Aerodynamics and Downwind-to-base turn

[SyncroScott]

At some point I could have sworn that someone once explained to me a phenomenon that occurs when turning downwind to base. I seem to recall it being about the loss of lift and a tendency to drop a wing. There was even an anecdote about WWI pilots having issues with this and that it tends to affect lighter AC.

The guy may have been way off, I could easily have gotten my wires crossed with the ol' turn TO downwind issue, or I could have imagined the whole thing. It has been bugging me for quite some time ever since I seem to have experienced it while flying a park flyer in a decent wind a few weeks ago. Anyone care to help me get to the bottom of it?

-Scott

[HerkS]

It's a boundary layer effect. The wind near the ground is moving slower than the air above.

It is not about the turn.

It's about climbing into layers of air that are moving faster than the those below it - the layers in which the plane is climbing. It's nothing to do with the ground, but rather the speed of the air above relative to the air in which the plane is climbing.

If the plane is climbing upwind into faster moving air there will be a transient gain airspeed, and if climbing downwind there is a transient loss of airspeed. Vice versa when descending.

It's usually only noticeable when the wind is pretty strong and when flying at low altitude.

[SyncroScott]

Thanks!

[Texas Buzzard]

I respect Herk and he may be right in the case he has in his mind, but maybe there is something to the downwind turn since iI lost two (2) Parkjets in the past year to the downwind turn. I know what I see. Here is what I saw:

My 16 oz. parkjet was flying down wind in a 15 to 18 mph wind ( the flags were straight out) Altitude was about 40 feet. We have trees on the downwind side of field. I had eased back the throttle to maybe 40%. This throttle in still ait is just a little above stall speed. I made a quick left turn and it turned OK, but as soon as it got close to the 180 degree point - -it wagged the wings, stopped flying and fell of quick in a little spiral to the ground -crash. It happened fast, I didn't have much time to hit the throttle and get out of there. It just gave a wing rock and PITUEEEEEE in a spiral.

Characterists of plane in calm air. Stalls at maybe 12 to 15 mph. Top speed maybe 30mph, Drag....yes drag is high, lots of equipment hanging out,

I know the quick turn added drag and at a 40 degree bank I needed more spee for lift.

I just poster this to say I think there is something to the downwind turn in a draggy, light wing loading and Flying at just a fey miles and hour abone stall.

[SyncroScott]

Tex: I have experienced similar stuff!

I don't know what caused what I saw either but while flying patterns with several different sized light park flyers (Kyosho Minimums and 30oz BL powered 182 at different times) in moderate winds (relative to both sized craft) nothing was out of the ordinary other that what seemed like a loss of airspeed and resulting altitude upon turning to BASE from CROSSWIND leg. It didn't surprise me because I could have sworn that there was a reason for this that was previously explained to me. I compensated and all was fine (sorry to hear you weren't so lucky), but either this was just dumb coincidence or there is something to it...I just don't know...

-Scott

[vintage1]

Texas, we all have the same experience, but we interpret it differently.

Two factors come flying slow, with wind, one is that there is a tendency to turn more sharply, because the model doesn't appear to be making the turn, and the other is wind shear and turbulence means that the model can - and does - encounter quite a high variation in airspeed whilst again it appears from the ground to be making a steady speed.

This is particularly bad downwind of trees, where you can get massive sink.

And even upwind, you can get a bit of lift and a roll in the air as it starts to climb to get over the trees.

Now you lost airspeed , stalled and crashed. The question is why. If you were flying INTO the wind, down wind, at that point, I'd say the wind simply dropped! If you were flying across the wind, likely as not you just hit turbulence.

I've had three models snap their wings on me flying straight and level, in modest wind.Now they may well have had internal damage as well, or been badly designed but the fact remains it wasn't 'high G manoeuvres' that caused them to die. Unless there were glitches I didn't see..it must have been a massive 'bump'.


Those of us who cut our teeth on free flight and single channel, with no elevators, and no proportional rudder. know that turns seem to take ages in a model flying down wind, and the model can, and does, dance all over the place in a 'hands off ' glide, and can, and does stall for no apparent reason apart from wind shear and turbulence.

You learnt to start turns to upwind very early..when the model was scudding past you, knowing it would be miles downwind by the time it was pointing into wind, and would make very little progress against the wind once it had. What we used to do - especially with trees, was to start high, and close and never let the model get too far downwind at all, and come on on a series of S bends which, with single channel, always put the model into a bit of a dive as well, keeping airspeed and descent speed fairly high, and the trick was to get the model close in low and pointing the right direction just before the ground, let go of the rudder button, and hope it JUST pulled out before it hit the ground.

And there is another factor when landing upwind. There is less wind near the ground, particularly at sunset where the lack of thermal activity means the wind shear is uniform - I have flown here with it flat calm at ground level, but 10-25mph at treetop height. This is nasty. As you drop below the tree line, the wind collapses, and so does the airspeed. This is VERY hard to deal with as you find yourself with too much height and a model that is falling fast and picking up airspeed - or stalling, as you come below 30 feet or so.

The best way out is a higher speed circuit at lower level and HOPE you can lose the speed enough to land. Or be prepared for a longer walk.

