helicopter silly questions

[baobab72]

hello folks
here i am again with a couple of not so clever questions, even though there is not such a thing as a stupid question in the aviation world!

just bear in mind that even though i have experience on fixed wing airplanes, i do not have any experience on helicopters, just a fascination with them.

Why is the downwash produced considering that the high pressure is on the lower side of the rotor disk and the lower pressure on the upper portion?

In a counterclockwise main rotor system, the sideway thrust generated by the tail rotor is oriented to the right swinging the helicopter to the left about its pivoting point to counteract the torque effect that makes the fuselage to spin clockwise?

Why do you have a yellow arc and a red line on the MAP gauge? is that due to prevent overboosting or does it have any aerodynamic implication?

what is the difference between the freewheel unit and the sprang clutch and is there a sprang clutch in the r22?

are the blades on the r22 and 44 symmetrycal or asymmetrycal?

Many thanks and more silly questions to come!!

Baobab72

[Rotorwashed]

I don't think any of those questions are silly at all!

The downwash of a helicopter is produced by the downward deflection of air from an airfoil that is creating lift. An airplane creates downwash too, I'm sure you've heard of wake turbulence. The high pressure air beneath the disk does want to move to the low pressure area on top, this creates (in airplane lingo) wingtip vortices. Of course in a helicopter the "wingtips" are moving around in a circle, so its more like a donut of vortices around the tips.

You're right on the money with the tail rotor statement.

The yellow arc and red line on the MAP gauge is just a warning line for the pilot. The amount of power we can use changes with the density of the air, so we have to calculate our max power so we dont pull too much power and put undue stress on the engine. I believe the yellow line starts at 21", and is the lowest point which your power could be limited to.

The free-wheel unit and sprag clutch are different words for the same thing. sprag clutch is just a little more specific. It allows the rotor system to turn freely in case of an engine failure. There is a sprag clutch on the R22, and I'm not sure if its called the same thing on other helicopters.

The blades are symmetrical because it makes them more stable. Their center of pressure remains the same regardless of their angle of attack.

Hope this helps!

[Aerobot]

I'd just add one thing: the yellow arc on the MAP instrument shows how hard the engine is working to maintain rotor RPM. By implication, then, it shows how much rotor-blade drag is being overcome by the engine: the higher into the yellow you are, the more pitch is on the rotor blades. So, the farther you are pulling into the yellow, the faster your rotor RPM will decay after an engine failure.

When you're up in the high yellow, get really spring-loaded on that collective!

[baobab72]

many thansk for your answers although i am not sure i have clearly understood the relationship between MAP, RPM and collective with particular reference to overtorque.

Is then a true statement that basically when you ass throttle as you increase the collective, you are compansating for the extra induced drag?

And during an autorotation, the reason why you lower the collective, is it to minimize the induced drag and thus the decaying of the rrpms? but why do you have to flare a well?
And how is the rotor spun up by the upwash?

Many thanks

Baobab72

[Rotorwashed]

As you increase collective, you increase angle of attack, and therefore induced drag. So yes, it's safe to say that as you increase collective, you must increase throttle to compensate for the extra induced drag.

In an airplane, as you bring the nose up, you are increasing the amount of lift being produced and start climbing, but as a result you create more drag and lose airspeed. If you want to increase your lift and maintain a constant speed at the same time, you must increase power. Its the same thing in a helicopter, the blades must be at a constant speed, so when you increase angle of attack you must increase power.

Autorotations are a little more complicated. Autorotations work because the air that is rushing vertically through the rotor system creates a very steep angle of relative wind. Lift acts perpendicular to relative wind, so if you imagine the relative wind being completely vertical, the lift is acting on the rotor blades in the same direction as their rotation. This keeps them spinning.

So in the beginning of an autorotation, we lower collective so that we can initiate a descent. The vertical relative wind then begins turning the rotors. Then we have to use the collective to control the amount of lift (and therefore drag) that is being produced. Too little lift, and the rpms will increase and overspeed, too much lift and the rpms will decay and stall.

At the end, we flare because it further increases the angle of attack of the rotor blades. We trade our forward airspeed for this increased lift to arrest our descent.

[Heli-Ice]

Here is a good book about this all:

FAA Rotorcraft Flying Handbook (http://www.faa.gov/library/manuals/aircraft/media/faa-h-8083-21.pdf)

Enjoy

[Shawn Coyle]

Heli-Ice:
I'm pouting that you didn't consider Cyclic and Collective
The author.

[Heli-Ice]

Shawn

I apologise, I have not read that book as of yet and would have pointed it out if I had done so. The RFH was just the one that jumped to mind and it is accessible on the interweb.

Since the word is out now, I feel a lot better and our colleague can have a look at your book and increase his knowledge. :ok:

Regards,

Karl.

[jetdriverbr]

Here is the better book It was composed long after the dinosaurs were extinct as opposed to the RFH. :ok:
(http://www.amazon.com/Cyclic-Collective-Shawn-Coyle/dp/0557090660)

[Rotorwashed]

Agreed, Cyclic & Collective is a much better book in every way. you'll outgrow the RFH very quickly. It seems like I find something new every time I open C&C, which is often.

[alouette3]

There, Shawn, you can quit pouting now!!:)

Alt3.

[muffin]

Agreed. Cyclic and Collective is the best of the lot.

[VeeAny]

Cyclic and Collective is the best one around IMHO.

Some of the others are OK, some are dire.

[Major Neb]

I found The Art and Science of Flying Helicopters by Shawn Coyle in my local library in Perth Australia, of all places! Awesome first read for me. Thoroughly recommend any by Shawn Coyle based on his ability to write readily understandable text.

Cyclic and Collective is on my Christmas List!