


Indoor News and Views October 1976
Here it is....






THAT'S THE ONE ! ! ! !
Thanks to all that helped us find the article and to dcloin for posting it. Note that this is a step up over the usual fixed neutral point as it produces a chart which provides for a flexible tail and nose length so you can simply move the wing this way and that instead of fixing the wing then moving the CG with added weight. The only catch is that it requires that the wing can be either balanced on the fuselage stick or the posts tack glued for measuring to get the final balance point correct as given by the positioning chart. 





What I do is cut a stick of balsa from say 3/32" or 1/8" square and ballast (ballast in the middle of the stick) to the same weight as the wing. Then you can slide it around to get the correct cg then you know where the wing should be.
The only issue with this method per this INAV issue is it makes some assumptions. One is that the drag of the wing times the wing height above the motor stick is zero. In many full size aircraft this is true or nearly true as L/D of airfoils is very high like 30 to 1. However for indoor model L/D is much worse, about 5 :1 so that term is no longer "near zero" also undercambered airfoils have a negative pitching moment that this method does not take into account. I use a spreadsheet that has these additional factors added and also allows you to input the weights of the sub components (wing stab etc) and it will give you approx location of the wing as well. I would post it but Excel files are not supported as an uploadable file type on RCGroups. Don 





Don, I'm not so sure that this is the case. Normally by setting the CMOS to 0% stability the model would not recover from a dive. But in the case of duration indoor models I think the idea was that one could safely set the stability margin to 0% and rely on the high center of drag force combined with a low center of gravity to provide the otherwise missing nose up pitching needed for stable pitch response. The pitching due to the drag being proportional to the flying speed it would substitute nicely for the neutral aerodynamic pitch stability.






In the spreadsheet I use you do not set the model up for 0% stability margin. You typically fly from 515% margin depending on the event. For EZB ~10% stability margin is good. All I am saying is that the equation to calculate the neutral point location really should include some terms which are missing in the INAV article method. The method in the INAV article is a classical version of the NP equation found in most Aero Engineering text books and is the way I was taught in my aero classes as well. But that equation has been simplified by assumptions to make the equations easier to work with and those assumptions are valid for many situations but the missing terms can add up when you have a model with low L/D airfoils and wing posts. Many current indoor classes, such as F1D have the stab located below the tail boom which will act opposite of the wing above the motorstick. By including all the terms in the equation then you have a more accurate NP calculation. For example on my old 65cm F1D, if I neglect the wing post height, then the calculated NP is 63.4% with the wing height added in the NP then becomes 69.8%. Yes it is only 6.4% difference but that is .58" difference in location and on a 50 minute flight that will result in about 45 seconds of lost time. I am not saying the INAV method is wrong and should not be used, just making people aware that it is not entirely 100% correct in the way it does the calculation, but it is way better than guessing a correct CG location.


Last edited by The Don; Jan 27, 2013 at 08:01 PM.




Don, I can only relate what I saw in the article I used some 35 years back. It definetly called for setting the model up to 0% and rely on the high drag center. And it worked fine that way for a number of my models at the time.
I would also suggest tht 15% is going to be FAR too stable a margin value for the best performance. Heck, by free flight standards even a 5% margin is high. Setting it up with that much is going to produce a strong power to pitch result which would have to be counteracted by putting the wing and tail at high positive angles so that the motor stick produces an effective downthrust amount to counter that much pitch stability under power. You are very right about the equations we use for model stability being a greatly simplified set. But that's what we've got. I can only suspect that trying to evaluate the drag to include that in the set would be a horrendous job since there's so many things that are going to affect this based on so many variables. For example data for a single surface arc shaped airfoil with Gurney flaps front and rear is singularly hard to come by.... But as they say, if it were a perfect world we'd have this sort of thing at our fingertips. 





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If you want to have a more in depth discussion on this topic I suggest we start a new thread as I don't really want to derail this EZB thread over NP calculations more than it already has. If anyone is interested in the spreadsheet, shoot me a PM with your email address and I will email the Excel file to you. 


Last edited by The Don; Jan 28, 2013 at 06:51 AM.





well, i altered my model a little bit, i lengthened the tail boom, moved the wing posts farther forwards and made a prop slightly larger diameter and the normal rotation so i could use my pitch gauge to set it. according to the prop calculator i found, i set the prop to 32 degrees at 3" from the hub which came to 12.55" of pitch. it flies across my living room quite nicely, gaining height on as little as 50 turns
so i am hoping to get a good chance to fly it at the next indoor flying this wednesday. 





some pictures of the changes. i tried something i read on another forum and dyed the prop green with food coloring when i wet them for baking. i think it looks good and will help distinguish mine if its ever flying with many aircraft like it
the new dimensions are: 13.5 overall 1.25 nose to leading edge of wing 7 wing trailing edge to tail leading edge also i bought a powered winder with a counter on it. including a pic of the pitch gauge i built too. 





here you go guys, took some video at work
none of the flights had more than 400 turns on it 






Trim looks really good. Nice job! I think you could up your prop pitch some. I typically run 1214" diameter and 2428" pitch on the full span EZBs, looks like your prop is about 10", so 20" pitch will still work OK.


Last edited by The Don; Jan 29, 2013 at 12:00 AM.




Here is my current EZB which is almost finished just need to put the wing posts on. I spent today rolling some tissue tubes.
Don 

