What goes up - questions

[Major]

Hi.

After reading the last article some questions appeared and there is coupple things I'd like to understand better.

First, angle of attack. In the article we read that "the angle of attack at which wing will stall (...) is typically 15-20 degrees". But how does it look like in case of jet fighters and their unrestricted climb or even some acrobats?

Stall. "No aeroplane ever stalled with the stick forward". Is fo, how appeared saying, that stall can appear at any speed and any angle of attack?

I've got some own theories and explainations, but will keep them for myself, not to introduce any unnecessary mess.

Cheers

Tom

[Concorde Speedbird]

I have one other thing to ask.

You talked about tailplanes, and how it keeps the plane from going nose down, what about deltas?

CS

Edited By Biggles' Elder Brother - Moderator on 24/01/2013 11:13:21

[Tim Mackey]

Reflex on the elevons CSB, just like flying wings.

Stall and AOA - when a 3d er, or very powerful 'plane is flying at "stupidly" high AOA its not "flying" as such, its just being hauled along - or pushed in the case of a tractor - by the huge power available. They do say, anything will fly with a big enough engine, using the word "fly" with some artistic licence

I'll wait for BEB to correct it all now LOL

[Steve Hargreaves - Moderator]

Well...BEB sems to be on a tea break so I'll have a go......

Tim is right about 3D'ers in so called "high alpha" flight....they are not really flying at all but the power of the engine is simply defying gravity......

With regard to Angle of Attack & stalling then we need to consider what the AoA actually is....its the angle at which the airflow meets the wing....in your vertical climb scenario Major the airflow is hitting the wing pretty much head on so the AoA experienced by the wing is pretty small.....where it starts to get dodgy is in the landing phase......the model is flying nose high, slowly & decending too so where is the airflow actually coming from? Thats right.....from below the wing so the AoA in this case is extremely high & the risk of a stall is also correspondingly high.

This is why talk of a "stalling speed" is to dangerously inaccurate. Stalling is not caused by speed as anyone who has flown a scale aerobat will tell you.....you can be flying quite quickly but change the AoA very rapidly by adding lots of "up" elevator & the model will stall & probably flick on you....

Does all this help me when I'm landing......?? Sadly not......my aircraft still manage to fall out of the sky when they are 2 feet off the ground......

Hope that helps....I'm sure BEB will explain it better than I can....

[Major]

Tim and Steve


thank You for explaination. I was not far off with my theories about engine power, but good point about AoA in vertical climb. That of course helps a lot, but what about the wind in this case?

Next question:

How to tell the difference between prop stall/wing stall? How does it look like in practise, how (RC) pilot can recognize which one occured? Is it possible to get plane into both?


I know these questions might look just stupid, but have to ask.

Cheers

Tom

[Steve Hargreaves - Moderator]

Well an aeroplane doesn't know about "wind".....wind is simply air moving relative to the ground.....an aircraft flying is simply flying in air. Wind will only affect its speed relative to the ground.....the aircraft is not "aware" of the wind at all. Think about carrying a fish in a tank of water......it is simply swimming in a body of water...its not really aware that its being cariied along by you.

Wind is dangerous to us RC pilots though because we are stuck on the ground & the wind changes our perception of the model......imagine a model flying into a 20mph wind at 50mph......the air is flowing past the model at 50mph but the model appears to us on the ground to be doing 30mph......now lets turn it around & fly with the wind.....again the model is flying through the air at 50mph but the air it is flying in is also moving...at 20mph...so the model looks to us on the ground like it is moving at 70mph. This is why the so called "down wind" turn is the graveyard of so many models......it flashes past & we then try & make it turn back into wind at a fixed spot.....since the air is moving away from this spot we need to turn the model around very tightly in an attempt to make the turn over that precise spot....this means its airspeed can drop dangerously low & the aeroplane can simply drop out of the sky. Note, this may or may not be a stall......we can certainly stall a model in this situation by increasing the AoA beyond the critical figure but it is also possible for the airspeed to drop to a level where the aerofoil simply won't support the weight of the model. This isn't a stall as such (although many will claim that it is)...the wing simply isn't moving fast enough to create any lift.....

The correct approach in the "downwind turn" (I hate that term...it just reinforces bad airmanship) is to accept that the model is being blown along relative to the ground & start to turn a bit earlier & allow the model to drift away from you. It will still turn.....it doesn't know its being blown along.....but it will appear to turn slower

This is a very important point Major & I urge you to take a bit of time to get your head around it....it will make you a better pilot....

