Tip Stall
 

After reading a good many posts here about tip stall there appears to be some misunderstanding of what really occurs, or what is thought to occur in such a stall.

We all should know that there a number of factors that can be involved to cause the tip stall often called a snap which most often occurs when our models are on approach, or a turn to final. There are also other regimes of flight where the same can occur even at higher speeds called an accelerated stall where drag exceeds lift available. Stalls can and will occur in very high G turns at any airspeed at any aircraft attitude.

Since there are many factors in stalls here are some thoughts with a link to provide additional information which should give you more insight into the aerodynamic principles involved.

With that said...........

If you are making a level turn to final approach as the angle of bank increases so does the "G" loading on the airplane requiring more lift to support flight. A level 45 degree turn has a loading of 1.41 G's, or a level 60 degree turn - 2 G's . This means if your airplane weighs 10 pounds in a 60 degree bank the wing now has to support 20 pounds of effective weight with requires more airspeed in order to support the induced weight.

If your airplane gets too slow in the turn a stall will occur and if the upper wing stalls first a snap like action will occur when the lower wing (still flying) goes over the top. The same can happen if your airplane gets too slow on an essentially straight in approach if one wing stalls before the other causing the stall not to be straight ahead. Since we do not have the advantage of being inside the aircraft with a method of knowing if the airplane is in coordinated flight one wing will most often stall first.

Here is a link with a lot of good information to review there is a lot of good information that may save you a crash.

http://selair.selkirk.bc.ca/Training...mics/turns.htm

RE: Tip Stall
 

Excellent insight, Steve. Thanks.

CGr

RE: Tip Stall
 

Give a heads up on this article people...... the man knows what he is talking about. Very good Steve. gphil

RE: Tip Stall
 

Not the normal definition of an accelerated stall. Normally it is defined as exceeding the critical angle of attack under a load greater than 1 g. Never seen "drag exceeds lift available' definition.

Kurt

RE: Tip Stall
 

Kurt,

There is a lift to drag ratio which is called L/D max the point where stall occurs. This can happen at any airspeed be it low A/S's or at a much more accelerated speed. To clarify this a bit if one were induce a high G loading on an airframe to where the level of drag increases to the point where enough lift can not be maintained thereby causing a stall (think wing loading and induced drag). When I was an Air Force pilot I once stalled a jet at over 500 knots in a high G maneuver heading straight towards the ground which was certainly accelerated. In order to recover I had to unload or reduce the G load to get the bird flying again I had exceeded L/D max for that airspeed. L/D max is something that was not discussed or taught to me in ground school when I was flying as a civilian pilot having 1600hrs before entering the military to fly. Your definition is certainly what most are taught.

Steve

RE: Tip Stall
 

Steve,

L/D max angle of attack is usually less than the critical angle of attack. I think you are blending L/D curves with Coeficient of Lift/AOA curves. L/D max AOA and critical AOA are different.. Tell us what you flew - I too, flew in the U.S. Air Force, like many others here on RCU.

Kurt

USAFA '88
Big Mouth Block 30!

RE: Tip Stall
 

Classic "tip" stall... 2:30 in
http://www.youtube.com/watch?v=nQJ3TzSZkpo

RE: Tip Stall
 

Classic "tip" stall... 2:30 into a snap
http://www.youtube.com/watch?v=nQJ3TzSZkpo

RE: Tip Stall
 

Had one today.  I was maidening a G-44 Widgeon (GP electric twin-engine seaplane) and slowing it down in anticipation of landing or some touch-and-goes.  She has a 28 oz/in sq wing loading and fully tapered wings.  As I lowered throttle at 50 ft to watch the glide characteristics she fooled me and tucked a wing under and went into a beautiful inverted flat spin.  Nothing I tried broke the spin and she landed updide-down with a Splash!  Underpowered for the weight - but scale so I should have known better and kept the speed up. 

Nothing damaged but one nacelle & prop and my pride.  Came down like a maple seed.

RE: Tip Stall
 

"A level 45 degree turn has a loading of 1.41 G's, or a level 60 degree turn - 2 G's"

G force is a function of speed and turn radius, not bank angle. I can make a turn banked at 45 degrees and have 1 G or 9 Gs depending speed. A change in direction is an acceleration, and that is then divided by 9.8 to get the factor of normal gravity you feel.

