My cap snap rolls when i pull back too hard on the elevator. The giant scale caps snap roll.Even the real cap snap rolls when you do that. Why do they do this?

There are many theories about this, so I'll make a guess . .

The relevant factors seem to be wing aspect ratio , leading edge straightness and wing thickness.

If you look at the popular aerobatic planes - Edge, Extra, CAP and Giles, they all snap, but it seems the straighter the leading edge, and the thicker the section, the less they are prone to snapping when you don't want them to.

They snap in this order (most prone to least prone) - Giles - CAP - Extra - Edge.

No doubt there will be many other theories, but that's my 2c worth!

-David C.

I'm leaning toward the leading edge sweep. I've flown models with similar aspect ratios and wing thicknesses without this tendency. The sweep really shows if the plane is out of rig or is flown uncoordinated. I have a nasty little Cap232 myself, and I never fly it as low as my other planes!

What is a snap roll?

Hi Jack (!)

A snap is what happens when one side of the wing provides different lift from the other for any reason. The aircraft then digs in one wing and 'falls over itself'.

It looks like a very vicious roll.

You can do one by applying full up elevator, full left rudder and full left aileron at the same time.

Try a fast climb, high up, and apply those controls and watch what happens (but only for a few seconds!)

An inverted snap is full down elevator accompanied by full left or right on both rudder and aileron.

Sometimes a snap happens when you don't want it to, especially on take off or landing when the wing partially stalls, because the model is flying too slowly, when the model flips over inverted.

It also sometimes happens in a loop, when it 'falls off' the loop and flips over sideways. This is normally caused by CG being too far off or, more commonly, too much elevator movement.

-David C.

I can do anything with it but wall it. However, i can wall it when i am inverted, so i just flip the plane over and do it. I just want to know why it does that.

Dave, ya got your inverted snap inputs wrong... I'm sure it was just a type-o. But you got your list right if you ask me... every Giles I've seen is an snap-o-matic surprise, even the 40% Carden.

Spaceclam: Inputs for inverted (also called negative or outside) snap should be elevator down, left aileron, right rudder for left hand snap ( or right aileron and left rudder for right hand snap.) Note the crossed aileron/rudder inputs. This differs from an uprights snap.

The snap with hard elevator can be caused by a lot of things, but ultimately it is because one wing stalls before the other. You are pushing the wing to stall by the high elevator input which increases the angle of attack on the wing. You don't have to be going slow. It is possible to stall at high speeds. One wing decides to stall and bam around you go. Pretty much all aerobatic planes will do this if you push them hard enough; however some are much more stable than others. Caps can get a little squirrelly, but that is what makes them awesome tumblers.

Don't own a CAP at the moment, but if it will inverted wall but not right side up wall, there are a few areas to look at. CAPs have a medium high stab. I suppose it is possible you are stalling the stabs upright due to down wash from the wing at high AOA. This would prevent the wall. You might try to increase up elevator to see if that helps.

Does it snap out of a upright wall? (I kinda get that impression but you haven't come out and said it.) If it snaps out only when upright you might have some funnies going on with wash-out on the wing. Check for wing warps. Try raising both ailerons slightly to reflex the wing some, this will make the wing less prone to snap when upright. Also be sure both elevators move the same distance. Hope this helps.

Cheers

Hi Monkeyboy.

Yep! It was a typ#op%ye^%*.

Outside / inverted snaps happen with aileron opposite to rudder (plus down elevator), in the direction of the aileron.

. . and, yes, I have stopped flying my Giles now, as I find I am so prepared for the snap that chicken out and don't give the right amount of elevator movement to get the desired effect.

-DC

I would say the difference is because of the way the vertical/rudder is presented to the airflow. In a positive pull, the vertical is deep in the turbulence of the wing, fuselage and the slip stream of the prop. These are all going to result in small ( or not so small) changes in yaw. A plane will snap a lot more readily when there is some yaw present to create a diffence between the wings, so one stalls first.

When you are inverted and push, the above mentioned turbulence factors shift towards the bottom of the plane while the vertical is projected farther into clean air. Thus the plane stays straighter to a high angle of attack and less tendency to snap.

