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Old 07-22-2019, 10:14 PM   #11
thrash
 
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Default Re: VE2 question regarding helicopters

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Originally Posted by Anthony View Post
The simple analysis is that the lift of a rotor is equal to (downdraft speed)^2 * (rotor area) * (density of air) (this is somewhat incorrect because of additional interaction with the air below the rotor), and the power requirement is 0.5 * lift * downdraft speed, ...
In practice, both downdraft speed and rotor area are strongly influenced by the behavior of the surrounding air mass (not just below the rotor). If the air is already moving downward (as in a stationary hover), the effective downdraft speed is reduced. If the rotor blade-tip vortices stabilize and grow (as at a high hover), the effective rotor area is reduced (a "vortex ring state").

Translational lift develops from flying into undisturbed air, and so increasing the effective downdraft speed. Hovering close to the ground ("in ground effect") restricts vortex development, also increasing lift.

In extreme cases, descending at low airspeeds from a high hover can result in a catastrophic loss of lift -- "settling with power." Increasing power (rotor pitch) actually exacerbates the problem. The solution is to forward the cyclic and accelerate out of the condition.
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Old 07-22-2019, 10:34 PM   #12
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Default Re: VE2 question regarding helicopters

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until you're in the velocity regime where treating it as a fixed wing aircraft is fine.
That is a significant help. My smallest helicopter can fly faster than stall Speed at 59% of maximum power. I will check all the others.
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Old 07-23-2019, 01:11 AM   #13
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Default Re: VE2 question regarding helicopters

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I do recall that in the Soviet invasion of Afghanistan, the high altitudes sometimes required Hind helicopters to make rolling takeoffs. That may have been because there wasn't sufficient safety margin for hover, rather than being literally impossible.
High and hot conditions cut into aircraft lifting capacities, and as helicopters are basically a brute force approach to providing lift, they notice it more than most. Helicopters quite commonly have rather lower ceilings for hovering outside of ground effect than they do for flight (where they get some lift from body and possibly stub wings), making it entirely possible to fly into places where you can't get back out if there isn't enough room to get up airspeed in ground effect.

As for Hinds, they are a heavily loaded airframe and use stub wings for additional lift in flight, so making rolling take-offs to utilise the lift from their wings would help them carry enough payload to make using them worthwhile.
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Old 07-23-2019, 01:52 AM   #14
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Default Re: VE2 question regarding helicopters

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The fastest helicopters have 2,200hp power plants for about a 10 ton machine and can achieve about 190mph (305 kph). In 1986 the Westland Lynx with a BERP rotor managed to set the record of 250mph (400kph) approx. In 2014 the Eurocopter X3 broke the record with 293mph (472kph) but this was not a single rotor aircraft.
The Mi-6 had two 5,500 shp engines, and a max take-off weight of 42.5 tonnes. It made the circuit speed record in the late 50s, doing ~340 km/h.

The A-10 (a version of the Mi-24 used for record-breaking in the mid-70s) had a couple of 2,220 shp engines, and made a lot of records, including the record that the Lynx broke (368 km/h for the A-10), and many of the A-10s other records still stand. The Lynx has two engines of about half the power, and weighs about half what a Mi-24 does.

So, really weight doesn't have a whole lot to do with it.
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Old 07-23-2019, 04:22 AM   #15
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Default Re: VE2 question regarding helicopters

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Originally Posted by RyanW View Post
As speed increases, the advancing side of the rotor moves through the air faster and the retreating side moves slower. That requires higher angle of attack on the retreating side to maintained balanced lift. Eventually, you get blade stall, which basically puts a hard cap on a design's safe operating speed.
I haven't studied it for the best part of 25 years. But that's the nub of it.

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The Mi-6 had two 5,500 shp engines, and a max take-off weight of 42.5 tonnes. It made the circuit speed record in the late 50s, doing ~340 km/h.

The A-10 (a version of the Mi-24 used for record-breaking in the mid-70s) had a couple of 2,220 shp engines, and made a lot of records, including the record that the Lynx broke (368 km/h for the A-10), and many of the A-10s other records still stand. The Lynx has two engines of about half the power, and weighs about half what a Mi-24 does.

So, really weight doesn't have a whole lot to do with it.
Power to weight ratio always counts. With the exception of the big lifters as you have mentioned, the Chinook etc, and having an armoured design, you mentioned 11,000 hp to move a 42 tonne vehicle 340km, The Mil-24 is 4,400hp for a 9 tonne vehicle. The Super Lynx with 2,200hp for a vehicle that is 3.5 tonnes. The smaller the vehicle the better power to weight ratio there is. Of course the effects of aerodynamics, drag coefficient needs to be considered too.
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Old 07-23-2019, 09:38 AM   #16
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Default Re: VE2 question regarding helicopters

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Power to weight ratio always counts. With the exception of the big lifters as you have mentioned, the Chinook etc, and having an armoured design, you mentioned 11,000 hp to move a 42 tonne vehicle 340km, The Mil-24 is 4,400hp for a 9 tonne vehicle. The Super Lynx with 2,200hp for a vehicle that is 3.5 tonnes. The smaller the vehicle the better power to weight ratio there is. Of course the effects of aerodynamics, drag coefficient needs to be considered too.
That's a partially-loaded weight for the Hind, and an almost unloaded weight for the Lynx.

Note that the standard (not lightened for records, etc.) Super Lynx is slower than the standard Hind, despite a rather better power/weight ratio and being smaller. Streamlining, etc. would appear to be as or more important than power/weight.

The reason I mentioned the big Soviet helicopters was because you seemed to be claiming that faster helicopters had to be of moderate weight, when many of the fast helicopters are actually quite large (and the Mi-6 doesn't have an especially good power/weight ratio either).
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