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Author Topic: LHX Program  (Read 47776 times)

Offline Sundog

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Re: LHX Program
« Reply #90 on: April 09, 2010, 11:29:41 am »
Someone who is more knowledgeable about rotor dynamics (I wish Ray Prouty was here!) could probably explain this better than me.   You are correct in that helo power is usually measured (or at least described) in terms of engine shp.  It's a convenient way to compare  theoretical power available (discounting friction, efficiencyand accessory losses), and doesn't depend or rotor design.    But remember, that power is delivered to the rotorhead.  It is not what lifts or propels the helicopter.  As that power is delivered, it turns the rotor.  That rotor, and the air it moves, is what produces the thrust, and it is that which lifts and propels the helicopter.    Couple of ways to visualize it would be to take a Bell 206 and put the blades in totally flat pitch ( or even remove them, as long as you had some drag mechanism to keep the gearbox from running away).  The engine would still produce its rated shp, but there'd be zero thrust.  Conversely, when a JetRanger is autorotating down after an engine failure , it's storing energy in the freewheeling rotor.  When the collective is pulled just before the flare, the rotor expends that energy to arrest the 'copter's descent.  Here, the rotor is briefly producing a lot of thrust even though there is no engine power at all. 

An autogyro works on a similar principle.  The forward motion of the vehicle sends air through the center section of the rotor and spins it up.  The center portion is powered by that air, and essentially procues no thrust.  But the inner portion is connected to the outer portion, and the outer spins faster, because it has to travel a larger arc in the same amount of time as the inner, and so it produces thrust.  In fact, all the thrust (= lift in this case) comes from the outer ~ 1/3 of the rotor in an autogyro, so the aircraft gets thrust and flies with a rotor that is receiving no power at all from the engine.  That's why an autogyro can't hover, has a staling speed and doesn't take off vertically unless there is an auxiliary power system to spin up the rotor  for a "jump" takeoff. 

As for the new tail rotor, it allows more of the engine power to be used at the rotorhead, allowing the new rotor to generate its greater thrust.  This also allows faster sideways flight into the wind because it can counteract not only the higher torque from the greater amount of engine power being used to power the rotor, but the ruddervane effect of the aircraft wanting to turn into the wind. 

Yeah, I understand the physics, I was just making sure I was on the same page as you. ;) Although, your last paragraph I hadn't considered in the sense that the more efficient and less power the tail rotor uses, the more is available for the main rotor; I mean I've known that, but I didn't think about it. I also hadn't considered the "sideward" flying part of the envelope.

I develop aircraft for flight simulator and work on mainly the flight dynamics, although I haven't worked on a helo. But I definitely get the relationship between the engine and how the thrust is actually generated as a lot of people get that wrong in FS. That's why I am always looking for good engine decks, which are damn near impossible to find. Because, it doesn't matter how much you know about the aircraft's flight dynamics/aerodynamics if the engine/propulsive modeling is incorrect. In that sense, I've worked a lot with trying to get propeller aero/thrust correct. It would be the same with a helo model, I would start with the engine and then the rotor aero/thrust, because if you don't get that right, you'll never get the right combo of performance for the given lift/drag, etc. Although, the FS standard Helo flight modeling isn't the best, my understanding is that the Dodo Sim 206 is very realistic, because of how well they were able to model the flight dynamics and systems.

Offline F-14D

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Re: LHX Program
« Reply #91 on: April 09, 2010, 12:37:16 pm »
Someone who is more knowledgeable about rotor dynamics (I wish Ray Prouty was here!) could probably explain this better than me.   You are correct in that helo power is usually measured (or at least described) in terms of engine shp.  It's a convenient way to compare  theoretical power available (discounting friction, efficiencyand accessory losses), and doesn't depend or rotor design.    But remember, that power is delivered to the rotorhead.  It is not what lifts or propels the helicopter.  As that power is delivered, it turns the rotor.  That rotor, and the air it moves, is what produces the thrust, and it is that which lifts and propels the helicopter.    Couple of ways to visualize it would be to take a Bell 206 and put the blades in totally flat pitch ( or even remove them, as long as you had some drag mechanism to keep the gearbox from running away).  The engine would still produce its rated shp, but there'd be zero thrust.  Conversely, when a JetRanger is autorotating down after an engine failure , it's storing energy in the freewheeling rotor.  When the collective is pulled just before the flare, the rotor expends that energy to arrest the 'copter's descent.  Here, the rotor is briefly producing a lot of thrust even though there is no engine power at all. 