To get an upright wall may just take a bit more trimming to be sure the plane does not yaw at all when pulled. If it always snaps the same direction, I would suspect thrust angle changes are in order - small ones.

Here are some of the facts. Snaping is all about stalling the wing.
You must first understand how stalls work and understand that stall can happen at any speed. In fact speed has little to nothing to do with stalls. When the Angle of attach become too high the air trys to separates from the airfoil, the wing stalls and stops flying. The wing tips of most aerobatic planes like the Cap are less efficient than the rout of the wing because the thickness, chord, or both are smaller. This lightens controls, allows fast roll rate, great for flat spins, snaps EXT. The airplane are also designed to be unstable on all three axes in order to do tumble and such. This lack of stability in particular direction stability(Yaw) in combination with prop torque means the plane rarely track in a perfectly straight line. When you put all this to together the answer is clear. You pull back too hard, the air starts to separate, the plane isn't tracking in a perfectly straight line thus one wing is advancing while the other retards. The retarding wing stalls first, drops and the other wing flys up and over the top. WHALA Tip stall!!! Thus Snap Roll. Why dose the retarding wing stall first you ask? The answer to that is Bernoulli’s principal.

I just noticed something. My elevator is warped.

That would do it.

-DC

when the tip that protrudes to the leading edge is flush with the stabilizer surface, the tip of the trailing edge (the highest point, where the elevator surface meets the fuselage) is up about an eighth of an inch, where the other one is down about an eighth of an inch. The question is though, because the aircraft snap rolls against the torque, and the elevator wants to snap it in the direction of the torque, What force is overcoming all that that does not show up in strait and level flight?

Wings and tail feathers stall in many mysterious ways. Any asymmetry like a warped elevator would tend to induce asymmetry in the lift. What happens as a result can be very complex and, unfortunately, vary with many things, including speed.

A lot of things about aerodynamics come as a surprise and can be difficult to model. That's why wind tunnels are used a lot in design.


-DC

Wow! Must be every Cap, Extra, EXT in the world has a warped Stab. LOL Thats why thay Snap. Silly me to think there was more to it.

Just Kidding Guys, Have fun with it:-)

The plane snaps to the right. The warped stabs would bake it snap left, as would the engine torque. So something is overriding those two factors. Keep in mind, i am runind a huge engine on that thing so there is a lot of torque.

Saceclam Go back, Read my post. If your plane was dead straight it would still snap or Tip stall if you miss treat the airplane. If you have warped surfaces and it was causing a problem it would show up in maneuvers like loops. that airplane would roll when you pull back or push forward on way or another. Don't pull hard! thats just improper flying technique with a Cap unless performing 3D

How about this:-

Since you have trimmed the aircraft to go straight, and therefore compensated for the left roll induced by the elevator warp, there may be a situation where the trim (maybe the ailerons?) actually has more effect than the warp it is trying to correct. For instance, if the stab stalls, the aileron trimmed to stop it turning will take over and the plane will roll in the opposite direction to the warp.

Just a thought!

-David C.

hmm, maybe. The elevator is made of ribs and covered with super-shrinky arf covering. Do you suppose that by heating it up, it would pull it back strait?

I always do an inverted snap roll with down elevator, right aileron, and left rudder. When the plane is inverted left aileron is still left, but left rudder becomes right. Right?

A snap roll is a fully-stalled condition in which directional control is lost, either intentionally or accidentally. It is a "horizontal spin," and the airplane will fall into a spin when the forward momentum runs out if no correctiive action is taken

Wash-in ( a positive angle of incidence at the tips) is a good way to get an unwanted snap roll. Wash-out (tips warped slightly downward) is a good way to keep an airplane stable in the stall.

The PICA 1/6th scale Focke Wulf has a half degree of wash-out built in and it is a very forgiving airplane when stalled.

Packy

unfortunatly, i don't have one.

Warps and all that can cause them, but having everything straight still won't prevent them. First off, it if snap rolls when you pull full elevator you have too much elevator throw. Tone it down until you can pull full elevator without a snap.

