Help understanding snap rolls

[DavidJ]

I have went through the links, but can not get a good understanding of what a snap roll is, and how it differs from a regular aileron roll. All I have found is that the plane must stall and change nose angle, but I still really do not understand.

Can someone explain a snap roll in layman's terms?

Thanks,
David J.

[PTLdom]

I'm also waiting for a "competent" answer - please, do not tell us that it's also a spin, because it ain't.

[Neil Morse]

Not sure if this is in layman's terms, but a snap roll differs from an aileron roll because it involves full up elevator and full rudder in the same direction as the ailerons. Technically, I believe this causes the inside wing to stall. It is similar to a spin -- only at velocity with power on. Does that help?

Neil

[Smash McCrash]

That is mostly right. Rudder doesn't have to go the same way as the ailerons and elevator doesn't have to be up. Each combination causes the snap roll, or flick roll, to look a little different. It requires a fair amount of control surface deflection to work. This kind of roll is caused when the "inner" wing, that is, the one that is moving downward, stalls. I usually do the standard "inside up" snap roll which is when the rudder moves the same direction as the ailerons and elevator is full up. Just seems to be the easiest for me to perform.

Mike

[PTLdom]

Hi Smash,
Is there any sequence to enter the snap? like elevator + elerons and more elevator + rudder after entering the knife edge position?

[Smash McCrash]

Nope. Just apply and hold till you are done, at least thats what I do. I *used* to just let the sticks go to recover until I heard that letting them snap can cause damage. Obviously do this at several mistakes high so you can recover before the possible high speed impact with the dirt. Caution: this is a violent maneuver in some cases that has caused serious damage in full scale aircraft, make sure your plane can handle very large stresses prior to executing it.

Mike

[Majortomski]

Well you're all on the right track. A technically correct snap is done without ailerons. ( I can hear the screams now). As asked the sequence is; pull enough elevator (either up or down) fast enough to change the angle of attack so that the wing stalls. Then add the rudder to start the rotation. Trouble is with most models the wing loading is so low or the elevator throw is so low that you can't get the plane to stall in the first place. So most people add aileron to cheat and get the wing stalled out. If you want to try one start at half throttle or less that will help the chances of the stall happening. And if you hold the controls that gets the snap the airplane will fall into a spin. the snap is just a high speed horizontal spin.:p

[DavidJ]

Excellent! Thanks to all. That helps alot.

This will be a great forum.

David J.

[Smash McCrash]

Thomas, I stand corrected. I forgot that the purists still do them rudder/elevator only. I haven't tried that way yet.

Mike

[PTLdom]

Thanks.
Nonetheless I do not understand how can one stall a wing in our model airplanes at relatively high speeds when rising the airplane nose (full size airplanes have properly desgigned blunt leading edge airfoils that helps with it). When flying at a relatively high speed, if you apply elevator you'll get the plane climbing quickly. I do belive that by start the rotation the inner wing will stall (it's moving backwards, right?). But when applying elevator we are only trying to get a stall ATITUDE (a judging aesthetic criteria), but never a stall, am I correct?

[BillK]

What would you call a roll that doesn't use rudder? Full up elevator and full left or right aileron that makes the plane roll upwards quickly to a level position? I know it's not a snap roll but it looks cool and I want to know what to call it.

[PTLdom]

Originally posted by BillK
What would you call a roll that doesn't use rudder? Full up elevator and full left or right aileron that makes the plane roll upwards quickly to a level position? I know it's not a snap roll but it looks cool and I want to know what to call it.


??? :confused: ... a barrel roll?

[BillK]

I guess it is an extreme form of a barrel roll when done with a 3D plane. Doesn't look like a barrel roll really because it's fast and tight and the plane gains a few feet of altitude.

[Andy W]

Originally posted by PTLdom
Nonetheless I do not understand how can one stall a wing in our model airplanes at relatively high speeds when rising the airplane nose (full size airplanes have properly desgigned blunt leading edge airfoils that helps with it).

You can stall any airfoil, model or full-scale, at any speed. If it was not possible to stall, spins would never occur and inexperienced pilots would not die. If the airflow over the airfoil is 'broken' in such a way that it no longer produces life, the airfoil is now stalled. This is induced during a snap-roll by applying sudden and drastic elevator.
Elavator does not make a model 'go up', it ONLY adjust the angle of attack to the relative wind (airflow across the airfoil). If sufficiently provoked, you can immediately break that flow, and stall the wing. As 99% of wings will stall asymetrically (one wing before the other, even if only a little bit), one side will 'fall' before the other, inducing a rolling tendancy. If the elevator throw is maintained, the wing remains stalled, and the roll will continue for as long as there is power (climbing) or altitude (falling) to maintain it.

