For WIMAC Aeronautical engineers.

[beto9]

A question for all the gurus out there.

Some years back many of us bought a Phoenix .46 size Sukhoi. Alix had one flight with mine and told me to put it away.
Everyone else pancaked on landing approach because the plane had a nasty tendence to tip stall at low speeds.
This was recognized by many people in different forums who had flown the plane.

Do you think the pictured winglets will improve the stall characteristics?

Explain your opinion, please...

Substitutions? Explain.

[Burning Head]

Well,if Alex try and say so.I see one in the past on a trainer but not so huge Maybe on 747 Duc jet...:cool:

[Mick]

Some long forgotten theory:

It looks to be a straight wing therefore the lift distribution is reasonably even provided the wing is not warped. Therefore the wing tip section is trying to generate its share of the lift but due to the over the tip flows, from the under side to the top side there is a tendency for the tip to stall. (This generates the tip vortex) This is exacerbated by the full span ailerons when you put in control deflections. Therefore, in theory, the wing tip fence should reduce the over tip leakage and therefore reduce the tendency of the tip to stall. Many wings have wing tips with reduced thickness sections to reduce lift at the tip, like a fence but parallel to the wing not perpendicular to it.

Additionally you could also shorten the aileron by making the last inch or two fixed, particularly if you make the outboard fixed section have some washout i.e. reduced camber. This will change the lift distribution. You can achieve a similar result by warping the aileron. Clearly both of these do not work once inverted.

Ultimately you will have to fly it to find out.

[Eric Marchand]

:iamwithstupid:

Top of the wing : low air pressure
Bottom of the wing : High air pressure

What you want to do is prevent the pressure from trying to equalize at the wing tip. Full scale planes have winglets, that extend upwards, but they don't fly inverted. Having it go up and down like in the picture should help in both situations.

However, I don't see the need to extend it forward of the wing leading edge, unless you are trying to generate extra lift in knife edges.

[PaulG-rc]

Alberto they do make a big difference,I have been using these for a long time even when I started as a young kid in RC I already understood what they would do.
Look at some of my planes that I had with fins and winglets some go back many years.
The ones you see on my Skycam Cessna made a huge difference as I carried a heavy camera and it flew very well.
Also the PT40 on floats ,after I install them the plane basically fly's itself at low speed. It all depends on what kind of wing and for what kind of flying you intend to do.

[Jeremy Cartlidge]

Albert,

I am not familiar with the Phoenix Sukhoi, but I suspect that it has a nearly scale tapered wing. If so, the wing is not well suited to a 46 size model. A rectangular wing will initially stall at the root, while a highly tapered wing may stall first at the tip - not good except for spectacular aerodynamic snaps. Reynolds number effects make this tendency much more pronounced on a model aircraft wing, especially for small models. Your tip fences are unlikely to have any useful effect and may make things worse if there is a yaw angle. The Hangar 9 solution for the P-51 PTS that allows for the use of a scale like wing planform is to use NACA droops. A more rounded symmetrical leading edge (top and bottom - no droop) at the tip may reduce tip stall tendencies while preserving inverted performance.

If you need more docile sport flying characteristics, go with the droops.

What is the taper ratio from root to tip? What is the thickness ratio from root to tip? Wing loading?

Air flows are extremely complex and hard to predict. Perhaps your fence will work. A test is the only way to know for sure (3 mistakes high).

[loflyer2001]

Albert;

If you look in Wikipedia and Yahoo on winglets you will
probably come out more confused than ever. I built
and installed a set on my full size cub. They were the
downward types to trap in the vortex has it flows under
the wing and create more thrust. However, the Cub
did not generate enough airspeed to notice how effective
they were and it created more drag thus negating the whole
experience. I think the upward winglet would act like
vertical stabilizers and prevent wing tip stalls. I built a
Sorrell Hyperbipe (model .40 size) and did experienced
on approach at slow speed uncontrolled rolls and wing stall.
The stabiliser/elevator on that model is blanketed by the
lifting fuselage. Maybe winglets would have help there.
At least, we dont hurt ourselves when we experience
those things with our models.:confused:
Guy L.