With regard to a wing stalling & a prop stalling.....a stalled wing is easy to spot....the aeroplane will simply drop until the AoA reduces to a lower level & the airflow can re-attach itself to the wing surface & it starts flying again......a prop stall is I would suggest, almost impossible to spot. Its certainly possible.....a prop blade is simply an aerofoil that is being rotated through the air so again if the AoA goes beyond the critical level it will stall.....what will happen & how you would spot it is beyond my knowledge I'm afraid.......we'll have to wait for BEB....

[Major]

Very informative Steve!!

I also hate "downwind turn". More then coupple of times I experienced it in local park, remember first time as very supprising as it was totally above all expectations. Other side is that I shouldn't really fly at those winds, but to me it is a great pleasure to fight a model few minutes and bring it back home in one piece.


Re prop stall

I did read BEB's article about props, remember some theory, but will have to return to it. The wing stall is simple to spot even to me, but I don't think I ever experienced prop stall. Tried few times with Spit and every single time model was just kicking forwards as soon as I opened throttle (as rapidly as I could). No any sign to me at all, but yes - let's wait for BEB, maybe I had to do something more

Once more thank You for Your effort Steve!

Tom

[John Cole]

When a wing stalls, it loses some but not all of its lift. You can think of the lift as coming from increased pressure below the wing and reduced pressure above it. Close to (but below) stalling speed most of the lift comes from the latter. At the stall, the lift from below the wing is slightly increased. That from above the wing is much reduced. So at and just beyond the stall, the wing does not generate enough lift to support the plane without increasing airspeed.

The Lift Coefficient just below stalling AoA may be about 1.0 (for e.g. NACA 0012) and drops at slightly higher AoA down to about 0.6, but it then rises to above 1.0 at AoA of 35 degrees. This data is for Re similar to ours, ignoring end effects.

So the stalled plane can continue to fly supported only by aerodynamic lift, but at a much higher AoA. The section drag coefficient at this high AoA is much increased. This is of course exactly the condition produced by a dethermaliser.

DB's statement that "at the critical point (i.e. at the stall), most of it (i.e. lift) disappears instantly." is I think slightlly misleading, but MANY authors claim ALL lift disappears at that point - nonsense. However you will need to look hard to find good wind-tunnel aerodynamic data on performance at AoA significantly beyond the stall.

 

Edited By John Cole on 24/01/2013 12:43:36

[Biggles' Elder Brother - Moderator]

Well, an interesting debate!

Dealing with the point about what the AoA actually is. There are a number of definitions that all amount to the same thing, but vary in detail. The most commonly accepted definition I would suggest is that it is:

"The angle between the direction of the on-coming airstream and the aerofoil's mean chord line"

The important bit there Tom is the "on-coming airstream" - often people get confused and think it is the angle between the chord line and the horizontal - this is not generally true. It is only true if the aeroplane is flying level. If, as Steve says, the aeroplane is flying in an upward direction then the angle of the on-coming air is effectively downward onto the aircraft and although the aircraft is "nose up" relative to the ground, it is not nose-up relative to the on-coming airstream and so the AoA under just circumstances might actually be quite small.

The problem comes when the aircraft is not travelling forward in the direction that its nose is pointing. Suppose its basically flying level but in a nose-up attitude, now the on-coming airstream is "level", (horizontal or parallel to the ground - how ever we wish to express it) but the nose is pointed away from this direction, upwards - so now the AoA will be large.

This happens when flying at slow speed, when we are trying to maintain lift with lower airspeeds, but it can also happen in tight turns were we are increasing the AoA, trying to generate more lift to use as a turning force.

One point Tom - you say " that stall can appear at any speed and any angle of attack". I think you are slightly confused here. It's true that I explain that a stall can occur at any speed (if the critical AoA is exceeded). But it not true that it can occur at any angle of attack. The wing will only truely stall if the crirical angle of attack is exceeded - below that it won't stall. But it will stall at any speed above when above that angle.

Final point, the questions about 3D. Well Tim and Steve are quite correct - this is not "flying" in the normal sense of the word. Take the extreme example, if the model is prop hanging. Clearly the wing is not contributing anything to keeping the model in the air in this case, it is the brute thrust of the engine which is doing this. The plane is behaving effectively as a helicopter!

