I have a few questions for you folks into RC Helicopters

frankthedruid · 01-15-2005, 08:55 PM

Q. How much lift does mone of these things get?
Reason
I want to make a robot that is somewhat pc based
Q. Would adding more props one the same stem increase lift. (the stem would be the spinny thing that holds the props
Reason
I saw an airplane set up like this on the history channel

I will ask more questions when I get some info

Peoples · 01-15-2005, 08:57 PM

the osprey right? they decommissioned all of them in the military. I think the only reason for the use wasnt its increase in lift, but for vertical takeoff, then they would rotate forward and have the spead of an airplane

frankthedruid · 01-15-2005, 10:05 PM

No, this was a wing shaped airplane. But if it works for thrust it will work for lift?

jcdillin · 01-16-2005, 10:52 AM

do you want to have it fly like a plane or just act like a helicopter?

frankthedruid · 01-16-2005, 03:16 PM

Quote: Originally Posted by jcdillin

do you want to have it fly like a plane or just act like a helicopter?

Eventualy both

EBFoxbat · 01-16-2005, 03:55 PM

More props = more lift/thrust but...

more props means mroe weights, slowing there spin meaning less lift. I suppose trial an error will let you know if there is a net gain or loss.

I vote for a second motor with a second set of props.... what are you up to?

Dominik · 01-16-2005, 04:36 PM

You can buy ducted fan kits, which state the thrust amount for each unit. But you would need some big bastard ducts to lift a plane which is carrying a PC (based on a *light* 1kg laptop!)

jcdillin · 01-16-2005, 08:38 PM

I was actually planning to do this same thing at one time, my thoughts were to use a dual rotor design like army helicopters. Or something like this design for stability

Scouse Monkey · 01-17-2005, 06:50 AM

My turn:

If you simply add another rotor on top of an existing one you won't get an increase in overall lift as thrust is dependant on power. To increase the thrust of a rotor you change the angle of attack (incidence) and this angle is limited by blade stall but also to keep the rotor angular velocity high you need a power to overcome the drag of the high incidence.

So if you add a second rotor you simply cannot use each blade as much as it can be and you will also have more balancing work.

The dual rotor systems you see used (both in that vertical takeoff plane) and the Russian helicopter (see picture) with no tail rotor are counter-rotating as this cancels out the torque you get from using a single rotor so you don't need to have a tail rotor like a conventional helicopter. In this setup you need both rotors to have the same pitch to you get the same thrust and torque.

This design makes it very compact as there is no dangerous tail rotor to hit things but is not as efficent per rotor as a single or side by side set up as the lower blades are int he wake of the upper blades making them slightly less efficient. The Chinnook system (fore and aft) also cancels out any torque so it doesnt need a tail rotor and also has the advantage of being able to accomodate a large change in the centre of mass along it's length which is why it is so useful but it cannot get certified for civil use because of the reliance of the gearbox inbetween the two rotors that stops them hitting each other which would destroy the aircraft.

Going back to a single rotor system, if you fit a more powerful engine you could add a second set of blades but you would have to couple the control links etc and it is also likely you would have to increase the power of your tail rotor to maintain good control due to the increased thrust and therefore torque of the main rotor. But don'e expect 2x the power from doubling the blades but do expect a lot more vibration and less reliability.

EBFoxbat · 01-18-2005, 09:36 AM

Quote: Originally Posted by Scouse Monkey

if you fit a more powerful engine... it is also likely you would have to increase the power of your tail rotor to maintain good control due to the increased thrust and therefore torque of the main rotor.

Good point

If you double the wing (blade) area, you (slightly less than) double the lift. This is only true if the rotational speed of stem is kept the same. Most likely you would need a stronger motor to keep the same rotation speed because of the increase in mass it's trying to rotate.

Simply adding more mass to the stem, spinning, means you'll have more rotational inertia that the tail rotor will need to overcome.

I've never built an RC helicopter, but you do have other options, I just don't know how doable they are.

1) Make the blades longer... more surface area=more lift... however it means more mass so you'd need a stronger motor.
2) Add aditional blades on the same plane... 4 instead of 3 or 5 instead of 4... more surface area=more lift... however it means more mass so you'd need a stronger motor.
3) Add an independent second motor and set of baldes... a la Chinnook . If you could regulate them some how to keep them rotating at the same speed, then you would have no net inertia causing rotation. Add a tail rotor and you could control rotation. And you would have one mean looking beast of a toy.

EBFoxbat · 01-18-2005, 09:39 AM

Come on, you can't look at that and not laugh.

Scouse Monkey · 01-18-2005, 03:16 PM

No you got it all wrong!!!!

Its not the mass thats the problem its the aerodynamic drag!

