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Feb 23, 2018, 04:42 PM
Build, crash...repeat
AleRRon's Avatar
Question

Tip stall


I recently became interested in the tip stall problem I've heard so much about. I've read various explanations about how it works and a few ways to prevent it. Wing shape factors in, but if you want to build a scale or semi-scale plane with an elliptical wing or swept back wing, you gotta build it's wing with it's shape, so you're stuck with that. I have flown two high wing trainers with dihedral (FT Simple Cub and the Blu Baby 33) so far and I have not run into this yet. But I also have a RTF A600 bush style foamie that flies ever so sweetly and gently on beginner mode, but instantly spins and spirals nose first into the ground as soon as I switch it to "advanced" mode. As far as I can tell, the advanced mode just increases throws, but I wonder if I'm running into a severe tip stall problem. I built some low wing planes over the winter; some have smaller tips than roots and could have a tip stall tendency-I don't know yet; weather is still crappy. I really don't want to crash these due to tip stalling after I put all that work into them

Apparently, real planes and many RTF planes and balsa kits are designed with "washout" in which the wing root and tip have different angles of attack so as to counteract the tendency for tip stalling. Stall fences can be installed to prevent airflow from flowing out towards the tips. Vortex generators are built on to real planes to create airflows that stick to the wing and flow in the right direction.

I have seen a few foamie planes (RTF or ARF) with stall fences along the wing. Seems easy enough. I have not seen vortex generators.

Does anyone worry about this much and/or build any of these defenses into wing design? If so, what is the best/easiest/most effective? Any good way to build washout into a foamie wing such as a FT airfoil or Armin wing, or KFm wing?

Thanks and looking forward to aerodynamic wisdom!
Bob
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Feb 23, 2018, 05:26 PM
Build straight - Fly twisty
Whiskers's Avatar
Quote: But I also have a RTF A600 bush style foamie that flies ever so sweetly and gently on beginner mode, but instantly spins and spirals nose first into the ground as soon as I switch it to "advanced" mode. As far as I can tell, the advanced mode just increases throws, but I wonder if I'm running into a severe tip stall problem.

Something is very wrong here.
Bench test how the control surfaces react when you switch modes. It seems that the control surfaces are reacting in an inappropriate way.

Re tip stalling: Google "wing cuffs."
I have found them to be super effective. I had an FPV plane that would sometimes tip-stall and drop into a spin on a landing approach. That really grabs your attention and tests your reflexes.
Wing cuffs cured this nasty behaviour.
Feb 23, 2018, 08:52 PM
When cows fly!
I have blamed sooooo many crashes on tip-stalls! But in reality, it was due to dumb thumbs/overactive thumbs! Combined with lack of airspeed!
Feb 23, 2018, 10:58 PM
Build straight - Fly twisty
Whiskers's Avatar
I've had another thought. Does "beginner mode" bring an electronic flight controller into play?

I've had no real experience with these except on a flight simulator that had an Apprentice as one of the planes.
With the beginner mode engaged the most inept stick banging resulted in lovely smooth flight.
Full aileron (for instance) would produce a mild bank. In beginner mode it was impossible to perform any aerobatics.

This was so pronounced that I thought any new pilot would have a hard time making the transition from Almost Impossible To Crash to Highly Likely To Crash

This seems to match your experience.
Feb 24, 2018, 09:33 AM
treefinder
springer's Avatar
Along that same line, we were flying out back and one guy had a new plane with that beginner setting engaged (but hadn't put selection on a switch yet) . First comment was "look it takes off by itself!" That was followed by " can't seem to get it to do anything". Followed by a long slow turn around to land and figure out how to program the selection switch so he could fly the plane himself.

I'm not a fan of stabilization or beginner "safe" stuff, figure the objective is for me to fly the plane.

To your original question, while each of the methods you mention have their place, for my foamies I find washout provides the best stability add. Depending on the wing construction I may heat warp the wing, or build it in during construction. If that doesn't eliminate roll instability (or tip stall if you like) I will try the LE cuffs as Whiskers mentioned. They work well at our scale /RE numbers from my experience, but look funny if on a scale plane that doesnt have them in full size. So, it depends on plane type.

Anecdote: when I built my 7 ft span wing, I built it up on the plane room floor and the process of weighting the symmetrical air foil down to glue skins put a nice couple inches of washout, symmetrically on each wing half. The plane flys a treat. I couldnt have done better if i had planned it!
Feb 24, 2018, 09:42 AM
Build, crash...repeat
AleRRon's Avatar
I thought so too but I didn't see anything besides receiver and ESC in the fuse. It's an XK DHC-2 A600 5Ch 3D6G. I'm not too worried about it though. The other planes I fly have been fine, but I've kept the throws low on those and have Expo set at 30%. I have dual rates set on those and I can fly them more spirited and they behave well. The XK comes with a transmitter and it's not adjustable like my regular FlySky transmitter is for the other planes.

