Aerodynamics?

[Varc]

As helicopters beat the air into submission, and their control surfaces are not like a fixed wing airplane, AND, with all the funky-ugly big and heavy helicopters that are out there, just how important are aerodynamics on a helicopter? It seems to me, they're just to make it look cool. But, do "aerodynamics" really play a roll in helicopters? I mean, look at the Apache, the Hind, to name a few birds.

[SurfSloper]

Helicopters do not beat the air into submission; I would say it is the other way around. If not, then why am I always replacing bearings and dampers?

Their control surfaces see the same viscous fluid as their fixed-wing counterparts.

I wouldn't say the hind or apache are aerodynamic atrocities, considering their role. The aerodynamics team often takes a backseat to other performance requirements in military aircraft.... and are left to clean up the mess. I'm sure the apache's structure is ridiculously lightweight for how tough and strong it is, and is therefore, efficient.

Clean aerodynamics is almost always paid for in structural weight, so it must be justified.

It should be noted that not all sleek-shapes are low drag, either. Fairings, canopies and fillets add surface area, and if done improperly, can add to the total drag, rather than reduce it. On the flip side, the effective drag reduction in some awkward, boxy designs could surprise you.

Aerodynamics aren't exactly as intuitive as one might think.

For a 3D model helicopter, i would say no. The pod and boom design is a most effective layout for lightweight, high rigidity and most importantly, modularity. Which direction would you streamline for, anyway?

For speed helicopters, it is obviously paramount. Similar to sailplanes (my area of expertise), I'm sure some hotshot aerodynamicist will eventually come along and blow away the competition with some seemingly trivial change in concept. Look how Mark Drela's airfoils revolutionized rc sailplane performance.... ever see a top-of-the-line discus launch glider get thrown nearly 300' straight up? Or the new breed of dynamic soaring machines do their thing?

I notice that you (varc) fly mostly small/micro helis. At this size and speed, air behaves like molasses and aerodynamics get really tricky (read unpredictable). Any unnecessary surface area will be a substantial source of drag and reduce performance. Weight is a far more controllable factor to boost performance. The benefit of a canopy is trivial and probably only serves for protection and visual orientation.... at these sizes.

It is all a compromise

[Varc]

Thanks for that awesome reply! And yes, I do tend to favor the micro's. But I do have a couple of larger birds that I tend to warm up to. They just terrify me at the moment with my current skill-level. I even have a Kyosho 700 that hasn't seen the skies since I first built it years ago.
As far as speed-helicopters go, I've really never understood those as all I see around me are 3D stuff where it's all tossing the helicopter every which way but loose. I don't see where aerodynamics would matter in such behavior's.
Do they actually RACE RC helicopters? I don't mean straight line drag-racing, but like circuits? This is where I can see aerodynamics playing a roll.
Also, look at the new Sichorski "spell?" co-axial helicopter with the pusher tail. That is one slick machine and it's ben said to be the fastest forward-flying helicopter yet.

[npomeroy]

Hey Varc: What are meaning by aerodynamics?
To most of us it means the physics of how a machine flies, i.e. rotor design etc etc. Are you just meaning streamlined body shape?

[SurfSloper]

Originally Posted by npomeroy

Hey Varc: What are meaning by aerodynamics?
To most of us it means the physics of how a machine flies, i.e. rotor design etc etc. Are you just meaning streamlined body shape?

Good point. I meant to bring that up. I just assumed he was referring specifically to aerodynamic streamlining..... otherwise it is quite a silly question. Are the dynamic interactions between the air and my heavier-than-air flying machine important? Uh... yeah, a little...LOL .

[Varc]

Just the basic shape of the fuselage. I see aerodynamics being more important in fast forward flight, like a fixed wing airplane, but most helicopters seem to be designed for heavy armor/weaponry or heavy duty like lifting ton of weight.
There are some really ugly helicopters out there that seem to defy aerodynamics.

[SurfSloper]

Thats correct.

except I would say..."There are some really ugly helicopters out there that seem to defy economics" ... as the cost of poor performance is power and fuel.

One of the first equations you learn is for dynamic pressure, or q.

q = 1/2pV^2
where
p = air density
V = velocity

q rises exponentially as V increases linearly.
The lift and drag of an object is directly related the the pressure of the air "blowing" against it; the dynamic pressure.

From this you can easily understand why ultralights and Learjets look so different.

This begs the question: What part of the helicopter should receive the most scrutiny? The fuselage traveling 40mph or the blades traveling 400mph?

