AH64 Apache - can they roll?

[Zippee]

I was wandering if anyone can help settle a query;
At an airshow a few years ago I swear I recall seeing an Apache doing a roll or loop during its display but my friend says the only helicopter capable of this is a Lynx. Whos right? Is an Apache capable of performing this kind of manouver?

[dirty doug]

Friend of mine is an Apache pilot based in Wattisham, Suffolk. Not a question I've ever felt the need to ask him, but next beer session the question will be asked. The Lynx CAN roll but iirc there are lots of modifications necessary to perform it - different torque settings on lots of bits & pieces - you can tell I normally get this info whilst beered up

[FourWheelDrift]

I'm sure they could if they can go inverted in a loop, here's one doing a full loop at Farnbrorough in 2004 - http://www.youtube.com/watch?v=us18o7qOXjI

[dirty doug]

Right just had update from a senior technician from REME. No the Apache cannot loop or roll. The Lynx has a baffle in the engine oil tank to keep a few seconds worth of oil at the top to provide lubrication during high G manoeuvres i.e. roll and loop. The pilots have to be qualified to do the manoeuvre so not just anyone is able to do it. HTH

[Zippee]

Thanks chaps! Though I'm a little confuzzed as that youtube clip clearly shows an Apache doing a loop??

[moleamol]

I thought the chopper needed a fixed rotor to perform aerobatics?

[dr_gn]

dirty doug said:

Right just had update from a senior technician from REME. No the Apache cannot loop or roll. The Lynx has a baffle in the engine oil tank to keep a few seconds worth of oil at the top to provide lubrication during high G manoeuvres i.e. roll and loop. The pilots have to be qualified to do the manoeuvre so not just anyone is able to do it. HTH

Don't understand that statement.

The way I see it, if it's a positive G loop, why is there a problem with oil strvation? It's effectively like being upright as far as the oil is concerned. Same with a barrel roll, again positive G will surely keep the fluids in the right place?

Axial rolls would be a different matter (depending on the position of the tanks and whether a helicopter can do a true axial roll), as would any manoever which would subject the aircraft to negative g for any significant amount of time (where baffles or pressure tank could be used) or require invertred flight. For this you'd need to be able to switch the pitch of the blades. A function commonly available on R/C helicopters these days.

Willing to be corrected as ever!

Edited by dr_gn on Tuesday 28th July 15:30

[FourWheelDrift]

dr_gn said:

The way I see it, if it's a positive G loop, why is there a problem with oil strvation? It's effectively like being upright as far as the oil is concerned. Same with a barrel roll, again positive G will surely keep the fluids in the right place?

As proven by Bob Hoover - http://www.youtube.com/watch?v=Xp2Uc9XvmjY

[dr_gn]

FourWheelDrift said:

dr_gn said:

The way I see it, if it's a positive G loop, why is there a problem with oil strvation? It's effectively like being upright as far as the oil is concerned. Same with a barrel roll, again positive G will surely keep the fluids in the right place?

As proven by Bob Hoover - http://www.youtube.com/watch?v=Xp2Uc9XvmjY

Exactly.

[Semi hemi]

This wee beauty can roll & loop as well


http://en.wikipedia.org/wiki/MBB_Bo_105

"The four-blade rigid main rotor, a worldwide first, with fiberglass blades ensures high maneuverability. A Bo 105CBS used for promotional purposes by Red Bull USA is fully aerobatic, performing loops, rolls, Immelmanns and other maneuvers normally regarded as for fixed-wing aircraft only.[3][4] All main systems (hydraulics, electric, fuel, lubrication) were designed to be fully redundant."

Shell used them for in field shuttle flights up in the Brent oil field back in the 80s and was a helluva way to got to work in the morning
they didnt roll or loop them there, but with a wee bit of encouragement the pilots would really chuck it about the sky.

[Mr Dave]

A Dutch AH-64 did a loop and a roll at Fairford a few years back. Took me by suprise.

Im pretty sure in one of the Apache books released last year it was alluded to that certain aerobatics have taken place,with AAC pilots but the aircraft arent cleared for it.

http://www.youtube.com/watch?v=nZOKecduNBQ

Video proof that the Dutch do do it at airshows.

[Taffer]

Remember that the WAH-64s used by the AAC differ from all other AH-64s in that they use Rolls Royce Turboshafts as opposed to GE ones fitted on all other models. There will probably be some difference in the two engines' lubrication systems that allow one to be cleared for limited aerobatics, but not the other.