In the case of a move like the harrier the picture is more complex. Yes the wing is (as far as our ideas of traditional flight are concerned) stalled. BUT,...the aircraft is flying basically due to four effects:

1. the power of the engine pulling upwards - as Tim and Steve say
2. the effect of the prop-wash over the inward third or so of the wings. The prop-wash is an "on-coming airstream" and acts just like the model was flying forward through the air. It is of course also at a very low AoA. This is I believe the main reason why 3D flyers favour relatvely high-pitch props - its not that they want to fly very fast (the usual reason for using higher pitch) but they want the prop to "shift a lot of air" to help provide low AoA lift in apparently high Alpha conditions.
3. As John correctly says not all of the lift is lost when the aircraft stalls. So this does contribute some lift - but I do think we can overplay this factor. For most aerofoils the lift drops off rapidly immediately after the stall. While there is evidence that for some aerfoils there is limited return of lift, there is as John says little data. The reason is that for wings this region is generally not of interest because the sharp loss experienced at the stall point by most aerofoils induces severe instability and so this is simply not a region we want our aircraft to enter. There is more interest lately in this post-stall region for aerofoil sections used on wind-turbines etc where the consequences of the sudden transistion at the stall are not so destabilising.

Regarding stalled props. Well not it is not easy to tell other than by measuring the propellor's efficiency, although I do think that on larger props you can hear the effect as a stalled prop tends to be noiser due to the air turbulance associated with the stall. But TBH with 99% of model props I think its very difficult to tell. The reason you can't detect it on your EP Spit is simply that the rotational inertia of your power system is very low compared to its output power - much lower than in bigger models or the full size. The consequence of this is that your motor powers the prop through any stall range in almost an instant!

BEB

[Major]

Amen!

The confusion about stall at any speed or angle of attack appeared after reading some other forums (mainly Polish ones) but now it is clear.

Thank You Dave! It is nice to have so good explaination from You (and others of course) and so good articles. Keep them coming please!

John,

thank You for pointing lift produced by wing at stall speed and aero tunnel data. Will kook for it!

[Tim Mackey]

Thanks BEB - mine was shorter...but still basically right LOL

And do I get points for my explanation of reflex substituting a tailplane on a delta / flying wing ?

 

Edited By Tim Mackey on 24/01/2013 13:57:14

[Myron Beaumont]

Tim

Where do I send a Yorkshire sticky bun to ? Do you remember a chuck glider called "Spook" from way back? It was a swept back flying wing with fins just inboard of the wing tips .The outboard bits beyond the fins were at quite a considerable negative angle of incidence thus giving stability .Quite an amazing glider for a wing in its time .

Nice to see your contributions back on the forum by the way

Myron YO13 Proper sticky bun dept.

[Concorde Speedbird]

Thanks Tim, could BEB describe to me in more detail how deltas work?

And what about vortex lift, the thing that Concorde used so successfully at low speeds? I know the wing was designed to create large, slow moving (I think) vortexes on the trailing edge so how did this transfer to lift? I'm assuming something to do with pressure. Am I completely wrong?

CS (budding aircraft engineer no1!)

[Biggles' Elder Brother - Moderator]

Wow! CSB that's a very big ask! Vortex lift is really advanced stuff.

OK here's an attempt at a non-mathematical description.

First of all, all wings generate a vortices (plural of vortex!) - its a natural consequence of how they work.

What I'm going to say now applies to "normal" wings - not deltas - but we need to understand this first. The vortex exists because the wing is not infinitely long. On a wing we normally think that the flow goes straight backwards over the wing - but in fact it doesn't, it travels over the wing at a slight angle. This happens because at the tip the higher pressure air beneath the wing leaks out and around the wing tip into the low pressure region above the wing. So can you see that this means there is a component of outward flow along the underside of the wing - and inward flow along the top surface. It looks a bit like the diagram I've drawn below:

 

The dotted arrows show the flow underneath the wing moving progressively more strongly outwards as we get nearer the tip. While the solid arrows shows the effect on the upper wing surface as the higher pressure air from below "pushes" in and so creates inward flow which is stronger the nearer to the tip you get. The effect is exagerated to make it clearer.