[Varc]

Good point there as blades are universal in design, but the fuselage is dependent on the roll of the helicopter.
Speaking of blades, are there real helicopters with bullet-blades? Or are those just on our models?

[SurfSloper]

I believe the bullet blades are an attempt at shifting the CG of the blade, which is impossible otherwise while being made of a uniform material. Full-scale airplanes employ a similar concept while trying to shift the CG of their control surfaces to prevent flutter.

http://en.m.wikipedia.org/wiki/Ailer...alance_weights

The blades are not universal in design, by a longshot. It is my suspicion that they may be the most under-developed part in our models. I've never heard one mention of airfoils, Cl/Cd, aspect-ratios, solidity, etc from the manufacturers. I suspect they are designed through pilot feedback, which is subjective, not to mention a slow, inaccurate process. All I hear from pro heli pilots is adjectives like "lively" and "crisp." This reminds me of a surfer describing how a new surfboard performed; it is useless information to the designer.

I want some numbers to crunch.

Even most intermediate sailplane pilots can describe the performance caracteristics of the airfoil they're using, their aspect-ratio and wing-loading.

There, is that enough personal opinion to drag someone else into a lively debate with us?

[extrapilot]

For one, Im not aware of any blade manufacturer who employs in-house a real aerodynamicist, or a real (degreed) CFD engineer, or owns a tunnel to calibrate reference models against. For two, Im not aware of any real data produced for any commercially-available blade design. And there is NO reason to not publish said data- I can test it myself and publish it.

But IMO, there are two base issues here:
First is a borrow from Carl Sagan: “It is suicidal to create a society dependent on science and technology in which hardly anyone [aside from a small, numerate elite] knows anything about science and technology.” Most people have absolutely no understanding of how these machines work. Worse, many of them, as we have seen on this forum, have an incorrect understanding- and they are so entrenched by ego that they refuse to learn.

Second is the very real paradox regarding blade design. Example, you cannot design a blade that is efficient at high speed and also in hover, because the physical requirements on chord/thickness are different. Likewise, we have to accept some downside as regards Cd to permit access to very high levels of thrust at low blade mass. And then there are all kinds of issues with blade pitching moments, chordwise CG, spanwise CG, aeroelasticity, etc, because you have different head geometries that will fly the blades differently based on lots of factors in the blade/head interaction (does it permit teeter? How much before spindle lock? What is the implication on dynamic stall? Does it have Delta3 or Alpha1? If DFC, how much lag is seen in the swash drive under certain cyclic/collective conditions? Etc).

I think the thing that bothers me is not that people don’t care to invest lots of hours learning about aero- but that some vendors are flat out lying to them about why their blades are ‘better,’ knowing that one in ten thousand may see the nonsense in it, and even fewer will go to the trouble of calling them out. I mean, a newer offering was talking about tip vortex being shed 45deg away from the tip-path-plane, so as to reduce BVI. Id love to see wind tunnel LDV on that. And- some nonsense about ‘proper’ lead/lag angle, when that changes constantly in flight.

Someone asked in this forum why blades cant just be classified like shoes- Size 12E, or whatever, so you could switch between brands and have the same characteristics. And it seems a reasonable question, until you look at the parameters that are involved. Even if that were plausible, most blades are wet hand-layup with inconsistent fiber orientation. Heck, in the countries where most blades are made, even the manufacturers cant be confident that the suppliers’ properties descriptions for raw materials are correct; when sources are willing to put melamine in baby food to save $.05 a pound, does anyone really think they care about diluting a hardener etc? Do the end-users (blade manufacturers) even have any mechanism to validate materials properties?

[Varc]

Originally Posted by SurfSloper

I believe the bullet blades are an attempt at shifting the CG of the blade, which is impossible otherwise while being made of a uniform material. Full-scale airplanes employ a similar concept while trying to shift the CG of their control surfaces to prevent flutter.

http://en.m.wikipedia.org/wiki/Ailer...alance_weights

The blades are not universal in design, by a longshot. It is my suspicion that they may be the most under-developed part in our models. I've never heard one mention of airfoils, Cl/Cd, aspect-ratios, solidity, etc from the manufacturers. I suspect they are designed through pilot feedback, which is subjective, not to mention a slow, inaccurate process. All I hear from pro heli pilots is adjectives like "lively" and "crisp." This reminds me of a surfer describing how a new surfboard performed; it is useless information to the designer.

I want some numbers to crunch.