As an interesting side note, we practice (full-scale sailplane) stalling and spinning at different speeds and attitudes. One of the most interesting lessons I learned was how it was possible to NOT stall the wing as the airspeed dropped to 0!!!!! It was a roller-coaster style ride over the top of a parabolic curve, although we had zero G, we never did stall, and coaxed the airframe over the top without incident. I was flying with a well known air force general who's flown everything in their inventory at some point, but he never mentioned aerobatics!

..a

[flyfalcons]

What Andy said was very good, and just in addition, the application of rudder will slow one wing, effectively increasing its angle of attack and causing it to stall before the other wing. That is how you can control the direction of the snap.

In addition, it is important for every pilot to know that a plane can be stalled at any speed* and any attitude simply by exceeding the critical angle of attack of the wing. One of the highlights of my aviation career so far was when I took an instructional flight in a Pitts S-2B with a German Aerobatic Team member as the instructor. To prove that the airplane can be stalled at any time, we did a 3/4 loop, waited for the speed to build to 100mph going straight down, and yanked back on the stick. The results were as predicted: the plane stalled.

*if you're going too fast the wing will fail before reaching the stall point, but if the wing had infinite strength then it truly could fail at any speed.

[Smash McCrash]

Another example of the stalling at any speed with a model is the dreaded Zagi Death Spiral. Induced by too much elevator input at any speed. I can get my e-Raider to do this, but I have to try to.

Mike

[Mike Parsons]

David,
I know this was a few weeks ago, but if you are going to be at the Hamilton Mill field on Sunday, I can show you this. I do it all the time with my Miniflash and PF mustang. There are four combinations, Left up, left down, right up and right down. Again , you may already be out there snap rolling the wings off of your planes;). I threw three consecutive snaprolls in the miniflash day before yesterday that I threw the wing bolt. Glad I brought it in when I did.


Cheers,
Mike

[swr@pobox.com]

Mike,

I just got my eRaider flying with a Promax Cobalt and it doesn't roll very well thus far. What kinds of tricks are you able to do with yours. My rolls are really slow. I may have too much nose weight. However, it seems to go fast, I'm guessing about 65. What kind of speed are you getting?

Steve
swr@pobox.com

[SkyHawke]

Originally posted by Thomas Solinski
Well you're all on the right track. A technically correct snap is done without ailerons. ( I can hear the screams now). The snap is just a high speed horizontal spin.:p

Yeap, I do them all the time on my backup 3 channel Stik .40 with "Uber-Throws". . .

Kind of freaks people out when I land and they realize I don't have Ailerons. . .

-=SkyHawke=-

[SheldonYoung]

A snap roll is a horizontal spin. All you're doing is using the elvator to force the wing to stall at flying speed.

Applying the rudder in a snap, besides pointing the plane in the desired direction, causes the inside wing to generate less lift. There's less lift because there wing presents a different profile to the relative wind. Rolling with the ailerons in the snap excagerates this effect. You can snap without ailerons or rudder, you just won't have much say over which direction it snaps.

With a bit of practice I could to pretty convincing snap rolls with an RCM&E Tucano, a model without a rudder.

And of course if I was paying attention I'd realize I just said what Thomas did a while ago...

[bob the builder]

excuse me while I flail around with this thought for a minute but it is my understanding that lift is generated mostly by the flow of air under the wing...so is a stall really when when the airflow OVER the wing is broken or is it something else?

bob

[SheldonYoung]

A stall is when a wing stops generating lift when it passes a critical angle of attack, but I'm not sure about the exact aerodynamic mechanism.

[flyfalcons]

It's when the flow on top of the wing separates and does not reattach, creating a loss of lift, or stall. Bob the builder is incorrect; according to Bernoulli's principal the majority of the lift comes from the difference in pressure between the top and bottom surfaces of the wing. The shape of the top of the wing is what creates the pressure differential, and when airflow separates on the top of the wing, lift is substantially decreased. You can be producing some lift and still be stalled.

[bob the builder]

ok, I'm going to go down with the ship so to speak. flat plate airfoils still generate lift and a flat bottomed airfoil will fly inverted so I'm still not sure...(aerodynamics and philosophy, two great topics:D ) btb

[flyfalcons]

There is still pressure differential in those situations due to the angle of attack of the wings. In addition, pressure differential is not the only source of lift. Up/down wash, deflection of air, etc, also help a plane fly.

