Aerodynamics

[SARBoy]

Hi all,

Loving this new forum. Great idea!

I fly the full size for a living and I love principals of flight(POF). Well more than all the other stuff I have to know which can be great bed time reading. I thought I'd just post and open up a discussion on the how's and whys of helicopter flight.

I'll be starting with the basic vector diagram. This forms the basis of most heli POF and a basic knowledge of this is esential.

Before I go on I'll stop and ask the question does anybody care and want to know? If not I'll stop right here but if there's any interest I'll go on.

Tom

[Sneeky_Pete]

I'm keen to learn!

[npomeroy]

Sounds good to me.
Excuse my ignorance but are you referring to POF as a particular book, or the subject in general?
I am interested in the "Bernoulli" vs "Newtonian" explanations for lift. (I'm in the Newtonian camp myself, but am not sure there is much argument about it these days (If there is , I'm sure it will surface here!)).

[extrapilot]

There is irony in the B vs N thing; neither had designs on explaining lift, and their principles do not fully encompass the mass/momentum/energy of a flow. Yea, Newton applies as conservation of momentum, and Bernoulli as conservation of energy. Neither are adequate to fully describe lift. For that you are looking at Navier-Stokes, etc, and even there, you have a slew of assumptions assigned. Long story short, I would not be too fixed in one camp or the other- both lead to the same result, and both are incomplete…

Its sort of a fun conundrum to ask Newtonian ‘air particle ricochet’ people how it is a wing can generate lift simply by flowing a fluid OVER the wing- no flow under it. It is not that Newton is wrong; it is that people are miss-applying it- not understanding that lift is a result of turning a flow (i.e. creating a downwards-flowing wake). Likewise with Bernoulli- the implication is that lift is a result of the difference in pressure above and below the wing, and that somehow- no vertical energy need be imparted to the flow to generate lift… How many Bernoulli diagrams show downflow trailing the foil?

[tribble]

I'm lurking here to see what I can learn as well... let's go!

Cheers,
Mike

[Fredo0709]

This is true! In brief, for 2-dimensional flows, circulation (captial gamma) and the Kutta-Joukowski theorem describes the lift per unit area. For 3-dimensional flows, you're talking about vorticity and how lift can be described by circulation per unit area through the Navier Stokes equations.

[npomeroy]

Originally Posted by extrapilot

There is irony in the B vs N thing; neither had designs on explaining lift, and their principles do not fully encompass the mass/momentum/energy of a flow. Yea, Newton applies as conservation of momentum, and Bernoulli as conservation of energy. Neither are adequate to fully describe lift. For that you are looking at Navier-Stokes, etc, and even there, you have a slew of assumptions assigned. Long story short, I would not be too fixed in one camp or the other- both lead to the same result, and both are incomplete…

Its sort of a fun conundrum to ask Newtonian ‘air particle ricochet’ people how it is a wing can generate lift simply by flowing a fluid OVER the wing- no flow under it. It is not that Newton is wrong; it is that people are miss-applying it- not understanding that lift is a result of turning a flow (i.e. creating a downwards-flowing wake). Likewise with Bernoulli- the implication is that lift is a result of the difference in pressure above and below the wing, and that somehow- no vertical energy need be imparted to the flow to generate lift… How many Bernoulli diagrams show downflow trailing the foil?

No argument with you there.
I didn't know anybody tried to explain lift merely by "top" flow: It has to result from top:bottom pressure difference. And in my view there is no such thing as suction - only the removal of a counterbalancing pressure. The problem I see is not with Bernoulli as such - it is the asymmetrical aerofoil argument. We in this hobby all know that while asymmetrical airfoils have more efficiency than symmetrical ones, symmetrical ones fly quite nicely thankyou. There are people for whom this is a surprise.

[SARBoy]

Ok,


I'll dip into the theory of lift, which will also provide a premise of a lot of what is to come and is I guess considered a given level of knowledge.

Right, lift can be described by several theories but it gets very complicated very fast if you move away from the stuff taught in school and to be honest beyond my expertise and understanding.

So you have an wing, which is passing through the air. Due to the shape of the wing the air passing below the foil maintains its speed and the air passing over the foil increases in speed due to the increased distance it has to travel. The air passing over the top drops in pressure due to its increased velocity creating a pressure differential between the lower and upper surfaces. This causes the wing to want to lift.

Additionally we can then increase the angle of attack (AoA) or Alpha. This is the angle of the mean chord to the direction of airflow. This causes the airflow passing under the wing to be deflected down and the airflow passing over the wing, which hugs (to coin a phrase) the upper surface to be deflected down from the trailing edge.