[JetPlaneFlyer]

If you were flying just above stall speed then executed a 'quick' turn than you should expect to stall, regardless of turn direction. Turning increases 'g' which effectivly increases wing loading and so increases stall speed. Herk's point about wind sheer would also contribute if the model was descending during the turn to base, which is very likely...
Bottom line is dont execute a tight turn when flying close to stall.. 'especially' at low level and 'especially especially' on the turn to base in windsheer conditions... In fact in windy conditions you should not be flying close to stall speed at any time..
Texas Buzzard; You fly full size ..I thought full size practice was to approach at about 1.3 x stall speed in any conditions and to add an additional few KTS (half the wind gust speed) when approaching in blustery conditions in order to account for gusts? Apply full size practice to your model flying and you should not have a problem.

When flying down wind it's difficult to assess true air speed.. We dont have airspeed indicators on RC models so there is a temptation to judge model flying speed based on a ground reference which can mean the model is close to the stall even though it 'looks like' it's flying fast. My 'solution' to this problem is let the model choose it's own airspeed, i try to avoid pulling back on the stick during the downwind leg and into the turn to base. My experience with free flighht tells me that if the model is trimmed with a little stability is will look after it's own flying speed quite nicely without my intervention.

Steve

[slipstick]

I just poster this to say I think there is something to the downwind turn in a draggy, light wing loading and Flying at just a fey miles and hour abone stall.
Like Steve (JPF) says that's all true....apart from the "downwind". If you perform a tight turn in any direction without adding power when you're flying only marginally above the stall you'll get in trouble.

The only thing "downwind" adds is the greater likelihood of making the mistake of turning too sharply with not enough air speed because to a ground-based pilot the plane "looks like" it's going quite fast. I.e. ground speed is high, air speed still isn't enough.

In lightly loaded full size aircraft like the old WW1 planes the biggest problem is more likely to be a gust catching you when you're banked. Happens all the time but it's worse when coming in to land simply because you're flying slower and are closer to the ground with less airspace to correct in.

Steve

[Andrew McGregor]

Full size, there was a problem early on when pilots were not trained to ignore their visual perception of speed. They got too slow flying downwind because their eyes were telling them they were going fast. It still kills the odd fullsize pilot when they forget their training, or never really internalised the lesson.

The solution for a plane with no instruments is to rely on throttle position, trim position and elevator position, which works just as well for a model. So, if you know you can fly level at, say, 30% throttle with three clicks of up trim, it's going to be safe to do that upwind or downwind. But, if that is just on the edge of a stall, you're going to have to add some power to turn at that speed... potentially quite a lot.

[HELModels]

I remember flying full scale and encountering my training. I always ended up pushing that nose down just a hair more than needed turning base. I also remember encountering a wind shear so bad the airspeed went from 40 kts to 100 kts, stall horn went off a few times without me changing anything, except pushing the nose down more in response.

[Julez]

It's a boundary layer effect. The wind near the ground is moving slower than the air above.

It is not about the turn.
It is also about the turn. As one wing is higher than the other, the higher wing experiences more wind than the lower. This increases the banking angle, which might be a problem for inexperienced pilots.

[ciurpita]

It is also about the turn. As one wing is higher than the other, the higher wing experiences more wind than the lower. This increases the banking angle, which might be a problem for inexperienced pilots.
I don't know how this increases the bank angle, but when banking the angle of attack on both wings may be different as the plane turns crosswind. Since the plane has excessive speed with the wind in order to maintain airspeed, the upper wing tip where the wind speed is the highest, will have a lower effective AOA, while the lower wing where the wind speed is the lowest will have a higher effective AOA of attack. If the lower wing were to stall, the plane would roll, increasing the bank angle making matters worst. Isn't this what leads to a spin?

[JaRaMW]

I don't know how this increases the bank angle, but when banking the angle of attack on both wings may be different as the plane turns crosswind. Since the plane has excessive speed with the wind in order to maintain airspeed, the upper wing tip where the wind speed is the highest, will have a lower effective AOA, while the lower wing where the wind speed is the lowest will have a higher effective AOA of attack. If the lower wing were to stall, the plane would roll, increasing the bank angle making matters worst. Isn't this what leads to a spin?

AOA would only change if there was a change in wind speed perpendicular to the incoming air. If the descend rate is neglible and the wind shear is horizontal, both wings see the same AOA unless the plane is already rolling.

As has already been said, aerodynamics do not care about ground speed. Our estimation of the model's airspeed based on the ground speed we actually see causes the trouble. Add wind shear and turbulence and such turns can become quite tricky.

It is true that in a horizontal wind shear like in the boundary layer near the ground, the higher wing creates more lift than the lower, but I think that the typical wind speed differences over a model's wing span in a turn are so small that they may only play a role in extreme cases. This should be no issue under moderate wind conditions.

Totally agree with Steve. Focus on how the model "feels" during any part of the approach. If the ground speed in the downwind leg is faster than usual in still air and you still have to keep the nose high to maintain altitude, you know that the plane has considerably less airspeed than ground speed. The higher load factor in the following turn may already cause a stall. Add a few possible turbulence effects and it gets even worse. Vertical wind can disturb the perception of the flight attitude even more. It doesn't need an exceptionally strong wind to create downdrafts in the order of 3 m/s (600 fpm) near a forest border for example, or similar updrafts on the other side of the rotor.
Like vintage1 said, downwind -> base turns can seem to take forever because the high ground speed makes the turn appear too wide, urging you to turn sharper.

[bwalt822]

Starting a new 300 page thread are we?

[JaRaMW]

No need to discuss any modeling science at all. Google knows it all.
As soon as this forum becomes overcrowded, I agree with you. Until then, you owe us a beer.