To get turn radius, you need either degrees per second and speed, RPM, or the arc length of the turn.

RE: Tip Stall
 

That is really what Mr. Steve said. LEVEL TURN he did say. You SIR are a tad short on your lift and YOU may need some extra to stay in the air!

Mr. Steve used an example of G - Force required to maintain level flight in a specific bank angle. After 20,000+/- hours and 41 years, Well HECK, even I was dang nye almost getting the knack of it. :D

RE: Tip Stall
 

?? G force doesn't make you maintain level flight. As for the rest of your post no idea what you are saying.

RE: Tip Stall
 

Great info! Thank you"

RE: Tip Stall
 

Oh, but it does. A tad shy of 2 g's at 60 degrees of bank and you will descend. A tad too much pull (more than 2 g's) at 60 degrees and you will climb. Trigonometry at it's finest.

Kurt

RE: Tip Stall
 

Steve,

The stall occurs at CL max not L/D max (by definition). A wing is stalled when any further increase in AOA results in a reduction in CL.

I agree that most modellers have a very sketchy understanding of stalling, stall speed, critical Angle of Attack, accelerated stalling, asymmetric (wing drop) stalls, and tip stalling.

I applaud you're attempt to improve the knowledge, good luck.

Some definitions that may help;

Stall= exceeding the critical angle of attack or CL max.
Stall Speed= the speed at which the stall will occur in level (1 G) flight, power off.
Accelerated Stall= stalling at more than 1 G, implies at a speed greater than 'stall speed'
Asymmetric stall= one wing stalls before the other, may be due to flying 'out of balance', manoeuvring (turning, rolling etc), wing warp or aileron deflection etc. not all asymmetric stalls are 'tip stall'. If the wing stalls from the root first then the wing drop from an asymmetric stall may be quite controllable with aileron and/or rudder.

Tip stall= if a wing stalls from the tip first (bad design feature) then any asymmetric stall will be much more violent. Any attempt to use aileron will likely make the wing drop worse.

Most modellers refer to all wing drop stalls as 'tip stalls' even though most of them are just asymmetric stalls.

This leads to forum questions along the lines of how do I fix tip stalling with answers along the lines of washout, change of section, root leading edge stall strips, just fly faster, etc.

If the question had been how do I fix wing drop stalling then the answers might have included; check wing symmetry, length, sweep, squareness to fuselage etc. try some rudder etc.

My advice to all modellers is stall your model lots, get used to where the elevator stick is when it stalls. Every time you put the stick in that position your model will stall. If you find yourself turning final low and slow or making a tight turn and you notice that the Angle of Attack (elevator) stick is near the stall position then be aware that you are near the stall. Speed or angles (bank or climb or descent) are irrelevant.

Not quite 100% true in theory but close enough in practice.

Dave H

RE: Tip Stall
 

G force is also a function of how hard you pull on the elevator. If I want to fly straight and level I will have to adjust the elevator to achieve exactly 1 G. If I roll to 60 degrees bank and I want to stay level then I will have to pull on the elevator until I have exactly 2 G. If I pulled to 9 G I would definitely wouldn't stay level.

So for level flight G force is a function of bank angle.

Now to achieve a specific turn radius does imply a given G dependant on speed. I can't just pull that G though, I need to bank, otherwise I'll just pitch up. If I match my bank angle to the required G then I can stay level ( if I'm not level the radius will be wrong anyway).

It's normal in flying to do it the other way round though, choose a bank angle and apply G to maintain level, climb, or descend as required. If a smaller radius is required then increase bank and adjust G to match.