An autogyro works on a similar principle.  The forward motion of the vehicle sends air through the center section of the rotor and spins it up.  The center portion is powered by that air, and essentially procues no thrust.  But the inner portion is connected to the outer portion, and the outer spins faster, because it has to travel a larger arc in the same amount of time as the inner, and so it produces thrust.  In fact, all the thrust (= lift in this case) comes from the outer ~ 1/3 of the rotor in an autogyro, so the aircraft gets thrust and flies with a rotor that is receiving no power at all from the engine.  That's why an autogyro can't hover, has a staling speed and doesn't take off vertically unless there is an auxiliary power system to spin up the rotor  for a "jump" takeoff. 

As for the new tail rotor, it allows more of the engine power to be used at the rotorhead, allowing the new rotor to generate its greater thrust.  This also allows faster sideways flight into the wind because it can counteract not only the higher torque from the greater amount of engine power being used to power the rotor, but the ruddervane effect of the aircraft wanting to turn into the wind. 

Yeah, I understand the physics, I was just making sure I was on the same page as you. ;) Although, your last paragraph I hadn't considered in the sense that the more efficient and less power the tail rotor uses, the more is available for the main rotor; I mean I've known that, but I didn't think about it. I also hadn't considered the "sideward" flying part of the envelope.

I develop aircraft for flight simulator and work on mainly the flight dynamics, although I haven't worked on a helo. But I definitely get the relationship between the engine and how the thrust is actually generated as a lot of people get that wrong in FS. That's why I am always looking for good engine decks, which are damn near impossible to find. Because, it doesn't matter how much you know about the aircraft's flight dynamics/aerodynamics if the engine/propulsive modeling is incorrect. In that sense, I've worked a lot with trying to get propeller aero/thrust correct. It would be the same with a helo model, I would start with the engine and then the rotor aero/thrust, because if you don't get that right, you'll never get the right combo of performance for the given lift/drag, etc. Although, the FS standard Helo flight modeling isn't the best, my understanding is that the Dodo Sim 206 is very realistic, because of how well they were able to model the flight dynamics and systems.

Remember I said there were others who explain this better than me?  What I was getting at was that rotor thrust wasn't the same as engine power and is not rigidly related.  The torque was generated by the engines, not the rotor thrust.  What the AH-1Z's new rotor/transmission does is along with its better design, allow the aircraft to absorb more power from the engines and in turn translate that into more thrust.  By absorbing more power you get more torque.   

The more efficient (higher thrust) a tail rotor, the less power the engines need to supply to it, which allows the use of smaller engines or the availability of greater power to the transmission if it can absorb it.  It also allows for more agility in military scenarios.  In the case of the 4BW, they ran those tests with the original Whiskey tail rotor, and found they were running out of tail rotor authority before the engine/rotor was running out of power.  It was decided that the extra cost of a new tail rotor was worth the cost by allowing the upgrade to use the full capabilities that it was delivering. 

Offline Sundog

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Re: LHX Program
« Reply #92 on: April 09, 2010, 02:44:53 pm »
Well, the torque is generated by the engines and transferred to the rotor, and the torque due to the rotor inertia is what has to be counteracted by the tail rotor. Of course, that will be a function of the mass properties of the main rotor and it's RPM's. Of course, something I never considered is where the main flight loads are transferred from the main rotor shaft to the airframe. I would assume it's at the main bearings, since you would definitely not want those forces transmitted to the transmission itself. I was just thinking about that, due to the increased performance of the AH-1Z when compared to earlier Cobras.

So why isn't the Army looking at AH-1Z's as a basis for advanced Scouts? Are they too afraid of what it would it would do to the Apache? ;D

Offline F-14D

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Re: LHX Program
« Reply #93 on: April 09, 2010, 04:26:47 pm »
Well, the torque is generated by the engines and transferred to the rotor, and the torque due to the rotor inertia is what has to be counteracted by the tail rotor. Of course, that will be a function of the mass properties of the main rotor and it's RPM's. Of course, something I never considered is where the main flight loads are transferred from the main rotor shaft to the airframe. I would assume it's at the main bearings, since you would definitely not want those forces transmitted to the transmission itself. I was just thinking about that, due to the increased performance of the AH-1Z when compared to earlier Cobras.