Dr gn, the manoeuvre carried out by Bob Hoover in the vid is a barrel roll, not a loop. A barrel roll will be a constant 1g when carried out but a loop will have higher G loading on entry and exit, and experience low/0g when inverted at the top - it's not just the oil systems that limit it; the blades have to be rigid enough not to strike the tail boom (helicopter airframe limits are less than fixed-wing aircraft mainly due to this reason).

Edited by Taffer on Tuesday 28th July 19:25

[dr_gn]

Taffer said:

Remember that the WAH-64s used by the AAC differ from all other AH-64s in that they use Rolls Royce Turboshafts as opposed to GE ones fitted on all other models. There will probably be some difference in the two engines' lubrication systems that allow one to be cleared for limited aerobatics, but not the other.

Dr gn, the manoeuvre carried out by Bob Hoover in the vid is a barrel roll, not a loop. A barrel roll will be a constant 1g when carried out but a loop will have higher G loading on entry and exit, and experience low/0g when inverted at the top - it's not just the oil systems that limit it; the blades have to be rigid enough not to strike the tail boom (helicopter airframe limits are less than fixed-wing aircraft mainly due to this reason).

Edited by Taffer on Tuesday 28th July 19:25

Interesting stuff, thanks for that - I realise it's a barrel roll in the vid, I think F-W-D posted that particular video because I mentioned in my first post that a barrel roll should be possible in a helicopter (at least in terms of g-loading for the reasons you outline).

Regarding your points about the loop - Assuming the pitch on the helicopter is not reversible for inverted flight, wouldn't (or couldn't) you always get downthrust from the rotors, such that the helicopter would be accelerating downwards while inverted, thereby giving positive G?

Regarding the boom strike issue - if the above assumptions I made were true, wouldn't the blades aways be coning away from the boom?

The video of the Apache looping proves that this isn't an issue for that particular helicopter, but it might be for one of these:

http://www.youtube.com/watch?v=I3Pp4vOr3SE

Cheers,

Edited by dr_gn on Tuesday 28th July 20:21

[moleamol]

Taffer said:

Dr gn, the manoeuvre carried out by Bob Hoover in the vid is a barrel roll, not a loop. A barrel roll will be a constant 1g when carried out but a loop will have higher G loading on entry and exit, and experience low/0g when inverted at the top - it's not just the oil systems that limit it; the blades have to be rigid enough not to strike the tail boom (helicopter airframe limits are less than fixed-wing aircraft mainly due to this reason).

I don't think it's that simple. I await correction but I'm pretty sure you could keep fairly constant positive G in a loop depending on how it's carried out. Even though the plane is inverted at the top the G will still be positive. After all a barrel roll still inverts the aircraft, it's just that it's easier to keep the G loading positive and minimal during that particular manoeuvre.

[dr_gn]

moleamol said:

Taffer said:

Dr gn, the manoeuvre carried out by Bob Hoover in the vid is a barrel roll, not a loop. A barrel roll will be a constant 1g when carried out but a loop will have higher G loading on entry and exit, and experience low/0g when inverted at the top - it's not just the oil systems that limit it; the blades have to be rigid enough not to strike the tail boom (helicopter airframe limits are less than fixed-wing aircraft mainly due to this reason).

I don't think it's that simple. I await correction but I'm pretty sure you could keep fairly constant positive G in a loop depending on how it's carried out. Even though the plane is inverted at the top the G will still be positive. After all a barrel roll still inverts the aircraft, it's just that it's easier to keep the G loading positive and minimal during that particular manoeuvre.

At the top of a loop, the G won't *necessarily* be positive, but could be.

Bet this one wasn't:

http://www.youtube.com/watch?v=bhbcxqKPXR4

Cheers,

[Taffer]

dr_gn said:

Regarding your points about the loop - Assuming the pitch on the helicopter is not reversible for inverted flight, wouldn't (or couldn't) you always get downthrust from the rotors, such that the helicopter would be accelerating downwards while inverted, thereby giving positive G?

Regarding the boom strike issue - if the above assumptions I made were true, wouldn't the blades aways be coning away from the boom?