Dave H

Who has only ever pulled 6 G

RE: Tip Stall
 

Obviously tip stalls occur when one wing reaches the critical angle before the other. Ailerons/flaps alter the lift of the wing by increasing or reducing the camber(angle of attack of the wing) and if you have flaps down during a stall the aircraft will have a less nose high attitude than what would be normal in a stall. This is because the wing reaches the criticle angle sooner because with the flaps down, the plane has a higher AOA  to start with. The stall will be slower but it will be more level and sometimes more violent. Ailerons work the same as flaps by increasing the AOA and consequently lift on one side and reducing the AOA on the other side thus decreasing lift.
Pretend that during a state of equilibrium you apply 'left' aileron. The left aileron will go up and the right will go down. This will reduce lift on the left side and increase lift on the right. this will initiate a roll to the left. Now pretend the the aircraft flying straight and level, slow it down so that it is just starting to get 'mushy' on the controls. Now again gradually apply left aileron as the plane slows right down. Eventually the plane will roll to the right.
This is because on the right side the aileron has gone down increasing the AOA and therefore stalling the right wing before the left one causing the plane to tip stall to the right. So even thought you have applied left aileron the plane rolls to the right. In full sized aircraft this has resulted in a destroyed airplane and a dead pilot because it is unexpected and surprising. Or for models a broken pane. This is why it should be stressed to new flyers that during the hold off and other slow phases of flight roll and yaw should be controlled with rudder only. If a tip stall happens during the hold off the plane will be headed for a cartwheel

RE: Tip Stall
 

Hi Flymad,

I guess you meant to reply to me?

I broadly agree with most of what you posted, but I think you missed my point a little.

In a normal symmetrical stall it's common for the stall to develop in one part of the wing before the rest. Full size aircraft designers mostly try for the root to stall first with the stall spreading towards the tip as angle of attack is increased further. In some models the opposite can occur with the tip stalling first. Depends on the design of the wing. So a 'tip stall' can be perfectly symmetrical. You are dead right when you point out that applying aileron, when the tips are close to the stall, will likely stall the tip with the down going aileron. There shouldn't be any full size pilots surprised by this phenomena though, we'll good ones anyway.

'Obviously tip stalls occur when one wing reaches the critical angle before the other'. Nope, that's when an asymmetric stall occurs. Now if the wing is one of those that tends to stall from the tip to the root then obviously one tip stalls first. The resultant difference in lift and drag is far worse than if one root stalled first. And that is an asymmetric tip stall. But not many wings stall tip first so even though one wing will drop it's not a 'tip stall'.

When the wing drops most modellers call it a tip stall but most times it's not, just an asymmetric stall.

For what it's worth I agree totally with the advise to be careful with the ailerons close to the stall and picking up the low wing with rudder is a better idea. Especially for beginners.

But there are plenty of models that will still respond to aileron well into the stall and this is because the tip is still flying, it's the root that has stalled.

Dave H

Who has spent a lot of time at and beyond the stall

RE: Tip Stall
 

I seem to remember another thread with an almost identical statement made in an almost identical fashion, with the poster failing to understand the bank angle to G force relationship expressed by the OP is a simplified case assuming ALL the lift is provided by the wing.

I think we all understand that the acceleration in a turn is a simple velocity squared divided by the radius, and that if the wing is the only source of the force to maintain a constant altitude during the turn then then it'd feel the forces described by the OP.

Now if people want to use the fuse to generate lift, like I do in a rolling circle then all bets are off for any given bank angle and the only relevent items are speed and radius.

Tip stall?? Pretty much what it says really, one wing stalls before the other and back in the day with poorly designed airplanes and airfoils it usually started at the tip first. Don't forget, simply rolling the airplane back to level from banked causes a large angle of attack increase at the tip of the downgoing wing which may already be close to the critical angle of attack.

Cheers

RE: Tip Stall
 

I sure don't feel 1.41 times as heavy when I make a 45 degree banked turn to final.. in a real GA aircraft... don't know if I have ever made a 60 degree banked turn.

RE: Tip Stall
 

ok,I was reading this quietly but upon reading this I had to jump in....

you can make a turn at any angle of bank and any speed but if you want to make a sustained LEVELturn, the G will have to be 1 divided by the cosine of the angle of Bank. IE G=1/cos(AoB)

45 degrees angle of bank will require 1.41G and 60 degrees angle of bank will require 2 G for a sustained level turn at the respective angles of bank..

Now the stall speed is a function of the 1 G stall speed multiplied by the Square root of the G

For example, an aircraft stalls at 100 kts at 1 G - in a 4 G turn (or manouver) it will stall at (100 x Square root of 4) IE 200 Kts...

