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sabato 30 gennaio 2010

"From the cycle: Mind your own business, Building an ultralight is easy, undestanding what are you doing is not, nor finding the money"

Little info for the amateurs, to be taken with "the gloves";
that is, those are hints that "apply" only in the exact given situation, which is not yours, so take them, recalculate them, test them in your plans, recalculate, and only than try a flight when shure about having eliminated any surprise.

Beware, you can never really eliminate any surprise!

Do not attempt tu build a helicopter that climbs by incremeting the rotations of the main rotor (more throthle, rpm, you call it how you want, same thing!) G1 is an example, and others!

Why? Giroscopic effect!

That is: High RPM big diameter rotating things (including your car's weels) have INERTIA. (go study what the heck taht is)
When RPM (that would be rotations per minute) reaches a stable speed, the inertia forces grouped in the disc (any rotating thing is a "disc") and the centrifugal forces reach an ecquilibrium of balances.
This ecquilibrium (balance) of forces leads to the Gyroscopic effect.

Example:
Take your bycilcle, put it weels up, take off from the front fork the front weel, keep it in your both hands by the central axis, away from your shirt or other objects, ask your friend to spin the weel by hand, as fast as he can.
Now your weel has a stable RPM speed, and you just try to tilt the axis left or right, so that you obtain different vertical and horisontal tilting of the rotating disk.
Surprise surpise, the disk will OPPOSE strongly to any change of position, and once reached another position will oppose to returning back!

Gyroscopic effect at some 40 to 80 RPM in your hands.

How much stronger is the gyroscopic effect at an 8 meters (yards) diameter rotating disk spinning 400 to 600 RPM? Don't try to do the math in your brain right now, you'll overheat your processor and smoke will come out of your head.

This is balancing your helicopter a lot, and keeps you alive, is a rotary wing!

Now, variations on the RPM speed brackes this gyroscopic effect, sending to hell all the balance, all the equilibriums goes down the drain, and you are at high risk.

Wether you climb or descend, breacking the Gyroscopic effect is dangerous.

Better having a stable RPM speed to the rotor, and variating the ATTACK ANGLE of any single blade of the disk to obtain lift variations, thus Gyroscopic effect remains untouched once reached.

Gyroscopic effect has influence on the stability of the propeller airplanes too, so beware!

Power per weight ratio!
Most ultralight helicopters and planes are slightly UNDERPOWERED.

So how do they fly?

This is the problem; they FLY,
in near perfect wether conditions, but once out of a perfect environment you are at high risk, so many peole crush!

The ratio that should be considered is at least ONE HP per every 5 Kg of take off weight.

As a matter of fact many fly with a ratio of 10 Kg per one HP, really dangerous. The slightes wind raffic coming from behind or above or lateral is putting at high challenge your piloting qualities!

Hot days in the mountiens would render your take off impossible, and so on and so forth.

Better if overpowerd, like 3 Kg per 1 Hp, to eliminate risk.

Example:
An European ultralight amphibious that has a MTOW of some 500 Kg, ecquipped with 100HP engine, every once in a while might not be able to take off in hot summer days on a high altitude airfield, sometimes will crush at landing (or if lucky have a "heavy" landing) or crush in take of for lack of engine power.

Some of those have even less HP.

Of course here enters in the dances the wing configuration, wing surface, wing stress, etc.

Same for your helicopter, better overpowerd than underpowerd, better having collective and cyclic rudders than just throttle, always better if RPM is mantained at a permanent prescribed speed by a GOVERNOR (like a cruise control for cars)

Enough for the day!

Enjoy digesting all the rubbish I wrote, than go to the books to comprehend those numbers in the light of the above!

Have fun!

Emil.

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