Piasecki X-49A Speedhawk

Discussion in 'Aviation' started by winnfield, Jan 13, 2008.

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  1. can anyone tell me what improvments it offers? Speed/Lift/Endurance?
  2. Speed? WAH!
    Lift? Dunno.
    Endurance? Dunno.
  3. Not for me thanks, looks v. dodgy.
    Is the main rotor redundant/set at zero pitch or has the advancing blade phenomenon gone away?
    Took off as if an elephant was hanging from the tail, and landed like a fixed wing.
    Would NOT fancy doing 177 kts low level (no great speed gain compared to current helos, Chinook 170 kts) with 'a slight descent'.
    Also given the axis of thrust from the tail fan, I could see any rapid nose-up input leading to an assymetric thrust couple ending in a very rapid back flip and early retirement without penesion or benefits.
    Only the septics.
  4. Both look total pieces of crap........That's done it , we will get either as Lynx replacement.
  5. I think its only being looked at as a technology demonstrator.

    As still21 says, I too can't see the point in it. It's whats called a compound helicopter but it will still suffer from all the limits to forward speed that a conventional rotor system suffers from. The only way to get around these problems is to somehow slow the main rotor down once the winglets have taken up most of the lift in forward flight. But that would still lead to some pretty bizarre advancing and retreating blade problems at speeds above about 250-270 knots. What its probably trying to overcome is a tail rotor encountering high speeds; ie, a conventional tail rotor suffers all the problems a main rotor does and this too limits its forward speed. A gain of only a few knots in this config doesn't really answer the question though...if anyone actually asked it.

    I think its yet again another dabble into trying to 'improve' the helicopter. So long as you have a disc being propelled through an airflow (not in a propellor sense), you will always encounter a wall when it comes to forward speed. Until someone gets off his arse and invents anti-gravity propulsion, we'll be stuck with helicopters doing slow stuff landing vertically and fixed wing doing faster stuff only able to land with forward speed.....and the MV22 doing neither very well except for sucking up money and generally falling out of the sky.
  6. Improving the helicopter is exactly what the US defence industries are attempting to do. - http://www.flightglobal.com/blogs/the-dewline/2008/01/will-army-aviation-finally-bre.html
  7. I thought I'd seen the idea before somewhere!
    A model from "Things to Come".

    If the aim is to mod the Apache could you do something similar to the solution that Fairey used here? Fairey Gyrodyne Video

    Alternatively, without going the full tilt rotor route could this configuration deal with the retreating blade problem? Side by Side
  8. The only benefit I can see is you have taken out 2 90 degree changes in drive to the tail rotor, but you have replaced it with a target the size of a half ton skip!

  9. The Fairey Gyrodyne used an asymmetric forward facing anti torque propeller for low speed with the rotors being powered by the engine. At higher speeds, the blades would autorotate (windmill) and forward power was provided by the side propeller. A good idea but one of the drawbacks was due to it being so close to the centre of axis of the main head, it proved quite ineffective at slow speeds and the power required to produce enough anti torque thrust of a usefully controllable nature was huge. The Gyrodyne was only about 2000kg but the larger and heavier you make the aircraft, the problem grows exponentially.

    The further away from the centre of axis you put your propeller or tail rotor means you can make it smaller therefore requiring less power to drive it because it will produce an equivalent amount of anti torque thrust as it gets further away. All about moment of levers.

    As for the Focke-Wulf Fw 61 twin rotor jobby. You will still get the same retreating blade problems at higher speeds. Although the problem may be pushed to a slightly right on the max speed side due to the main rotors spinning slower because there are two of them therefore same amount of lift for a reduce rotor speed. You certainly get around the problems of the need of a tail or anti torque system because the two main rotors counter each other. The disadvantages are you now have two complex rotor heads and the associated gearboxes to stop them hitting each other and the weight penalties that come with that. Think of the Chinook as the modern development of that idea. Yep, its fast and can lift lots of stuff but it requires a complex control system and still suffers from all the rotor problems associated with high speed flight.

    The only way to get around the limits to forward speed in a helicopter but still have all the advantages of a helicopter is to get rid of the main rotor all together. Get rid of that and you'll have no need for an anti torque system. Until we can produce a very light, efficient propulsion unit to effectively use vectored thrust (a small compact turbofan that has a huge power to weight ratio and doesnt use lots of fuel), we will just have to mildly improve the current configurations with little over all benefit and as always, putting up with a compromise. The trouble with a helicopter is the faster you want to go, the more you compromise the advantages a helicopter has and the less it stays a helicopter. The Holy Grail of aircraft design is to make an aircraft that is as versatile as a helicopter but has the performance of a fixed wing.

    At present, the better and probably the most practical idea is a compound helicopter. One that has a main rotor, a ducted tail rotor and stub wings. Once it gets to a speed were the stub wings can produce enough lift, you can slow down the main rotors therefore increasing the point were retreating blade stall will occur. The ducted fan on the tail (still pointing 90 deg from the airflow) is less prone to the problems associated with sticking a disc that spinning tres fast into a horizontal airflow. Once you get above about 60-70knots, a cambered vertical fin will actually produce enough sideways lift to take over anti torque reaction (just like a Gazelle) and therefore negating the need for the tail rotor plus youve slowed the main rotor down too so it will also be slower.
  10. I read around a little on the retreating blade thing and googled up this thread in the process. LINK


    The Kmax is a different configuration again but still susceptible to RB issues. I had assumed that addressing RB was simply a matter of dealing with the loss of lift but there is clearly more than that to it.

    It rang a bell though and a bit more googling brought me to Kmax's granny the Husky.


    Comparing their performance i was surprised to see that the Husky tops out at 120 kts where the Kmax does 100 kts unloaded.
  11. The Kaman solution wasn't and isn't really designed to overcome high speed RB problems but more to reduce disc loading which in turn will enable it to carry a heavier load. The way to think of disc loading and how it affects the disc is to view it from above and look at where lift across the disc is produced. Useful lift is generally produced in the mid portion of the disc as the tip and root dont tend to give much lift, mainily just drag. Increase the disc loading (by increasing weight or reducing the radius of the disc) and the useful lift area reduces up to a point where the rotors slow down due to drag overcoming thrust and which will lead to stall.

    If it had a single disc, it would require a higher angle of attack for a given weight. This higher angle of attack would obviously be more critical on the retreating blade side and would be closer to its stall crit angle/speed. It would get to a point where the stall would spread along the length of the blade the higher the disc loading. Having the config the Kaman utilises, it pretty much halfs the disc loading on each disc therefore bringing the crit angle of attack down and reducing the stall speed (or increaing the margin). The Kamans (especially the KMax) are designed to lift fecking huge loads with no real requirement for high speeds. Again, all a compromise.

    Just read a few of the posts on the link you provided and a few on there say that it is impossible to get RBS on dual or twin rotor systems. Not true. ANY rotating disc will suffer from RBS at some stage due to the differences in velocity of advancing and retreating blades and the inherant AoA of the retreating blade. Its just not as big a factor on these systems due to (as Ive said above) the disc loading being reduced therefore reducing the AoA on advancing and retreating blades. You'd have to fling the heli through the air pretty quick to make it happen or load the disc up to a massive extent (quite often well outside the usual envolope of the aircraft) but it will still occur, just less prone than a conventional system.
  12. You are freaking me out now.

    Can you make a reference to skiffing or something?
  13. I'm drinking my own piss if that helps?