True it will be harder to spin up due to the interia of the heavier rotor system but once it is going it won't matter - infact more mass can be a good thing. Mass=inertia which is only a problem when you want to change the speed of something turning. True the helicopter will weigh more due to added blades but on a hobby helicopter this is nothing compared to the wieght of the fuel and engine.

On a real helicopter the mass of the rotor head is a big problem as it is so substantial but on models the forces are many magnitudes lower than the scaling would suggest.

The more blades you have the more drag you have. For the same lift you can simply use less collective pitch with twice as many blades which gives you less drag per blade but there is still a basic drag of each blade x the number of blades:

now for the science bit (as they say in adverts):

For a basic aerofoil section (we are doing strip theory so neglecting area and all the rotational helicopter crap)

Lift = cl0 + 1/2 x air density x alpha x cla x velocity^2

where cl0 is the zero angle lift, alpha is the angle the section relative to the direction it is moving (incidence/pitch) and cla is the lift/angle value for the section ie the amount of lift change it can produce for a 1 degree of andgle change (incidence or pitch), and velocity is in m/s and is squared so has a bit effect!

Drag = cd0 + 1/2 x air denisty x alpha x cda x velocity^2

where cd0 is the zero angle drag and cda is is the change in drag per degree of incidence change.

As you can see there is therefore a relationship between lift and drag so as you increase the lift you increase the drag.

This drag will slow your engine velocity.

If you have excess power in your engine then you will get some benefit from adding blades but you will equally get some benefit from making them bigger or setting them at a higher incidence. As blades rotate they produce much more lift at the outer edges however the outer parts are also responsible for more drag as the blade is rotating but the drag coming from lift:drag coming from profile (cd0) ratio is much less so larger diameter or faster rotational speed is better.

If you add a second rotor head like the chinook you don't need a tail rotor and a tail rotor would not provide pure rotation response in thsi situation - it would miainly provide translation with a rotation about a point well in front of the helicopter.

If you control your lift by simply making the blades spin faster then more mass will hinder this response time however if you bags of excess power and fixed collective pitch it could work. In this case it may simply be easier to increase the gearing so the blades spin faster. I have never built an RC helicopter so don't know how optimised the blades are, if the pitch is variable, if they can go faster without problems etc byt if you want more lift then you have to increase the power. A chinook has two rotors but this is due to longitudinal stability requirements of having troops moving around inside and being able to stick any load in and not worry about the thing pitching up or down uncontrollably.

The most powerful helicopter in the world is the sky crane and this has a single rotor with large blades.

Scouse Monkey · 01-18-2005, 03:30 PM

Going back to your original point....

I suspect it will be very hard to add a second rotor head to the rotor shaft but it will increase the lift as long as you can maintain the rotor speed. YOu will also have a nightmate fitting it and keeping it all straight and balanced. But it is possible. You may have to then fit a more powerful engine but this will then use more fuel! If you have a helicopter see how much it can lift compared to the weight of the complete unit and then measure the weight increase of a more powerful engine and a second rotor system and see how much you will gain. Large blades are better for overall lift - you don't see helicopters with small blades but you probably have a size restiction so all you can try is increasing the blade and angle and engine power and then adding more blades.

Bugbyte · 01-18-2005, 06:04 PM

Quote: Originally Posted by frankthedruid

Q. How much lift does mone of these things get?

About 10 years ago a friend of mine who was an ace R/C helicopter pilot picked up my Sony 8mm handycam with a .60 R/C chopper. I'd guess it weighed at least 3 or 4 pounds. He used a sling system that we rigged up and he took off to hover, then lifted the camera off the ground, flew it around slowly and set it back down without busting it.

You are aware that R/C helicopters are extraordinarily difficult to fly, aren't you? I have the Great Planes R/C simulator with helicopters on it. According to my friend, it flies very much like a real chopper. I have practiced on it quite a bit and can manage a hover and circle but landings are pretty dodgy.

Scouse Monkey · 01-18-2005, 07:44 PM

Quote: Originally Posted by Bugbyte

You are aware that R/C helicopters are extraordinarily difficult to fly, aren't you?

The shop that used to sponsor me when i raced R/C cars used to specialise is planes and helicopters - the guys working there spent more time rebuilding their choppers than flying them! If you want something easy to control and stable build a 4 rotor rig with the rotors out on booms - like those kids toys you see. If you are good with gearing then you could actaully have them slightly overlapping ie two clockwise and two anticlockwise to enable you to get the maximum rotor diameter as small blades is normally the downfall of those things but they are very stable. Use engine speed to to control lift and then use a mass transfer system to control the direction - it wont be the fastest or most responsive craft but it will hover well and be very stable. Should be much easier to control with a PC. all you would need is a simple 2 axis servo system with some gyros etc for measuring movement.

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