[Ronald Longtin]

Something else to try. Quote from the Robin by Andy Lennon, available from Mosquito.

"Ahead of the ailerons, the E197 airfoil is modified to NASA development leading edge extensions and droop that do three things; the stall is delayed a further 9 degrees, aileron control at high angles of attack (AOA) is effective and there is no tip-stalling, important for STOL landings."

I also remember twisting he wings by selectively heating the Monokote. If memory serves me right the leading edge has to be lower at the tip. It could be that this was only valid for flat bottomed wings!!

PS I finally found the right term (I was stuck on warp) and Wikipedia has this to say

'Wing twist is an aerodynamic feature added to aircraft wings to ensure that the wing tip is the last part of the wing surface to stall, for example when executing a roll or steep climb; it involves twisting the wingtip a small amount downwards in relation to the rest of the wing. This ensures that the effective angle of attack is always lower at the wingtip than at the root, meaning the root will stall before the tip. The reason this is desirable is because the aircraft's Flight control surfaces are often located at the wingtip, and the variable stall characteristics of a twisted wing alert the pilot to the advancing stall while still allowing the control surfaces to remain effective, meaning the pilot can usually rescue the aircraft from the stall before control is completely lost. It is also sometimes referred to as washout.

[beto9]

OK guys, thank you very much by your postings...
Ionel is missing to post but already he gave me an earful at the field about this subject...
However... as you can see by reading, I got from the strong encouragement to the strong discouragement on my purported "solution" and some steps in between.

I have a bit to add to the confusion: I read in R/C Report about exactly the same plane that after a vicious snap and repair the author added some inches of triangular stock at the wing roots "to have the wingtips stall before the wingroot". And I quote because in my mind a stall is a stall is a stall.... but maybe if the wingtips stall the plane can leave them behind and continue flying...?:D

So, yes, I am with the only option left... fly it with and without and find out... 3 mistakes high (for me that's a mile high!) But I still have to bring it down to sea level AND LAND IT!

I also thought about adding wing washout (Ronald!) by some additional contraption (a detergent soaked rag at the wing tip?).

Oh well, I am tired of the plane gathering dust and spiders in the wall... it will fly and decide its own fate...

More to follow...

[AlixB]

Alberto, A short answer is yes it makes a diff.
The real question: will it really change something if the stall speed is lowered by say 10% ?

The reality is that this plane LOVES to be flown fast. On take off, on landing and in the downwind leg (god forbid if you are too slow when turning downwind).
My bet is also it that it will snap out of a loop if you let the airspeed decay as you are returning to level flight.
Not funny if you want to keep clean underwear....

The funny thing is that the wing area is barely bigger than the total wing area
of a big 3D indoor biplane (around 460 sq/inch if I remember well) and the A/C weight over 6 1/4 pounds loaded compared to 12 oz for the foamy...

Mine was stripped down to minimal (4 1/2 lbs) and I added 20% of wing area with extra large aileron. (750 mah AAA cells,carbon LG, and remove LOTS of ply)
It flew VERY well in that configuration.
This was my last glow airplane and I sold it.

[Sunfly]

Albert you read in RC report "The author added some inches of triangular stock at the wing roots to have the wingtips stall before the wingroot"

This is surely a mistake in their statement.
Adding a sharp edge or a rope in the middle of the leading edge will make the air flowing toward the top turbulent at an high angle of attack ( 7 to 15 degrees). Then the affected surface will stall before the rest of the wing.

Therefore
I think the last part of the sentence should read:
"The author added some inches of triangular stock at the wing roots to have the wingroot stall before the wingtip."

This was my 2 cents :)

[beto9]

No, it was a mistake in my quoting... wingtips to stall before wingroot.

[xed]

Well, really, you don't need wingtips to fly... If this guy (Jim, owner of RCGroups.com website) can fly with an entire wing gone missing in-flight, I'm sure you can go without any fancy wingtip gizmos...

YouTube - RC Yak 54 AMAZING!

[beto9]

What an opportune video!

We need no stinking wings! (from the dark side...)