Now when all these arrows (flows) reach the trailing edge air from the top is going slightly to the left (as seen in the diagram) and air from the bottom is going slightly to the right and so the effect, when they mix, is to create a swirl in the air we call a vortex. These votices initially exist all along the wing trailing edge. On an ordinary wing all the vorteces gather towards the tip and are by far strongest at the very wing tip itself.

If you want to see the effect then sometimes the vortices are made visible by the aircraft flying through localised cloud as in the picture below

 

Or sometimes they can be seen in "con trails" as in this case:

 

Whether we can see them or not they are there all the time.

These vortices are a big problem to the aircraft designer - they represent a lot of drag. Ideally he'd like to lose them. But while the wing tip exists he can't. But suppose he could build a wing that didn't have a tip? Sounds daft? Well that's exactly what a delta is. If you think about it there is no wing tip, the leading edge just sweeps back and meets the traling edge! Neat eh?

Does this mean there are no votices? Ah, I wish. But the big vortices at the tips don't form the same way. This means the little vortices that are all along the wing are both smaller (because the sideways flow is smaller as there is little "leakage" at the tip) and, more importantly, they are "trapped" - they can't move to the tip to make the big vortex. In fact they sit along the swept back leading edge.

So they just sit there. Does this have any effect? You bet it does. The presence of the vortices effectively changes the the aerofoil profile. Now the air flow has to flow not only around the wing itself - but also around the trapped vortices stuck to the wing. This effectively incerases the camber the oncoming air experiences and so, hey presto, more lift!

I hope this is useful - its a difficult concept to explain but I think this catches the basic idea.

BEB

Edited By Biggles' Elder Brother - Moderator on 24/01/2013 22:43:12

[Concorde Speedbird]

Great BEB, that is very interesting. I found this picture of an Air France Concorde, is the condensation the vortexes in action (as I have heard many say it is)?

And this video shows it at 0:20 when the BA Concorde lands, the tire smoke spirals- vortexes?

I love all this stuff, hence the reason I want to be an aviation engineer! Brilliant!

CS

[Biggles' Elder Brother - Moderator]

Yes it is. The top picture is a particularly good example. You can see the way in which the vortices are now trapped and line up with the leading edge as I stated above (after a small addition!)

That's a good photo I haven't seen before - I might use that!

In the video - what is happening is the tyre smoke is moving up into the vortex region immediately behind the wing and making it visible. We often use smoke in the wind tunnel to make flows visible - here Concorde has done it for us.

BEB

Edited By Biggles' Elder Brother - Moderator on 24/01/2013 22:48:33

[Concorde Speedbird]

That is a good photo, this is another one I have found but not so good quality.

Clever blokes those engineers were! I love learning how it all really works!

CS

Edited By Biggles' Elder Brother - Moderator on 25/01/2013 11:16:42

[Biggles' Elder Brother - Moderator]

You'll make a good engineer one day - just keep at the Maths and Physics - that's the key to getting into university to study engineering. Your interest in RC planes will help as well - we are always pleased to see candidates that have hobby that shows a genuine interest in the subject.

BEB

[Concorde Speedbird]

I love Maths and Physics, and the predicted grades are A* for both for my GCSE's and that is what I am going on to do at A level too, so I think I am on the right lines. It is just brilliant how everything works in flight, great.

CS

[Tony K]

CS, this picture might help you visualise the flow.

[Concorde Speedbird]

That's great thanks!

In this video at the take off at 07:18, it is quite clear to see the votexes in action.

CS

[Gordon Tarling]

CS and BEB - don't forget that the Concorde has vortex generators on the side of the fuselage, just below the cockpit side windows - my understanding is that these were fitted in order to be able to reliably 'kick start' the vortices on the main wing.

[Biggles' Elder Brother - Moderator]

Ah, I didn't know that Gordon - well there you go, the vortices should form naturally of course, but I suppose they're not leaving anything to chance. Wise move!

BEB

[fly boy3]

Hi all, thanks BeB and others for very interesting reading put in a way most of us can understand. I always thought wintertime was for building, but this stuff is great.Thanks. One query,is a vortex speed related ?

Edited By fly boy3 on 26/01/2013 16:12:23

[Gordon Tarling]

BEB - I'm fairly certain that the vortex generators were added during the development of Conc, to alleviate some sort of problem at low speeds - I think the vortices tended to become detached, but don't quote me on it!