But we digress...

Mostly I'm wondering if and how I need to deal with my scratchbuilt foamies IF they have a tendency to tip stall. Does anyone build their wings to combat tip stalling?

I think it's true that if you keep your throws moderate and keep your thumbs light on the sticks, and have reasonable skill, you probably will be fine. I'm just trying to move up to more advanced planes and would rather they behave nice!

thanks
Bob
Feb 24, 2018, 10:38 AM
An itch?. Scratch build.
eflightray's Avatar
Generally a stall is a stall, it just so happens that some models will drop a wing tip, some will drop with the wings level.

The first thing to determine is why did it stall ?

Stall occur when the wing reaches a critical angle of attack . That CAoA can easily happen when you fly slowly and add some elevator, but can also happen at higher speed by suddenly adding elevator.

If you fly aerobatics with a symmetrical wing section, you don't want wash-out, basically because it becomes wash-in when inverted.

Yes wash-out will help for a semi-symmetrical wing section on a trainer type model, mainly because people have a habit of flying too slow, adding elevator, usually in an turn when the inboard wing is travelling slower to start with.

A model flown straight and level at reduced speed, and deliberately stalled using up elevator, should stall and drop wings level. If it doesn't, then check for uneven warps, twists, or ailerons not level, lateral balance, (one side heavier than the other). You're looking for mismatch faults.

As soon as a model turns there is going to be a mismatch of the airflow over each side. Just don't upset that airflow at low speeds and put some elevator.

Also beware of watching a wing tip start to drop and give opposite aileron to try and lift it back up. All you are doing is adding a load of wash-in with the down going aileron, potential making things worse.

Basically make sure you plane is 'straight', not over-weight, has the CG slightly back not forward, and keep the speed up especially entering a turn.

.
Feb 24, 2018, 02:53 PM
Build straight - Fly twisty
Whiskers's Avatar
A pilot error I've often seen is to fly down-wind at a dangerously low airspeed.
The plane is going at a nice speed everything looks fine, but in reality its airspeed is just above stalling speed.
Usually, under these circumstances, entering a turn causes the wing to stall.
We model plane pilots fly without an airspeed indicator, which is probably the most valuable instrument for the full-size plane pilot.
We modellers don't even get feed-back through the stick to help us. Well almost.
We can observe when the controls go a bit sloppy and when the plane loses that crisp reaction to stick input. I find that this is the only way I can judge airspeed when flying FPV. The indicated ground speed is useless here. I've had instances when the plane is flying just fine at zero ground speed. This tells us that going down-wind at normal ground speed would be problematical, to say the least.
Last edited by Whiskers; Feb 24, 2018 at 03:48 PM.
Feb 24, 2018, 03:13 PM
When cows fly!
All good points guys!
Feb 24, 2018, 06:43 PM
Jack
jackerbes's Avatar
Quote:
Originally Posted by AleRRon
I thought so too but I didn't see anything besides receiver and ESC in the fuse. It's an XK DHC-2 A600 5Ch 3D6G. I'm not too worried about it though. The other planes I fly have been fine, but I've kept the throws low on those and have Expo set at 30%. I have dual rates set on those and I can fly them more spirited and they behave well. The XK comes with a transmitter and it's not adjustable like my regular FlySky transmitter is for the other planes.

But we digress...

Mostly I'm wondering if and how I need to deal with my scratchbuilt foamies IF they have a tendency to tip stall. Does anyone build their wings to combat tip stalling?

I think it's true that if you keep your throws moderate and keep your thumbs light on the sticks, and have reasonable skill, you probably will be fine. I'm just trying to move up to more advanced planes and would rather they behave nice!

thanks
Bob
I have not built any foamies that were not KF airfoils. And not one of them ever showed any tendency to tip stall. And I have have a couple of RTF planes that showed vicious tip stalling traits so I know what you are talking about.

The KF wings tame the stall behavior down remarkably. It turns low speed stalls into a slight downward nose dip and then an instant recovery from the small speed increase. That behavior gets better as you add dihedral to KF wings.

And if I had to single out one KF wing style that is the most stable and forgiving, without it showing any loss in performance because of it, I would say is is the polyhedral tipped KF wings. Building those give you a wide and flat and strong center section on the wing and the tip do not have to be very big or have very much rise. Here is one example of them:

** Blu Baby 33" KFm3P Polyhedral Tip Wing Build ** - https://www.rcgroups.com/forums/showthread.php?1474665

Let credit be given where it is due, this is the plane that inspired me to give the polyhedral tip wing a try:

Trainer type plane - https://www.rcgroups.com/forums/showthread.php?1281056

Keep in mind that the polyhedral tips can be relatively small and with only a little rise to perform their miracle. It bears experimenting with to sort out how much is needed for different style of planes.