So why isn't the Army looking at AH-1Z's as a basis for advanced Scouts? Are they too afraid of what it would it would do to the Apache? ;D


The performance increase comes mainly from the new rotor's dynamics and its and the transmission's ability  to absorb and use more of the power the engines can put out and its thrust and dynamics.  The 4BW testbed was a straight -1W and it realized a big performance gain.  In fact, originally the Zulu was to use the same T700 as the -1W because it provided enough power.  Now it looks like they'll use the slightly more powerful engine in the UH-1Y primarily for logistics and commonality reasons. 

As far as your question, although there were some people talking about putting Comanche avionics in an AH-1Z for the Army, it really didn't go anywhere.  It would be hard to justify having both the Apache and the Cobra (which the Army just got rid of) in the inventory.   For one thing, why would a -1Z scout for an AH-64?  Since the -1Z was already faster, more agile and carried a heavier load, just do it all themselves.   Also, supposedly the Army needed a smaller helo for the role envisioned for LHX.  Problem there is Comanche never was required to do all the things originally envisioned for LHX. 

Not to be discounted, of course is the famous "Not Invented Here".   It's outside the scope of this topic, but I'll give you a related classic example of this going on right now:  USAF is starting another big competition to select a helicopter to replace its UH-1Ns (a requirement that has been criminally neglected for too many years).  Instead of doing that, why not just look to how the Marines' idea for replacing their old UH-1Ns: convert them into UH-1Ys?  Yes, I know the Marines later decided to go new build with their -1Ys, but that's because -1N operations abroad have worn out many Ns to where the conversion costs as much as new build, and also operational temp abroad precluded Hueys airframes being out of service for the conversion.   USAF probably wouldn't have that problem, but even if they did I'll wager new-build -1Ys would be the better way to go.  Oops!  Can't do that: N. I. H.

Offline yasotay

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Re: LHX Program
« Reply #94 on: April 10, 2010, 02:23:23 pm »
So why isn't the Army looking at AH-1Z's as a basis for advanced Scouts? Are they too afraid of what it would it would do to the Apache? ;D

Certainly not any fear of what it would do to the Apache, better performance or not.  The Army is well satisfied with the performance of the Apache both from a weapons platform and as a survivable combat helicopter perspective.  Both AH-1Z and AH-64D are larger aircraft (in many aspects) than what the US Army wants for a LIGHT reconnaissance platform.  Logistically both are much more thirsty than OH-58D and parts are $$$ in comparison. As has been discussed above LHX was to be a light weight inexpensive scout (because historically they got... shot down more) that was relatively easy to make and operate from the farmers fields of Central Europe.  Today that is still the case... other than Central Europe.  There is even less necessity for the protections that caused LHX/RAH to grow in protective measures to be more of an attack helicopter.  While it must still deal with the traditional threats to rotorcraft, light to medium caliber guns and MANPADS, they are not in the density that LHX expected to try and survive in.

On top of the logistics cost I mentioned, I also suspect that AH-1Z and AH-64D are both likely about twice the acquistion cost the Army would want to pay for a scout helicopter.

Offline Sundog

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Re: LHX Program
« Reply #95 on: April 10, 2010, 02:37:35 pm »
So why not new build OH-58's with new engines, avionics, and rotors? I mean, they like the platform, they just need something newer, right?

Offline F-14D

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Re: LHX Program
« Reply #96 on: April 10, 2010, 03:09:07 pm »
So why not new build OH-58's with new engines, avionics, and rotors? I mean, they like the platform, they just need something newer, right?

They did.  It was called the ARH-70 Arapaho.  That was canceled in 2008.  As to liking the OH-58 platform, that's a subject for a topic elsewhere. 

Offline F-14D

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Re: LHX Program
« Reply #97 on: April 10, 2010, 03:27:36 pm »
So why isn't the Army looking at AH-1Z's as a basis for advanced Scouts? Are they too afraid of what it would it would do to the Apache? ;D

Certainly not any fear of what it would do to the Apache, better performance or not.  The Army is well satisfied with the performance of the Apache both from a weapons platform and as a survivable combat helicopter perspective.  Both AH-1Z and AH-64D are larger aircraft (in many aspects) than what the US Army wants for a LIGHT reconnaissance platform.  Logistically both are much more thirsty than OH-58D and parts are $$$ in comparison. As has been discussed above LHX was to be a light weight inexpensive scout (because historically they got... shot down more) that was relatively easy to make and operate from the farmers fields of Central Europe.  Today that is still the case... other than Central Europe.  There is even less necessity for the protections that caused LHX/RAH to grow in protective measures to be more of an attack helicopter.  While it must still deal with the traditional threats to rotorcraft, light to medium caliber guns and MANPADS, they are not in the density that LHX expected to try and survive in.