If it's a small Cap it's going to tip stall easily no matter what, unless you get the airplane incredibly light. It's all in the wing design, stab design, wingloading, and many other things. For one thing, the elevators are HUGE. It's easy to pull too much.

The triple taper gives you most of it's bad habits for normal flying, but is also what makes the Cap one of the best aerobats that you can find.

It's just a characteristic of that wing to stall at the tip and that is what it's going to do...You can try things like tip plates, or tri stock near the root to help the problem. Basically, you've just got too much elevator throw!

If you decrease elevator throw, you may prevent stalling when you pull, but then you will not be doing a 'wall'... you'd be doing a quarter loop as the plane would never stall. The objective of a 'wall' is to transition from flying on the wing to flying on the prop/thrust vectoring.

My experience with 3Ding Caps is that it is very difficult to transition from flying to stalled 3D flight gradually. You get a lot back and forth stalling oscillation. Once it is completely stalled and not even thinking about flying (hover, harrier, etc.), it becomes relatively stable again. Therefore, the more abrupt transition, the easier, all other variables aside.

This is just my experience and observations. A lot will depend on your particular plane, setup, CG, etc.

So far, this thread has seemed to produce different descriptions of what is happening during an elevator induced snap roll, but not the reason why. I will try my best to explain the why and how it happens. The guy talking about separation of the air over a wing has got it right, sort of... The air separates starting rearward on the cord. When the critical angle of attack is achieved the separation has moved almost entirely forward toward the leading edge of the wing. This is an explanation of a stall but not a reason why the elevator can induce this.

First, a little fact that has to be understood.

A given airfoil (wing) will alway stall at the same angle relative to the airflow across it. This is called the "critical angle of attack". All wings have a 'CAA' and the angle varies from wing to wing depending on what purpose it was designed to achieve. Again, if you believe that a wing will stall when you achieve "IT'S" CAA then you can continue on... This has nothing to do with airspeed so it's hard to make this assumption without some aerodynamic theory background.

Remember that the airflow across a wing is relative to the wing and nothing else.

OK, let's continue. Let's say our test 'wing' has a CAA of 18 degrees. It's a relatively common, but high number actually, a trainer wing. While flying slow, as you pull up elevator and approach 18 degrees nose up a stall develops. That is because the air hitting the wing is near parallel to the ground and the wing is 18 degrees relative to it. Just prior to the stall you add full power. The airplane accelerates on an 18 degree up line. As it accelerates the air over the wing becomes more relative to the cord line. The angle of attack begins to approach 0 degrees even though you are 18 degrees relative to the ground. OK, if you understand this then the elevator induced stall (snap) will be easy to understand.

Get back into your hotrod model airplane. Lets say the Extra has a CAA of 15 degrees. You fly your model at a very high speed of say 90 knots. You have a huge overly authoritative elevator with lot's of throw. While flying straight and level, 90 knots, you crank in 45 degrees up elevator. What happens is this. The airplane nose rises very quickly, let's say to 60 degrees nose up. If you approached this flight angle slowly the Extra with it's big motor would crawl right up that 60 degree up line. The relative wind angle would remain near 0 degrees is why. But when you yank in this 60 degree nose high angle the inertia of the airplane makes it momentarily want to continue on the level flight path it was on parallel to the ground. With the wings 60 degrees nose up and the aircraft trying to push straight and level for a few split seconds you have "WAY" exceeded the CAA. A stall is the result because the law is that when you exceed CAA it will stall, regardless of orientation. It snaps because one wing always stalls before the other (unless purely built identical IE. mirrors, not to mention P factor) and because of the high airspeed the difference is exagerated. Thus a snap roll is the result.

There are two ways to reduce this tendency.
1. Reduce the speed in which the elevator is applied so the CAA is not exceeded. Gives time for the inertia to be overcome.
2. Reduce the wing-loading (aircraft weight) so there is less inertia to overcome. This allows the airplane to change directions faster keeping the relative airflow over the wing less than CAA.

I hope this helps explain "WHY" elevator induced snaps occur.

Chuck,
Very clear explanation. Good stuff.