And some of the bad things that happen to control responses when you use center joint/half span dihedral never show up with the polyhedral tips. There is no need to fool around with differential on aileron servos with wings like the KFm3P. As an example, you can take that KFm3P wing with ailerons and turn it into a good RET trainer wing by simply disconnecting the aileron servos and reconfiguring the TX to put the rudder on the right stick with the elevator.

Jack
Feb 24, 2018, 07:25 PM
Build straight - Fly twisty
Whiskers's Avatar
On the other hand, the worst tip-stalling plane I ever owned was a KFm2, flat centre section, dihedral tips, no ailerons, 60 inch span powered glider.
Mind you it flew very nicely if kept in the right flight envelope.
One of the "things" I'd do with that plane was that if I was flying it when a friend came to the field I'd walk up and greet them with the Tx dangling by me side, fingers off the sticks.
Then I'd casually turn, find the plane and resume the leisurely flight.
A bit silly? Yep! I know...
Feb 26, 2018, 06:40 AM
Warbirds Lover
Dreamcatcher's Avatar
Quote:
Originally Posted by AleRRon
I thought so too but I didn't see anything besides receiver and ESC in the fuse. It's an XK DHC-2 A600 5Ch 3D6G. I'm not too worried about it though. The other planes I fly have been fine, but I've kept the throws low on those and have Expo set at 30%. I have dual rates set on those and I can fly them more spirited and they behave well. The XK comes with a transmitter and it's not adjustable like my regular FlySky transmitter is for the other planes.

But we digress...

Mostly I'm wondering if and how I need to deal with my scratchbuilt foamies IF they have a tendency to tip stall. Does anyone build their wings to combat tip stalling?

I think it's true that if you keep your throws moderate and keep your thumbs light on the sticks, and have reasonable skill, you probably will be fine. I'm just trying to move up to more advanced planes and would rather they behave nice!

thanks
Bob
Hi Bob,
An excessive control Throw can create a dynamic stall of the wing or, most frequently, the elevator : i experienced this with a little Cap 232 made from depron. I would like that plane to be a good 3D flyer but i built all the surfaces as scale as possible compared to the real Cap 232 (that is a nice aerobatic plane but not a 3D... of course). Too much control Throw on the ailerons gave me a plane very sensitive on roll axis but nothing impossible to pilot. But when my elevator control throw was increased up to +/- 45 and above... pulling or pushing the elevator stick over 2/3 of the curse gave me instantaneously a kind of very violent snap roll that was very hard to recover (specialy if the plane was flying low...).
The problem was also present at the end of the loopings, in the last 1/4 of the loop : If the looping's radius was too small or the airspeed was to high or to low... the plane started immediately a kind of half snap roll that placed her in inverted flight with an airspeed near to Zero - and most of the time finished as a crash...
Feb 27, 2018, 12:52 AM
flyin' fool
goldguy's Avatar
The staggered LE used here acts as an anti tip stall devise to allow for extreme elevator input.

Also, if you can't twist washout into the wing, adding a triangle of balsa or foam to the LE at the tips that's about 25% of the half span works as well.
Feb 27, 2018, 01:59 AM
Warbirds Lover
Dreamcatcher's Avatar
Thank you so much Goldguy for the tips !

This is very interesting

Wow that Yak wing is full of aerodynamic tricks that should improve a lot the flight characteristics... I would like to know what is the purpose of the triangular cuts in the trailing edge from the Wing root and up to 50% of the span. I guess it's used for improving the airflow that feed the elevator, as they are the same level (i guess it's designed to improve the elevator reactions because the triangular cuts seems to be the same size as the elevator). They may also be here to avoid the wing tips to stall before the wing root ?

This "step" in the leading edge is intersting too. It seems to be used to change the lift distribution on the wing or something like that that effectively can acts as an anti tip stall - I see that the step on the LE start where the triangular cuts stop on the TE.. That is probably not a random too.

Anyway, i had never thought taht such things could be applied to a (not so) simple flat plank shaped wing.

I may give a try to all these things on a "mini or nano foamie" one of these days !

Thanks al lot once again !
Last edited by Dreamcatcher; Feb 27, 2018 at 02:18 AM.
Feb 27, 2018, 11:15 AM
flyin' fool
goldguy's Avatar
That 3D floamy has been developed over the years and has some very innovative features that make it about the best out there.

It may not look like it, but we've come along way from the original flat 3D design the Shock Flyer.

Flat is more technical than it looks ....................... https://www.rcgroups.com/forums/show...&postcount=422


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