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Oct 14, 2001, 09:59 PM
an earth bound misfit, I
Basketcase's Avatar
Discussion

Please explain tip stall


Please excuse my ignorance. Although I find a lot of references to tip stalls and I know what power on and power off stalls are. I cannot find an actual definition to tell me what makes a tip stall and what the consequences are.
Thanks in advance.

Wayne
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Oct 14, 2001, 11:09 PM
Pacific Models

Tip Stalls


Ignorance or not,
when an aiplane stalls the tip is the last part of the wing which stalls. It appears that in the RC lingo everybody involved complaints of a tip stall.
Having discussed this with a few people it means that when an aiplane stalls in a turn everybody calls it a tip stall.
Airplane will stall in any position, it all depends of a input of controls.
Airplane will stall if a angle of attack will increase to a point of boundary layer separation.
John
Oct 14, 2001, 11:15 PM
Registered User
Wayne,

A tip stall is where the tip stalls first.

hehehehehehe

Sorry, couldn't help it.

On a more serious note, it really is just that, but it is a nasty thing. When the tip of one wing stalls ( and its always only one wing), that tip loses lift and the plane flips over on its back and goes into a spin. Almost always fatal (litterally in full scale) unless you have a ton of altitude.

This is why most models ( and real ones ) have "washout" built into the wings.
Washout is where the wing is twisted slightly so that the trailing edge is higher at the tips than in the center. This means that the root is flying at a higher angle of attack and will stall before the tips. This is a much more gentle and controllable stall. Usually almost straight ahead. It doesn't take much, just 1/8" higher at the tips on a 36" wing does it. More just makes the plane fly bad.

Flat bottom wings are especially prone to this behavior, IMHO. So some washout will make your model live longer.

The " tip stall" usually occurs at slower speeds, ( like when you are close to the ground getting ready to land) which means you don't have a chance to recover.

Hope this helps
Dave
Oct 14, 2001, 11:50 PM
Pacific Models

Tip Stalls


Dave,
how do you get the tip to stall before the rest of the wing stalls???
Oct 15, 2001, 12:06 AM
Our Daddy and Heli Junkie
Fred Bronk's Avatar
With the different angles of attack between the tip and the root of the wing.

The same thing (washout) can be achieved with different airfoils on the tips or a touch of up aileron, even with turbolators and slats.
Oct 15, 2001, 12:16 AM
Registered User
The washout on the wings is designed to keep the tips at a lower angle of attack than the roots. When the root is at the critical angle of attack and is beginning to stall, the tips are still a few degrees below critical. When there is no washout, some wing designs and planforms tip stall. The angle of attack is constant throught the entire wing. As it approaches the critical angle of attack, a yaw will make a tip stall. This is because as the plane yaws (turns) the inside tip is going slower than the outside tip. This causes the inside tip to stall and then the plane has one stalled wing and one flying tip. Since there is a stalled condition, this causes a spin. For some reason, delta planforms tip stall more readily than others.
Oct 15, 2001, 12:28 AM
Pacific Models

Tip stalls


Most of the tip stalls I see in the forums seem to relate a "ZAGI" style of the flying wing. Having measure the actual washout of the Zagi style flying wing it appears to be about 7 mm. Having said that, it is impossible for the tip to stall before the root of the wing stalls. So the tip stall in this case is a myth.
Any comments?????
Avro
Oct 15, 2001, 12:45 AM
Registered User
My understanding, and correct me if I'm wrong: A tapered wing is more prone to a tip stall due to the tip being thinner... (???)

Anyway, from my experience, Zagi's don't Tip-stall, they are just prone to spin. You can get a zagi to spin pretty easily. Recovering before "deceleration trauma" is the trick...


To paraphrase what everybody has been saying: A normal stall progresses from the fuselage outward to the tips. This is stable due to the tips being farther out.. (think lever-arm in terms of effective force). When the tips stall, you rapidly lose control of the roll...

BTW: JK Aerotech has a good fix for planes that tip-stall... Sharpen the edge for the inboard third of the wing. A sharp leading edge is more prone to stall... Thus causing the stall to progress "normally". This worked wonders on my 1/20th scale P-51 (speed 400). The easiest way to do this is to take the plastic from a retail "blister pack" that has a slide-in cardboard insert. The plastic has a 3/8" Folded section that cuts nicely and can be opened up . Simply tape the plastic to the LE of the wing, closest to the fuse. When making the piece, taper it, the thickest pt. goes against the fuselage...

(now I'm rambling.....)
Oct 15, 2001, 12:58 AM
Pacific Models

Tip Stalls


Well this is getting real good. Zagi does spin easy??
There is only one way to spin a Zagi. Hi speed with a tight turn ( must have a lot of travel on the elevons) and pull. When it goes into a spin pull power of and keep the elevators(elevons) in full up. It will demonstrate for you what a spin looks like. To recover realease up elevator and apply power.
If you do it any other way you may develop what people say is a TIP STALLL. All what that is is a spiral dive.
I guess I am working again testing a Flight instructors fo a last 25 years. Oh well.
Oct 15, 2001, 02:20 AM
Boffin
rpage53's Avatar
Modellers sometimes (frequently??) say a wing tip stalls first. As John and "Spam" have pointed out that never happens. A stall that progresses rapidly from the root to the wing tip on one wing only, we have come to call a tip stall. I don't think you'll find the term in any real aerodynamics book.

Ideally you want the wing tip to continue to provide lift after the root stalls to provide a gentle stall. Wings on full scale aircraft often have stall fences to prevent "tip stall". These would make the situation worse if the stall was actually happening at the tips first. The fence stops the spanwise flow from propogating from the root to the tip.

