Snap / Spin exit
 

I have a Symphony that continues rotating 60 - 90 degrees after releasing controls on a snap or spin. I have to pop opposite rudder to get it to stop. I also have an Integral that stops instantly when the controls are released. One person suggested shortening the stab as a solution. The stab span is about the same on both planes. If that is a solution, can someone explain why? Is there another solution? The wing LE might not be as sharp as on some planes, but I assume that affects entry and not exit. Due to paint problems with the Symphony, the tail ended up heavy -- would that contribute?

Ken

RE: Snap / Spin exit
 

Hi ken
your tail heavy ,,, or you are using too much rudder try a lower rate rudder first then,
increase inc. on the main wing and move your g/g forward
stab. size has nothing to do with this problem.
let us know the results
B

RE: Snap / Spin exit
 

Agree with Bryan.

The model is too deeply stalled in the spin or snap. The deeper the stall the harder it is to recover. Tail heavy will play a huge role as the model will be easier to stall, hence it ill get "too Stalled".


Too much control throws will keep the model really stalled deep as well, mainly the rudder throw. Especially if its already tail heavy the Rudder can actually keep the model stalled much longer as it becomes more effective as the CG starts moving back.

Reduced Rudder throw and moving the Cg forward should help it. If the Cg is too far aft you should be fixing that anyway not just for the snaps and spins. The more forward the CG the more predictable the snaps and spins become. However the more control throw it takes to get the model to get to the stalled state. The more aft the CG is the easier the model is to stall and the harder it is to recover from the stall. However if the CG is so far aft that its really keeping the plane stalled that deep you are suffering on your other sides of the models trim. CG plays such a huge role in our models.

Troy Newman

RE: Snap / Spin exit
 

Question, because it has been tried:

Same plane, same CG, 2 different stabs. Plane snaps cleanly with smaller stab, plane does not want to break and snap at all with larger stab. Same wing, same CG, same set up. Throws do not seem to help, it only aggrivates the problem as described in original post.

Why? Theoretical question, but could stand explaining.

-M

RE: Snap / Spin exit
 

The number of variables for snap comparisions is insane. We can't ignore rotational moment of inertia. The plabe with the lighter stab/tail would more easily rotate so it's hard to say that everything was the same for the purpose of comparison.

Colin.

RE: Snap / Spin exit
 

My point being everything else was the same. Smaller stabs built a bit heavier so they were within .1 oz of the larger pair.

-Mike

RE: Snap / Spin exit
 

Thanks for the suggestions.

Troy, I meant that the tail is physically heavy, not that it flies tail heavy.

I'll experiment with less rudder, and shift the CG forward to see what I can learn.

Mike, do you have any experience with reducing stab span on the Symphony? Compared to the Integral, the stab is 1/4" wider, 1" more at the root, and 1/4" more at the tip. I agree that it would be intersting to understand the relationship between stab size and snap/spen exit.


Ken

RE: Snap / Spin exit
 

I'm gonna guess because I don't know s**t about what is happening during a snap but here it goes. The smaller stab needs less yawing of the fuse to remove the air flow from it and the unstalled side has less surface area due to i'ts smaller size therefor can not control pitch by it's self. The larger stabs need the fuse to yaw further in order for it blanket that stab from air flow, the side that is in the airflow is large enough to still control pitch on it's own to a point. Ok jsut my guess.

RE: Snap / Spin exit
 

I know of it being done, and that's what I was told (regarding that specific airframe). I don't know how many other variables were involved, just that it helped dramatically. Take that for exactly what it's worth. I've done the same expirements with my own designs and found the same thing to a small extent. At some point it will snap like a bandit, but it's touchy to get it locked in straight and level (less area=less stability on that axis). The weights of my stabs was the same and hence my Q. I don't actually understand all of what I've "discovered". Which is why generally I don't go too deep into aerodynamic discussions anymore. Way too many opinions and too many variables. I'd trust Bryan's advice on trimming for certain. He hasn't lead me astray yet!

And yes on a Symphony. I'm not recommending that mod, because it does sound like you are tail heavy exactly as Bryan and Troy describe. I would fix that before I did anything else. Just try it. Adjust the throws like Troy said...make SURE you don't have too much rudder, that can kill you and you'll chase your tail forever.

-Mike

RE: Snap / Spin exit
 

Just as a start point, when I used to do O/D pattern planes, I used an old formula for calculating the C of G:

CG= MAC/7or6, + (2or 3 x TAIL AREAx moment arm length)/8x wing area.

This gave a front-to back range(and a comfy c of g was usually 2/3 back along the range,for me anyway).

So,the bigger the tail area,the further back the c.of g.

If you use the same c.of g. for two different tail areas, the larger one will give more stability, and if that (same)c.of g. is far enough forward, the plane won't want to depart, as you said.