[bob the builder]

okay, I think I'm with you now. thanks for the clarification. bob

[raptor22]

Originally posted by flyfalcons
It's when the flow on top of the wing separates and does not reattach, creating a loss of lift, or stall. Bob the builder is incorrect; according to Bernoulli's principal the majority of the lift comes from the difference in pressure between the top and bottom surfaces of the wing..

Incorrect!

Lift is created by downwash, as newton's principle states that every action creates an equal and opposite reaction. The majority of the downwash is not, however, created the the bottom of the wing on most airfoils. The majority of the downwash is created by curve on the upper surface of the wing forcing air downwards. In fact, scientific studies have concluded that the lift created by bernoulli's principle is so miniscule that a 747 airfoil would need to be half as high as the chord length to produce enough lift to leave terra ferma. Also, bernoullies theory would force all symmetrical airfoils not to fly.

A stall occurs when the upper portion of the airfoil breaks its flow and no longer produces downwash.

:eek: Suprised to see the truth?:eek:

--Alex

[raptor22]

The cool thing about flat-plate airfoils is that, since thay have no flow over the top of the wing, are perpetually stalled. Therefore, what feels like a stall to the pilot is (in theory, that is) impossible on a flat-plate wing.

--Alex

[flyfalcons]

Originally posted by raptor22
Incorrect!
Also, bernoullies theory would force all symmetrical airfoils not to fly.


No, symmetrical airfoils just can't produce lift when at 0 AoA. They have to be ever so slightly angled up to create the lift that they do. Up/Down wash is not the only force causing a wing to produce lift.

[raptor22]

Flyfalcons,

It is not the only force, but it is the most prodominating one; considering that the lift provided by bernoullis principlae is so slight.

--Alex

[oldpilot]

ALEX is right.

The miriad of wing sections which are available are concerned with reducing DRAG for any given or required situation.
Don't forget, planes are called AIRPLANES for a very good reason. They PLANE on the AIR. Boats plane on the water. Hydrofoils plane IN the water. Sailboats Centreboards and Rudders do too. And sailboats most of the time sail with their sails stalled, Ice yachts and some very fast Catamarans excepted.
All these have one thing in common. They deflect the medium they are operating in to obtain a force in the direction they want it to work in. The penalty is drag, which brings us back to Airfoil sections again.

Cheers. Patrick.

[erashby]

Anyone know where there is video of a snap roll?

[opualuan]

got my brushless x250 to do this, didn't know for sure it WAS a snaproll until I read this thread. is the direction the fuse points supposed to change a lot through the roll? I'm able to do it with opposite rudder/aileron and down elevator, only thing that seems to work. the fuse angle probably changes +-10 degrees during the move. normal or no?

so now... what's the technical definition of a 'spin'?

[raptor22]

opualuan,
spin is one wing stalled slightly, but another stalled completely, causing roll.

it is experience by full size airplanes accidentally when the pilot performs low speed slipping and skidding turns to help align w/ the runway in the pattern......usually w/ disastrous results. Sooo.... any wing stalled more on one side than another, causung roll w/out aileron is a spin. A snap roll is just a horizontal spin.

--Alex

[opualuan]

I'm just confused, someonw earlier in the thread emphatically pointed out a spin and snap roll are NOT the same....

[flyfalcons]

Originally posted by erashby
Anyone know where there is video of a snap roll?

http://www.rcsites.net/flyfalcons/RyansMWExtra.wmv

Toward the end of this vid I do a rolling circle, followed immediately by a snap roll, then a 1-1/2 snap to inverted.

For IMAC judging a roll is considered a snap when the plane's heading and pitch deviates from the straight line, and there is some autorotation. The snaps in this vid are considered IMAC-standard snaps.

[opualuan]

won't work on my windows media player... any special codec?

[flyfalcons]

Try downloading the latest version of WM off the Microsoft site... that seems to take care of it for most people.

[opualuan]

mac os X version, no dice. what codec is it in? I'll have to DL the video at work...

[flyfalcons]

I'm not sure what codec it is in as I didn't edit the video; all I know is for PC users downloading the latest version of Windows Media seems to work. The guy that made the vid goes by the handle here of JohnVH. Maybe he can answer your question.

[Jim T. Graham]

Have you done a snap roll yet?

[opualuan]

you talkin' to me?

I've actually been forcing them on my x250 for a few flights now, with throttle at full even... i gotta stop that, airframe is not built for it... waiting for thunder power 3s2p 4200 pack and repair to the motor mount (prop strike on landing)...

fun stuff! mini laser is next...