension, or both. If you try the "more line tension" approach, that means you have to compromise the trim in other ways. And, start taking whatever Barry Bonds was on, because the kind of muscle required to horse it around a competitive pattern is in the realm of the Incredible Hulk.
You would probably do better to increase the size of the stabilizer instead of the elevator, but I don't suggest any changes to the existing airplane. If it seems like something you want to pursue, then you can address it next time.
What you need to convert the Strega into a "90-sized" design is not more flap or elevator area, but 85-90' lines - which is illegal. This was the big problem with all of these gigantic bombs, you can get it to fly OK but the available space is too small for it. Even then, you can't actually use the full capability of a 90, and if you wanted to, you would run into the other problem that Windy discovered (after having other people, myself included, trying to explain it for years before) - that you can't control it. With no pipe to act as a regulator, you are trying to run a 90 at 10% of its capability. But if you get off a bit and end up with 11%, it's way too fast, and 9%, it's way too slow. Windy tried this at the NATs one year, found exactly that problem, and had to switch, in the middle of the week, to a 76 - where you need maybe 15-20%, and a slight variation wasn't a disaster. That's the airplane that later flamed out inverted and flew into the light socket pole.
David and others can use a 75 in his tiny airplane (small even for an ST46) only because he has a pipe to regulate it. The original airplane that the Strega is based on was the Patternmaster, which was intended to be big enough to cover the tolerance/variations that you get from an ST60 - not a 90 with 50% more displacement. It's (in my opinion) already too large for the 70' line length limit.
We shall see, of course, but I think you are likely to run into difficulties with power control and control loads even as it is. I am not sure (nor is it clear to me that anyone is sure) whether the 81 is detuned enough to be controllable in this situation. Making the flaps and elevators larger would just compound the problem with the control loads, and you might be looking to shave the flaps, for sure, just to be able to get it to maneuver in the cramped space available.
At this point, just get it going as it is and see what you wind up with.
Brett
Topic: My new Strega ARF to battle with in 2017 (Read 10353 times)