Highly swept, tapered wings and retreating blades of helicopters are special cases.

Rick.
Oct 15, 2001, 03:33 AM
Registered User
Sture Smidt's Avatar

Tip Stall


Hi,

A tip stall is what its name implies, the wing tip stalls first! This is due to the wings local lift pr unit area. A highly tapered wing has a higher or equal wing loading at the tip than at the root (wing tip chord half that of the root chord, P51, FW190, Me109 etc...). These planes have a nasty stall, especially in a turn, both models and full size. The stall is provoked with up elevator, enough elevator always gets you into a stall. This is true for all speeds!

The remedy is either wash-out (as mentioned in earlier posts) or having a more rectangular plan form. A rectangular wing (typical RC trainer) will stall with the root first, which is nice for the beginner as control will not be lost.

The type of stall is also strongly influenced by the wing section. Some sections have a nice gradual stall and some have a sharp and sudden stall. This has nothing to do with flat bottom sections !

On RC models the Reynolds number (Re) (the ration between speed times size and the air viscosity ("stickyness") is in a critical area, especially at landing speeds. On tapered wings the wing tip has a lower and more critical Re number, which doesn't help either.

So what do we? Place the centre of gravity CG correct, tune the elevator deflection so that you do not get a stall even with full deflection, add 1 to 3 degrees of wash out, choose nice wing sections for RC models and fly!!!!!!!!!!

Sture
RC-Pilot
Oct 15, 2001, 05:30 AM
Sloping off....
leccyflyer's Avatar
Well in the rather sloppy use that some of us RC pilots adopt terminology to descibe our observations as to how our aeroplanes react then tip stalls are undoubtedly real.

In common usage they describe a situation where on stalling the aeroplane will drop one or other wing and may flick or enter a spin. This is usually attributed to highly tapered wings or warped wings where the angle of attack at the tip is greater than at the root - wash-in - and is not restricted to swept wings or Zagis.

In fact I hadn't noticed that the Zagi was particularly prone to tip stalls at all until reading these forums - my Razor on the other hand does the detah flick and spiral dive unless handled with kid gloves. This has been attributed to a sharp leading edge promoting this type of stall. Of course dropping a tip in a stall may be an issue with the lateral balance of a model or full sized aircraft.

Benign stalling characteristics in rectangular wings in our models are encountered with a gentle bob of the nose, and are instantly curable by putting the nose down and regainng stable flight. In my particular case the only prerequisite for a tip stall is that I have hold of the sticks They need not occur in a turn and their most unfortunate habit would seem to be them occuring when the model is ten feet off the ground on a landing approach.

Washout is frequently recommended as a cure for tip stall, supposedly by ensuring that the tips of the wings are still flying due to their lower angle of attack after the critical AoA has been exceeded in the root section.

I'm not an aerodynamicist but there is some reference to tip stalling in the full size world, under the general category of unpleasant stalling characteristics.

http://www.navygouge.com/api/basics/ELOs/aero.html

cheers

Leccy
Oct 15, 2001, 05:57 AM
Vic
Vic
Registered User
I haven't seen a reply that has explained why, only how it behaves, so here's my feeble attempt to explain why...

When you make a tight turn with your plane, imagine the turnradius being a half circle, the the wing pointing towards the centre of this circle will be travelling slower than the wing pointing out of the circle, therefore the wing pointing inwards will be able to stall/loose ability to lift, before the outer wing does...

If you turn left, you'd be prone to tipstall on the left wing and visa versa...

Imagine flying close to the stall point of your plane, make a moderately tight turn and you will experience the wing travelling slower (pointing inwards of your turn circle) than the other will stall, the faster will not, thus sending the plane into a spin or invert the plane if executed fast enough.

Of course this is simplified, it's not a matter of the entire left or right wing dropping depending on turn, but a matter of how close a part of the wing is to the centre of the circle, thus gradually loose more lift the the closer a part of the wing is the the center of the turn radius.. The part of the wing that is the closest to this center, is of course one of the wing tips.

The reason washout will help reducing tipstall characteristics is that washout improves low-speed lift, if you look at indoor or park fliers, they almost always have a lot of camber and/or washout...

The tighter you turn, the more likely you are to tipstall - even at higher speeds, due to higher drag when turning you also slow the plane down (Thereby also slowing the inwards tip even further down).. The stall point of course depends on the aerofoils profile...

Hope these rambling make sense..

Best regards
Vic
Last edited by Vic; Oct 15, 2001 at 06:25 AM.
Oct 15, 2001, 07:45 AM
an earth bound misfit, I
Basketcase's Avatar

Wow, thanks everyone


Thank you to all who replied. I think I understand now. Quite a lesson in aerodynamics.
That explains why when I try to stretch the battery too far and am coming in real slow for a landing and the wind gets me or I turn a little too sharp, and all of a sudden I'm parked on the grass. Until now I never realized I had ever experienced tip stall.

I would like to add how much I enjoy EZone and appreciate all the knowledge that is so readily available.

Thanks again,
Wayne
Oct 15, 2001, 10:18 AM
Ascended Master
Sparky Paul's Avatar
One more note: Tip stalls are frequently encountered on landings, with highly loaded airplanes. When the airspeed is close to stall speed, and an aileron is deflected, the down-going side goes past the stall angle of attack and the tip stall occurs. The plane flips opposite the input command, which can be disorienting.
People who learned to fly on slow trainers encounter these a lot when trying to land their faster planes at the speed the trainer used.
Keep the speed up.


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