[Michael V]

Is you hero :)

[beto9]

This is what the author of the article in R/C Report sent me as an answer to my mail:


Since we aren't in an RC plane and can't feel the approach to a stall, it is important that the wing tips do not stall first. If a tip stalls first, the wing tends to drop quickly. Near the ground, this can be fatal.

Rectangular wings stall at the root first, then the stall, as the speed slows more, moves outward toward the tip. As you taper a wing or sweep it, the initial stall area moves outward from the root to the tip. This is the reason trainers have rectangular wings.

The Sukhoi has a tapered wing with an airfoil that is not too thick, especially at the tip, as I recall. This means if you get too slow, one wing tip will tend to drop out from under you.

Stall strips. It is known that a sharp leading edge stalls more quickly than a rounded one. To make the root stall first, before the tip, you sharpen the leading edge near the root. This is used on full scale planes. The old T-33, jet trainer that I learned to fly in, had angle iron stall strips about a foot long on its wing roots.

For RC, we normally use triangle stock. About 6 inches on each side works pretty well. You can flatten the leading edge and glue it on or glue it on and fill in behind it. Doesn't seem to matter. Once you paint it or cover it to match, it is hardly noticeable. The plane does land marginally faster, usually the pilot doesn't notice it since he is so happy that his bird isn't falling out of the sky.

[Michael V]

Well if you had the answer the whole time why, did you ask us?

[Michael V]

:wacky::confused::wacky::confused::wacky::confused ::wacky:
Well if you had the answer the whole time why, did you ask us?

[beto9]

Because there could have been more than one solution to the problem.

And besides, I do not necessarily believe everything I see in print.

And do not tell me that you didn't enjoy the input from everybody.

[Jeremy Cartlidge]

Alberto, the stall strips will increase the stall speed by inducing an earlier stall at the wing roots so that the tips do not stall. In your first post you said that others pancaked on approach.

"Everyone else pancaked on landing approach"

They would not have pancaked if there was a tip stall. Are you sure that the problem is tip stalling. If they pancaked. it seems that they just got too slow and stalled. Alex has the experience and answers - reduce weight or fly fast.

Add stall strips and you will need an even higher approach speed. Let me know when you plan to fly it!

[Michael V]

Because there could have been more than one solution to the problem.

And besides, I do not necessarily believe everything I see in print.

And do not tell me that you didn't enjoy the input from everybody.

Boy did I ever, :)

[beto9]

Wrong words used... pancaked would be they dropped the plane flat on the runway... too slow approach speed... NO! The plane snapped viciously one wing and re-kitted.

[Patrick Fortin]

Hey Al,
Please post something when you do fly it......As I was one of the
pancake makers back then, I am curious as to how it turns out.
It was a very well built plane as far as esthetics where concerned,
but I remember Alix's words very clearly: "Put it away and never
touch it again"......Well I did put it away....in a dumpster!!!

Have fun:):)

Pat

[beto9]

The first attempt at flying the Sukhoi was aborted due to erratic running of the engine.
Fuel tank problems suspected...

More to come...

Keep your fingers and toes crossed... and your knees if you have to go!

[beto9]

I want to re-open the original discussion... stall strips, winglets, washout...

How come nobody suggested flaperons for landing?:wacky:

[Jeremy Cartlidge]

Because they may make the tip stall worse if you have ailerons that go all the way to the wingtips.

[Sunfly]

I want to add:
Wash out can be good when flying normally (Up side Up !) but it will have an adverse effect on increasing the tip stall tendency on an aerobatic plane flying inverted at low speed.
In that case I think the suitable wing tip config is to have the tip thinner with no washout.

My little grain of salt.

[Eric Marchand]

Just...Fly the thing bloody fast, :eek:and go pick it up in the grass at the end of the runway...Problem solved...:D

[xed]

Just...Fly the thing bloody fast, :eek:and go pick it up in the grass at the end of the runway...Problem solved...:D

Let me fix that statement:

Just...Fly the thing bloody fast, :eek:and go pick up the pieces in the grass at the end of the runway...Problem solved...:D

[beto9]

Eric and Xed... I wish your dead sticks high up.... upwind!


Will post when flown....