How come a toy helo can fly inverted and a real one can't?

[collectivefriction]

Above question from a buddy of mine and my knowledge is too old/isn't enough to give him the technical answer he would like.

[leemind]

upside down hovering

because a model helo can get enough negative pitch on the rotor baldes to do so, where as a real one can't (aside from all the other secondary requirements of the fuel/oil/hydraulic systems to do so)

There was a thread a while back about how some Navy helos could put on a bit of negative pitch to help the stick to the deck of a rolling ship, but someone else can confirm/deny that...

[puntosaurus]

Who says ? See here. I think the reason no-one sets them up for sustained inverted flight is (a) because there's no point, and (b) allowing negative collective pitch would give you one too many things to think about in autorotation.

[R22DRIVER]

Hi Guys,

As a model heli pilot i am able to set up my heli with 9 degrees of pitch and both the positive and negative planes.

This allows me to sustain inverted flight for as long as i want. The power to weight ratio of the model helis is fantastic and there is no way i would like to be sat in a full size heli doing the things i can do with my model!



R22

[NickLappos]

A real helo is designed to do what real helos do. The cost of adding the control travel, negative g oil and fuel systems and puke bags to a real helo is too high when you consider that the maneuver won't help make a dollar more.

In other words, what helo operator wants to lose 2 passengers so it can fly upside down? Might be a private aerobatic helo, I guess, but development costs are quite high, so there must be hundreds of sales to support the capability.

Otherwise, it can be done, and the small RC models prove it!

[diginagain]

There was a thread a while back about how some Navy helos could put on a bit of negative pitch to help the stick to the deck of a rolling ship, but someone else can confirm/deny that...

Lynx, all models, 'sub-min pitch', collective is pushed below MPOG detent. (Lynx uses 212 bar of hydraulics to bend the rigid titanium rotor hub arms). Useful tool for pinning the a/c down on a moving flight deck, useful tool on AH for doing power matching checks without launching skywards.

Not sure if it still applies today, but when we first got the Mk 9, this activity was not recommended, as the fear was that the tyres would come off the rims.


(All from distant memory, so apologies if slightly off).

[Simon853]

I guess another advantage a model has over a full-size helicopter is the medium in which it operates. i.e. Even if you could scale a model up to full size, you can't scale air molecules accordingly.

Plus, the combustion engine in a model heli is optimised for the fuel it burns, burn rate across the surface of the piston, etc. and that kind of stuff just doesn't scale. Even if it did, what good is a full size helicopter that can only fly for 15 minutes on a fll tank?

Si

[NickLappos]

Simon,

Believe it or not, the biggest scaling problem when going from little to big is not the air (which scales pretty well when you are working subsonically), it is the structural materials. As a beam gets longer, the stresses go up by the cube of the length, but the strength goes up by a square, so rapidly the object, rotor blade, fuselage, gets weaker and weaker. That's why you can pick up a 1 yard long stick, but if you scale its dimensions to 20 feet, it will not support its weight. Godzilla can pick up those train cars with impunity because they are 1 foot long. If they were 60 feet long, their own weight would crush them.

For that reason, scale planes can pull eye-watering G and make box turns that real airplanes cannot do.

[Capn Notarious]

Just for the record. Have Nick, Shawn or any other regulars; ever flown or done a tandem with a radio controlled model?

[puntosaurus]

The HCGB did a trip to Aerial Training at Draycott Farm. Huge fun, and a lot harder than the real thing IMHO. It's also a lot cheaper to get it out of your system here than building and crashing the damn things yourself. One of the instructors did a breathtaking demo of competition aerobatics - these boys practice for hours every day to get this good. V. impressive.

They also do this Gnat shooting, where you blast model a/c out of the sky with shotguns. Much more my style !

[IFMU]

Don't forget twist...

The R/C helicopter I own & have flown had untwisted blades. This made them equally good right side up or upside down. But, if you put enough pitch range & inverted systems in your current full size mount to fly inverted, the twist would cause most of the lift to be at the tips of the rotor, and you would get huge bending moments. This is bad. And, don't forget that some of the helicopters we fly have assymetric airfoils optomized for performance right side up.

I haven't been active in R/C in almost 10 years, so if they have twisted assymetric blades now then I'm just out of the loop.

-- IFMU

[cyclicmicky]

Captain Have a look here.....Ady is a friend of mine who flies with me occasionally...Nice guy and absolutely superb with the models, he teaches you to fly models with a tandem set.
ADY

[NickLappos]

Capn,

I flew a tandem with a friend about 15 years ago, and it was an embarassment! He outflew me hands down in every way, I barely controlled the thing. Hover? Like day 1 at Flight School! The skill needed to control an RC helo is awesome!

[Rich Lee]

We can build a full sized helicopter that can fly aerobatics like the model helicopters, but that would exceed the physical ability of anyone flying it. Scaled up from RC helicopter sizes, the G-loading during many RC aerobatic maneuvers would be incredible and most difficult to endure.