A stall is when alpha is too great and the airflow on the upper surface breaks away and we get turbulence and a loss of lift. A stall can happen at any airspeed its alpha that matters. The amount of lift an wing produces is due to many factors, profile, chord, and on, but the velocity of the air passing over it is by far the most important.

Ok we have lift. There is so much more but it’s not really necessary to go further understand heli PoF.

Right lets take our rotor blade. Like a wing we generate lift by passing an airflow over it, we just spin it around a fixed point to do this rather than attach it to an engine and push it trough the air with the rest of the plane.

However our airplane wing has a constant airspeed over it and along its length, where as our rotor blade has a far higher airspeed at the tip compared to the root. If we did nothing this would cause the tip to create more lift than the root. Ideally we would like a uniform amount of lift along the blade so we do a couple of things. The root has an increased alpha compared to the tip and the aero foil section is designed for slower airspeed. There are other methods like reducing the blade area toward the tip hence tapering blades and sweeping the tips like a fast jet wing. This is an area that RC departs a little from the full size as most of us do aerobatics with our helis and require it to perform just as well upside down as the right way up. This leads to a symmetrical section. Also on the full size we are concerned with the integrity of the blade and its better to have uniform lift to achieve this.

Ok it’s late and my head is getting a little fuzzy. I’ll get on to a vector diagram another day and we can move on from there. Please chip in on lift if you know more, otherwise please keep it to your self, as the above is all you need to understand to move on. By the way I should just say that most heli PoF is a lie we tell ourselves so we’ll get in the aircraft and fly. Anyone with a physics degree will smile and say “gosh isn’t that a cute way of describing it! Wrong, but nice.”

Tom

P.S. a thread with no pics is pointless so here is what happens when you get lift wrong.

[npomeroy]

Originally Posted by SARBoy

... Due to the shape of the wing the air passing below the foil maintains its speed and the air passing over the foil increases in speed due to the increased distance it has to travel. ... .

Sorry but this is one of the most common misconceptions.
More later
No time now ...

[extrapilot]

Originally Posted by npomeroy

No argument with you there.
I didn't know anybody tried to explain lift merely by "top" flow: It has to result from top:bottom pressure difference. And in my view there is no such thing as suction - only the removal of a counterbalancing pressure. The problem I see is not with Bernoulli as such - it is the asymmetrical aerofoil argument. We in this hobby all know that while asymmetrical airfoils have more efficiency than symmetrical ones, symmetrical ones fly quite nicely thankyou. There are people for whom this is a surprise.

Firstly, you cannot make a blanket statement like ‘we all know asymmetrical airfoils have more efficiency than symmetrical ones.’ We don’t all know that- because it is not necessarily true. You have to define the airfoil and the flow regime.

Secondly, I don’t see why you or anyone is under the impression that a symmetrical foil violates Bernoulli. How? If the foil is at 0 AOA, it will not produce lift. That’s fine- but there is an AOA for ALL airfoils where they produce no net lift. Once you add AOA to a symmetrical foil, you have asymmetry.

Conversely, look at Newtonian ‘air particle ricochet’ theory. If you present a scenario where the wing has a flat bottom, and the AOA is such that the streamlines are parallel with the bottom of the wing- the wing should not produce upwards lift- it seems it should be forced downwards if you are a believer in this theory. Attached are a few CFD plots- flat-bottom wing, with the bottom parallel to the airflow streamlines. Surface pressure is lower with cooler colors, streamline velocity is higher with hotter colors. This wing is producing substantial +Z force (20” span at 50kts at STP generating 6Lbf +Z). In the head-on view, you can see the resulting downwash- which is F=MA in action.

This doesn’t disprove the Newtonian approach- it simply illustrates that there is a misapplication of the principles- something that is sadly very common in this niche.

[npomeroy]

Originally Posted by extrapilot

Firstly, you cannot make a blanket statement like ‘we all know asymmetrical airfoils have more efficiency than symmetrical ones.’ We don’t all know that- because it is not necessarily true. You have to define the airfoil and the flow regime. .

Excuse my science teacher shorthand. I thought you would recognise that I meant at matching and realistic AOAs and airspeeds, and the commonly used airfoils used on our hobby.

Originally Posted by extrapilot

Secondly, I don’t see why you or anyone is under the impression that a symmetrical foil violates Bernoulli. How? If the foil is at 0 AOA, it will not produce lift. That’s fine- but there is an AOA for ALL airfoils where they produce no net lift. Once you add AOA to a symmetrical foil, you have asymmetry. .