Even most intermediate sailplane pilots can describe the performance caracteristics of the airfoil they're using, their aspect-ratio and wing-loading.

There, is that enough personal opinion to drag someone else into a lively debate with us?

Uh, what?

[deebee]

Ok so i'm not to proud to admit I don't know but a thimble to the ocean about aero, It is most certainly an interesting subject. I am able to follow the thread up until here:

(Does it have Delta3 or Alpha1? If DFC, how much lag is seen in the swash drive under certain cyclic/collective conditions? Etc).

What does Delta3 / Alpha1 represent?
How Does DFC differ from say standard FBL in the amount of lag present in the swash drive? are you speaking of lag in terms or gyroscopic procession??

The paragraph following the one in question is foreign to me as well. Most of us aren't engineers (Why didn't I finish that degree oh so many years ago) , please break it down so we can understand .

I will agree that the average consumer tends to be quite ignorant and proud of it.
Johnny learning to hover might spend $100 a pair on blades b/c Bert 3D wiz and the pretty girl beside him gives them a thumbs up and brag about how savvy he is.
But wouldn't even give a second glance at a $30 set of blades designed with 50,000 hours of R&D geared toward his specific machine and intended use.

[Varc]

Originally Posted by deebee

Ok so i'm not to proud to admit I don't know but a thimble to the ocean about aero, It is most certainly an interesting subject. I am able to follow the thread up until here:


What does Delta3 / Alpha1 represent?
How Does DFC differ from say standard FBL in the amount of lag present in the swash drive? are you speaking of lag in terms or gyroscopic procession??

The paragraph following the one in question is foreign to me as well. Most of us aren't engineers (Why didn't I finish that degree oh so many years ago) , please break it down so we can understand .

I will agree that the average consumer tends to be quite ignorant and proud of it.
Johnny learning to hover might spend $100 a pair on blades b/c Bert 3D wiz and the pretty girl beside him gives them a thumbs up and brag about how savvy he is.
But wouldn't even give a second glance at a $30 set of blades designed with 50,000 hours of R&D geared toward his specific machine and intended use.

I know of a particular lady who would be more impressed with me spending $30, then $100 on ANYTHING. LOL!

As helicopters seem to defy physics in general, it still amazes me to see these fly. Especially the bigger, heavier ones designed for lifting gargatuant ton's of equipment.

[James Recard]

Is the biggest restriction on blade design the fact that we expect the blades to perform equally inverted?.. I would imagine any advantage gained on the topside, we would pay equal by having to apply it to the underside...

[SurfSloper]

EP- Right on; Eloquently put, as always. As far as blade design goes, I was not envisioning the in-depth optimisation you described. I suspect, and tell me if I'm wrong, that in designing blades, these manufacturers simply employ one of a hand full of appropriate, but generic, NACA foils. I was thinking they could simply refine their airfoils a bit for the more specific conditions they see on model helicopter blades... But then I began to wonder how you would go about doing this. Again, my main area of expertise as far as aerodynamics, is model sailplanes. Optimising an elliptical wing with no sweep is one thing, but a swept flying wing, with all that spanwise flow, is something entirely different. A rotary wing.... well, I don't even know where to begin. I was over-simplifying the issue.... I was wrong. And yes, these theoretical blades would probably have to run about $3000 to pay off all the lab time!

Varc- I hope I didn't throw you for a loop with the surfer reference. I think faster than I type and that was WAAAY OT, but I should explain a little... In my teens and early 20's, I built fairly advanced, composite surfboards for a living, trying to improve them with with a scientific approach. I was fueled on by all the ridiculous and blasphemous explanations I heard from pro surfers and top manufacturers about the, uh, well, "hydrodynamics" of their surfboards.... some of which was truly laughables. I had to fight for years before people would even accept epoxy over polyester resin.

Again, EP, thanks for the great insight.

[Varc]

Hydrodyanmics behave like aerodynamics don't they? I mean, don't they use fluid's in a tub to test aerodynamics?
Anyway, I'm still thinking more towards the fuselage than the blades.

[TTMR]

So drag reduction on the fuselage is what you are after.

Boils down to cost benefit.

There are two types of drag out there, parasite and induced. Induced is as a result of creating lift and heli fuselages usually aren't concerned with that so we can disregard that.

Parasite drag is affected by the airspeed, less speed means less drag (duh right) but what it means is that on slow things the shape doesn't matter much and let's face it MOST helis aren't rocket ships. Add in that most full scale helis are speed limited by their rotors and you have a recipe for not needing to bother with uber slick fuselages.