Really quite simple math..

This math is used when determining manouver speed for an aircraft.. Utilsing the structural designed G load of the aircraft in this formula... IE, for an aicraft has a 4 G structural limit..

If it stalls at 100 Kts then you can safely fly it up to 200 kts and it is physically impossible to break the aircraft, because at or below 200Kts it will stall at or below 4 G... so up to this maximum manouver speed you can use full control deflection Eg full up elevator and the wings will not fall off.. the plane will stall prior to reaching the design load limit..

Above 200 knots in the above example you can generate more than 4 G and therefore you can break the wings in flight..

This is a simplification, but essentially sound theory (yes I know rolling G is a factor and Design load limit is not ultimate load limit but I'm keeping it simple....)

As for tip stalling..

The propogation of a stall on a wing depends on a number of things, Aspect ratio, airfoil design, washout, sweepback, wing taper.... In a purely rectangular wing, the stall will propogate from the root and work outwards... a higher aspect ratio wing will generally stall at a lower crticail angle, so a tapered wing effectively makes the tip higher aspect ratio than the root and a tapered wing can stall at the tip before the root..

It one wing stalls before the other, there will be a roll or "snap" effect and this will be far more pronounced if the tip of the wing stalls first, simple leverage explains this.. an imbalance of lift at the tip of the wing will cause a faster roll than an imbalance of lift at the root..

Other factors are any input of aileron (changing angle of attack) and also any yaw present...

If you experience a tip stall (or any stall for that matter, just relax the elevator a few millimeters - yes millimeters and the aircraft wing will unstall and the tip stall (snapping) effect will disappear...

Your elevator stick is really a direct angle of attack control device.. you will only stall if you use too much angle of attack which means too much back elevator.. relaxing elevator in any stall situation will usually be enough to recover instantly...

The greatest design feature in a wing to prevent a stall commencing at the tip is "Washout" where the wing is essentially twisted from root to tip, so that the angle of attack of the tip is always less than the root.. this virtually guarantees the wing root stalls first and the tips (with ailerons) remain at a lower angle of attack and therefore unstalled.

(3500 hours of teaching aerobatics in real aircraft) the theory works just the same in real planes as it does in our models.

RE: Tip Stall
 

The wisest words written in this thread, . Yes this was the most fundamental concept we would teach aerobatic students.. Your elevator is an angle of attack control device.. the wing always stalls at the same critical angle and therefore the elevator joystick (RC or fullsize) will be in the same position each time you stall.. if stall is encountered, you only need a few mm forward stick to unstall..

Very refreshing to see someone else say this.. Ahh.. New Zealander... Ex RNZAF?

RE: Tip Stall
 

The aircraft will NEVER stall by itself..... A stall will never just Happen to you.. It is a result of what you are doing wtih the elevator. any aircraft will stall if you apply too much elevator..

Depending on a number of factors (control throw, CofG, wing shape, elevator size etc) the elevator position for the stall may be 30% deflection or it may be 90% deflection, but for a given aircraft, and without changing the CofG, the elevator position will always be the same .

The tapered wings will usually stall at the tips first,

When I hear people say.. "The plane just stalled on me, the true answer is.. NO.. you stalled the airplane by using too much elevator... you just may not have realised you were doing it"

RE: Tip Stall
 

Turning final you are probably descending... Try a level 45 degree turn for 360 heading change and if you maintained, height, angle of bank and speed I promise you will have experienced 1.41 G.

RE: Tip Stall
 

Thank you everyone for all the imput!!!! :D

This post to a great extent did exactly what I'd hoped it would accomplish which is to bring forth issues and principles related to stalls. From this discussion hopefully many here will gain more insight into how to not fly their model into an envelope of flight that could result in a stall ending in a crash.

I admit its been a while since I sat in ground school so there are some cobwebs in the bell tower. A lot of good information presented much of which I agree with. With that said I think its time for us to sift through what has been presented form a simple list of pilot considerations that will help our fellow fliers prevent future crashes. As many have pointed out a lot depends upon the type and design of the aircraft flown.

Since I am the instigator(devils advocate) if you will of this discussion/debate I'll reserve my input towards the end.

Please share your thoughts on stall prevention!