The video of the Apache looping proves that this isn't an issue for that particular helicopter, but it might be for one of these:

http://www.youtube.com/watch?v=I3Pp4vOr3SE

Cheers,

Edited by dr_gn on Tuesday 28th July 20:21

I think that happens already with helicopters that do perform the loop; however, after their gentle entry to the loop, their airspeed is so low at the top that the second half seems to be relatively short to keep a small positive g load (a full 'circular' loop in a light aircraft generates about 3-5g on entry and exit, which would ruin a helicopter pilot's day). It's been 5 years since I did any rotorcraft aeromechanics and even then we really didn't cover aerobatics, so I am rusty on this stuff!

The rotor blades will be coning if they are under load, but even with rotational stiffening there is enough flexing of the blades (and of the airframe) as they rotate that, even if the pilot nails the manoeuvre, contact may still occur. Rigid and semi-rigid rotors (as in the Lynx and Apache) aren't as at risk of tail strikes, but on 'conventional' designs such as the Sea King, fairly strong downward gusts are enough to cause contact, even when the rotors are under load. Some of the accident reports of light helicopter rotor strike are shocking - Robinson R44s have suffered enough blade flexing to have cabin and tail boom strikes!

[dr_gn]

Taffer said:

dr_gn said:

Regarding your points about the loop - Assuming the pitch on the helicopter is not reversible for inverted flight, wouldn't (or couldn't) you always get downthrust from the rotors, such that the helicopter would be accelerating downwards while inverted, thereby giving positive G?

Regarding the boom strike issue - if the above assumptions I made were true, wouldn't the blades aways be coning away from the boom?

The video of the Apache looping proves that this isn't an issue for that particular helicopter, but it might be for one of these:

http://www.youtube.com/watch?v=I3Pp4vOr3SE

Cheers,

Edited by dr_gn on Tuesday 28th July 20:21

I think that happens already with helicopters that do perform the loop; however, after their gentle entry to the loop, their airspeed is so low at the top that the second half seems to be relatively short to keep a small positive g load (a full 'circular' loop in a light aircraft generates about 3-5g on entry and exit, which would ruin a helicopter pilot's day). It's been 5 years since I did any rotorcraft aeromechanics and even then we really didn't cover aerobatics, so I am rusty on this stuff!

The rotor blades will be coning if they are under load, but even with rotational stiffening there is enough flexing of the blades (and of the airframe) as they rotate that, even if the pilot nails the manoeuvre, contact may still occur. Rigid and semi-rigid rotors (as in the Lynx and Apache) aren't as at risk of tail strikes, but on 'conventional' designs such as the Sea King, fairly strong downward gusts are enough to cause contact, even when the rotors are under load. Some of the accident reports of light helicopter rotor strike are shocking - Robinson R44s have suffered enough blade flexing to have cabin and tail boom strikes!

Beats me how anyone in their right mind would step in to a Robinson helicopter. They look like a school design project gone wrong. Even the control column looks like a bent paper clip.

[Taffer]

moleamol said:

I don't think it's that simple. I await correction but I'm pretty sure you could keep fairly constant positive G in a loop depending on how it's carried out. Even though the plane is inverted at the top the G will still be positive. After all a barrel roll still inverts the aircraft, it's just that it's easier to keep the G loading positive and minimal during that particular manoeuvre.

To pull tightly enough to keep positive g throughout the whole loop you'll stall the aircraft as your airspeed decreases and angle of attack increases towards the top of the loop or just over the top. When I was taught loops, you realise why you should tighten your shoulder straps, as at the top of the loop you experience -g when pulling a symmetrical loop. The 'loops' that helicopters perform are visually messy (though still impressive!) as they cannot risk -g or the excessive +g that comes with the exit of a normal loop.

[strudel]

If the barrier to looping is lubrication - I don't think an apache has an issue!

[moleamol]

Taffer said:

moleamol said:

I don't think it's that simple. I await correction but I'm pretty sure you could keep fairly constant positive G in a loop depending on how it's carried out. Even though the plane is inverted at the top the G will still be positive. After all a barrel roll still inverts the aircraft, it's just that it's easier to keep the G loading positive and minimal during that particular manoeuvre.

To pull tightly enough to keep positive g throughout the whole loop you'll stall the aircraft as your airspeed decreases and angle of attack increases towards the top of the loop or just over the top. When I was taught loops, you realise why you should tighten your shoulder straps, as at the top of the loop you experience -g when pulling a symmetrical loop. The 'loops' that helicopters perform are visually messy (though still impressive!) as they cannot risk -g or the excessive +g that comes with the exit of a normal loop.

Well that's why it's more of a flop than a loop I suppose. Thought the Red Bull chopper does lots of impressive looking loops and things.