On top of the logistics cost I mentioned, I also suspect that AH-1Z and AH-64D are both likely about twice the acquisition cost the Army would want to pay for a scout helicopter.

REmeember:  The original purpose for LHX was to be more than just a scout helicopter.  In fact, even the dumbed down requirement for RAH-66 said it was to replace both the OH-58 and the AH-1.  I wonder if, by the time they canceled it, whether the acquisition and logistics costs really were going to be less than, say, an AH-1Z or AH-64--which would be a good reason why they killed it.  
« Last Edit: April 11, 2010, 02:13:13 pm by F-14D »

Offline yasotay

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Re: LHX Program
« Reply #98 on: April 10, 2010, 07:44:15 pm »
So why isn't the Army looking at AH-1Z's as a basis for advanced Scouts? Are they too afraid of what it would it would do to the Apache? ;D

Certainly not any fear of what it would do to the Apache, better performance or not.  The Army is well satisfied with the performance of the Apache both from a weapons platform and as a survivable combat helicopter perspective.  Both AH-1Z and AH-64D are larger aircraft (in many aspects) than what the US Army wants for a LIGHT reconnaissance platform.  Logistically both are much more thirsty than OH-58D and parts are $$$ in comparison. As has been discussed above LHX was to be a light weight inexpensive scout (because historically they got... shot down more) that was relatively easy to make and operate from the farmers fields of Central Europe.  Today that is still the case... other than Central Europe.  There is even less necessity for the protections that caused LHX/RAH to grow in protective measures to be more of an attack helicopter.  While it must still deal with the traditional threats to rotorcraft, light to medium caliber guns and MANPADS, they are not in the density that LHX expected to try and survive in.

On top of the logistics cost I mentioned, I also suspect that AH-1Z and AH-64D are both likely about twice the acquisition cost the Army would want to pay for a scout helicopter.

REmeember:  The original purpose for LHX was to be more than just a scout helicopter.  In fact, even the dumbed down requirement for RAH-66 said it was to replace both the OH-58 and the AH-1.  I wonder if, by the time they canceled it, whether the acquisition and logistics costs really were going to be less than, say, an AH-1XcZ--which would be a good reason why they killed it. 

Comanche was cancelled because it was consuming a huge part of the Army Aviation budget and the Army was in combat operations.  With the same money the Army got the entire UH-72 program, UH-60M program, CH-47F program, AH-64D Block 3,  functioning ASE, MTADS and a slew of ground support equipment.  Gives you an idea of the size of the investment.  To be clear some of the programs got funding boost that accelerated their acquisition vice being entirely new efforts.  Some had been waiting in the wings for more funding (a.k.a. a miracle).

As to to refit of the OH-58D, it is getting refit... again.  The ARH program demonstrated that trying to modify civil platforms  (Bell 407) can be as difficult as coming up with a purpose built military aircraft.  You have to change out fuel tanks, add armor, strengthen structures, build a new transmission, etc.  These can be as expensive in redesign as new start.

Offline Sundog

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Re: LHX Program
« Reply #99 on: April 10, 2010, 08:55:28 pm »
So, what is it you guys are recommending, based on your understanding of what is needed? Is it something similar to the Comanche, but without the low RCS requirements? A militarized, slightly scaled up X2 (Assuming the tech works)? Or do you guys see the LHX/Light Scout mission as just not required anymore?

Offline F-14D

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Re: LHX Program
« Reply #100 on: April 11, 2010, 02:15:09 pm »
So why isn't the Army looking at AH-1Z's as a basis for advanced Scouts? Are they too afraid of what it would it would do to the Apache? ;D

Certainly not any fear of what it would do to the Apache, better performance or not.  The Army is well satisfied with the performance of the Apache both from a weapons platform and as a survivable combat helicopter perspective.  Both AH-1Z and AH-64D are larger aircraft (in many aspects) than what the US Army wants for a LIGHT reconnaissance platform.  Logistically both are much more thirsty than OH-58D and parts are $$$ in comparison. As has been discussed above LHX was to be a light weight inexpensive scout (because historically they got... shot down more) that was relatively easy to make and operate from the farmers fields of Central Europe.  Today that is still the case... other than Central Europe.  There is even less necessity for the protections that caused LHX/RAH to grow in protective measures to be more of an attack helicopter.  While it must still deal with the traditional threats to rotorcraft, light to medium caliber guns and MANPADS, they are not in the density that LHX expected to try and survive in.