RE: Snap / Spin exit
 

The Symphony is speed sensitive for snaps ( at least it is for me). . Having flown it for the past two years in advanced I can say for certain that if my downline speed is too fast and all things being equal I can wrap it up is a pretty wicked snap. But if I slow my entry speed down the snap is very predictable. Also my C.G. is pretty far forward and my incidence is different than most flying Symphony's. Spins are pretty predicatable as well set up properly. I would look at rudder throw as well like Brian suggested. I do a rudder elevator only spin and the rotation is nice and slow and will come out where I want it to.
Good luck with it, the Symphony really is a great flying plane. I will be sporting 2 of them for the 2008 Masters season in D3
Erik

RE: Snap / Spin exit
 

Living in California, working at home, and having a light schedule today I was able to go put in a couple of flights. (The cross wind got a bit too nasty to keep going) Less rudder, more aileron, and CG farther forward made a huge difference. The 45 down positive snap in advanced is much better. The avalanche with negative snap still needs work - it goes from decent air speed to almost stopped in the second it akes for the snap, and kind of wallows through. It tried keeping half throttle, and it was a little better. I've tried the "check-mark" approach, but the timing is pretty critical. I'll have to experiment some more with the throws and maybe move the CG even farther. I didn't check 45 or straight inverted -- but hope I don't need too much down at the bottom of the 6-sided outside!

Thanks for all the help.

Ken

RE: Snap / Spin exit
 

Yes...more stability from the bigger stab and also more control authority so we should be able to rotate the fuse to a high enough AOA to stall the wing and deliver the snap more effectively than a small stab. Unless they are both able to create the stalled condition and the extra size/weight/volume of the bigger stab impeads the rotation.

RE: Snap / Spin exit
 

ohhh boy!!! Lions and Tigers and Bears, Oh My. Colin, Sounds intuitive, but it's wrong thinking

MattK

RE: Snap / Spin exit
 

I was hoping you'd show up Matt ;)

-Mike

RE: Snap / Spin exit
 

Hi Mikey,

Just happened to be browsing after a hiatus of a couple months from this site. Regardless, it amazes me the things some guys think and truly believe as fact.

I have discussed the main reasons before a bunch times but not everybody reads it and comprehends it I suppose. One more time seems in order.

When it comes to snaps and spins, it totally depends on what kind of model you want to fly. Do you want a spirited, spritely, agile aerobat or do you want a slow to respond, slow to exit, clumsy model? Do you want the model to initiate and STOP exactly where you want it and when, or do you like a horsed entry and over-rotated exit? In car terms, a Ferrarri of a Truck??

Stabs ....well...stabilize. Stabs are either horizontal of vertical, it matters not at all...same rules apply. Very large stabs stabilize exceedingly well and smaller stabs stabilize to a lower degree. Large stabs work great on trainers.

Lets talk horizontal stabs for a moment. Large stabs, those that are greater than about 26-27% of the wing (rule of thumb), are really larger than they should be for a really agile, effortless, efficient aerobatic design. When a model uses larger stab area than this rule of thumb, it will require unecessarily large amounts of elevator command (and elevator area as percent of the stab area) to overcome it's designed in stability. A stronger, quicker servo will help you, but my point is, why work you equipment that hard? Your exits will not be as predictable and precise and you will likely wind up chasing the exit. My personal preference is stab area at around 20-22% of the wing. Typical elevator area is approx 40-45% of the stab area so a 200 sq in stab has about 80-90 squares of elevator.

Wing leading edge radius plays an important role. The outboard 12-15" of the panel (about 1/3 span) should have a fairly sharp LE...I prefer no greater than 1/8"-3/16" radius. The inboard LE is less critical to spirited performance.

Rudder and fin typically stabilize to a greater degree in yaw than the horizontal stab and elevator do in pitch....there is much less vertical fuse area to stabilize (compared to wing vs stab area), so the amount of vertical area in the tail is almost always large. HOWEVER, this is necessary to counter the spiralling prop blast and it's effects. Have you noticed some of the guys using some vertical area at mid-ships? My good friend Nat Penton pioneered this idea some 25 years ago. Well, that essentially destabilizes the fuselage in yaw a bit, again making for spirited performance from the model. It makes rudder input more effective. Think of it as having the identical purpose as dorsal and ventral fins have on a shark at mid-body. These surfaces make the fish extremely agile and without them, it would not be capable of turning on its prey as quickly as it can. Nature invented the concept eons ago.

This is getting as long winded as some of Troy's stuff so I'll cut it short. Just busting you Troy, so don't be offended...please!!

I'll leave this topic with a key parameter to chew on regarding pitch stability for aerobats. Tail Volume Coefficient, TVC, is a dimensionless number that brings wing, stab, and tail moment together in a neat package that is easy to calculate. It is essentially the ratio of the stab to wing areas times the ratio of tail moment to wing MAC. The tail moment should be determined aerodynamic center (AC) of wing to aerodynamic center (AC) of stab. It's generally accepted as being located at 25% MAC for either surface. MAC for either stab or wing can be determined graphically....or look it up on the internet. I really like the way my designs fly at TVC of around 0.65 and all who have flown my stuff can attest to their effortless flying ability. Many present day designs have TVC greater than 0.8 which would not be my first choice. From about 0.6 to 0.75 is a good all around range to shoot for.

There are some other factors that trim this main factor but once you have a handle on this major factor you will begin to understand why you model behaves one way or another. For example, I have found that high taper ratios on wings, as high as 3:1, tend to help the model's agility. Some fellows have opted to reduce the model's span instead. I prefer to keep the span as long as possible but use a high taper ratio. I get larger wing area and greater agility at the same time, since the tip chord is much shorter than usual which means I can drive to a smaller LE radius.

Hope this helps guys and doesn't confuse

MattK

RE: Snap / Spin exit
 

Yep, probably true, perhaps you can lose the "lions and tigers stuff" in your comments. Sounds alot like someting my 8 year would say.