Again, I don't think I said that "a symmetrical foil violates Bernoulli". I meant it violates the common assumption that the asymmetry is the CAUSE of the lift BECAUSE of the dofference in path length.

Originally Posted by extrapilot

Conversely, look at Newtonian ‘air particle ricochet’ theory. If you present a scenario where the wing has a flat bottom, and the AOA is such that the streamlines are parallel with the bottom of the wing- the wing should not produce upwards lift- it seems it should be forced downwards if you are a believer in this theory. Attached are a few CFD plots- flat-bottom wing, with the bottom parallel to the airflow streamlines. Surface pressure is lower with cooler colors, streamline velocity is higher with hotter colors. This wing is producing substantial +Z force (20” span at 50kts at STP generating 6Lbf +Z). In the head-on view, you can see the resulting downwash- which is F=MA in action.

Originally Posted by extrapilot

This doesn’t disprove the Newtonian approach- it simply illustrates that there is a misapplication of the principles- something that is sadly very common in this niche.

I don't know about the "Newtonian ‘air particle ricochet’ theory". I am merely acknowledging the the combination of F=ma plus action = reaction. Yiu can actually do some quite nice calculations (yes, with some assumptions) where you treat a wing as an inclined plane and thus calculate from its cord, length and airspeed, the mass and acceleration of the air it must deflect downwards. The fuzziest assumption is the depth of air mass deflected (I think I took about 2 x cord distance). This simple f = ma calculation yields results that are similar to the flight performance of say a Piper Cub.

Hey, I realise there are simplifications being made, and I know that these will make some explanations "wrong". But some things are more wrong than others, and there are a lot of very wrong explanations put out at school level. And I'm afraid the OP made one of them in referring to the path length causing the difference in air flow velocity.

[koppterx]

i warned you guys about this.............

[npomeroy]

Originally Posted by koppterx

i warned you guys about this.............

About what?
OK there's some different approaches, and opinions, and access to facts, but it's not too bad compared to some the rubbish that goes back in forth in the other forums.

[desertstalker]

Originally Posted by koppterx

i warned you guys about this.............

Nothing wrong with an argument, as long as it stays about the facts/theories...

[SARBoy]

Hey guys. Yup, nout wrong with good debate. As I said if you know more please add. My only concern is that it's factual and adding to people's understanding with out confusing them.

I know some of my explanations are wrong for certain amounts of wrong but it is not necessary to go into more depth to grasp the next concept.

Should have it up by tonight or tomorrow.

Tom

[SARBoy]

Right I have 30 mins before work.

The basis for the Vector diagram is the 4 main eliments of flight.

Thrust
Drag
Lift
Weight

These all act in the X and Y plane. Thrust overcomes/balances Drag in the X plane and Lift overcomes/balances Weight in the Y plane.

Additionally we have a force called Induced Flow(IF) which you do not get on an airplane wing. This is created by the rotor blades rotating around the disc. As each successive blade passes the same point(any piont around the disc), it pulls the air down a little bit and as the next blade pases it does the same. This builds into a flow and can be changed by the disc accending, decending, the AoA and forward airflow over the disc. we'll see how they affect IF later.

As always a pic to help.

Tom

[Side]

Does anybody have a E-Mail-Adress of the user called desertstalker?????

Please send me am pm.

Thank you.

Alex

[SARBoy]

Hi all,

Welcome to the vector diagram. See PDF.

I have tried to paste the diagrams into this post but I’m buggered if I can do it. So I have saved it as a PDF and included it with this post. Not quite what I wanted and if there is a computer whiz who knows how to post directly I have the original Word doc available.

Alright, a pic of hovering and IF, well kind of.

Tom

[npomeroy]

Now here's what I have found difficult to find an understandable and credible cause-effect explanation for:

What causes the air above the airfoil to accelerate downwards?

The nearest I have read refers to the "Coanda effect" whereby the air flow "sticks" to the wing. Clearly if the air did not "come down" there would be a void or vacuum created. This would naturally be filled by the expansion of the air above, but we are told that the air behaves as an incompressible fluid.

[ame]http://www.youtube.com/watch?v=UqBmdZ-BNig[/ame]
[ame]http://www.youtube.com/watch?v=6UlsArvbTeo[/ame]

[Elios000]

my understanding was that Bernoulli is what keeps flow laminar till high alpha
and that this makes the down wash
then the down wash + the AoA makes lift based on the Newtonian model

its not so much one or the other as both working in concert that makes the system work
but a lot of aircraft get a good bit of lift from body shape now as much as from the wings

also i have never head of treating air as compressible this would cause all kinds of issues as you model things getting in the trans and super sonic regimes