On top of the logistics cost I mentioned, I also suspect that AH-1Z and AH-64D are both likely about twice the acquisition cost the Army would want to pay for a scout helicopter.

REmeember:  The original purpose for LHX was to be more than just a scout helicopter.  In fact, even the dumbed down requirement for RAH-66 said it was to replace both the OH-58 and the AH-1.  I wonder if, by the time they canceled it, whether the acquisition and logistics costs really were going to be less than, say, an AH-1XcZ--which would be a good reason why they killed it. 

Comanche was cancelled because it was consuming a huge part of the Army Aviation budget and the Army was in combat operations.  With the same money the Army got the entire UH-72 program, UH-60M program, CH-47F program, AH-64D Block 3,  functioning ASE, MTADS and a slew of ground support equipment.  Gives you an idea of the size of the investment.  To be clear some of the programs got funding boost that accelerated their acquisition vice being entirely new efforts.  Some had been waiting in the wings for more funding (a.k.a. a miracle).

As to to refit of the OH-58D, it is getting refit... again.  The ARH program demonstrated that trying to modify civil platforms  (Bell 407) can be as difficult as coming up with a purpose built military aircraft.  You have to change out fuel tanks, add armor, strengthen structures, build a new transmission, etc.  These can be as expensive in redesign as new start.

The ARH-70 selection was also quite controversial (and surprising).  Story is that almost all the operational interst wanted the H-6 based entry. 

Offline F-14D

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Re: LHX Program
« Reply #101 on: April 11, 2010, 02:20:37 pm »
So, what is it you guys are recommending, based on your understanding of what is needed? Is it something similar to the Comanche, but without the low RCS requirements? A militarized, slightly scaled up X2 (Assuming the tech works)? Or do you guys see the LHX/Light Scout mission as just not required anymore?

It'll be quite some time before X2 technology would be trusted enough to be considered for an operational role such as this.  Personally, and this represents only me, I'd either go back to the original LHX specification before the dumb down, or do nothing.   Something like UCARS for the scout role, if we really need such a scout (and what happened to the AH-1 replacement requirement?), and buy more attack helos.   Of course I doubt if the interest, and therefore budget, is there to deal with the situation at all.  

Offline Stargazer

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Re: LHX Program
« Reply #102 on: April 11, 2010, 02:30:33 pm »
It'll be quite some time before X2 technology would be trusted enough to be considered for an operational role such as this.

Maybe. But I believe one way of shortening that timespan would be to find applications in unmanned vehicles. Once the technology is proven in unmanned configuration, it will be easier to "sell" the manned version of it to the armed forces.

Offline F-14D

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Re: LHX Program
« Reply #103 on: April 11, 2010, 02:54:37 pm »
It'll be quite some time before X2 technology would be trusted enough to be considered for an operational role such as this.

Maybe. But I believe one way of shortening that timespan would be to find applications in unmanned vehicles. Once the technology is proven in unmanned configuration, it will be easier to "sell" the manned version of it to the armed forces.

Personally, I don't think manned or unmanned will make much difference.  If it proves itself practical and economical, and it's still a long way from doing that, then it'll be a viable candidate for roles requiring that level of performance, manned or unmanned.   I don't see there being a worry about man-rating it, as we do with rockets.   It either works or it doesn't

What we need is some program that benefits from advanced rotorcraft technology, like  next generation powered lift attack vehicle, Osprey escort, etc.   The Marines' low-key VMAO program or the original "pre-dumbing" LHX requirements would be ideal. 

Offline sferrin

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Re: LHX Program
« Reply #104 on: April 11, 2010, 02:55:55 pm »
It'll be quite some time before X2 technology would be trusted enough to be considered for an operational role such as this.

X2 "technology" is not exactly revolutiuonary.  Better than what there is now but it's not like you're jumping to anything fundamentally different.  
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