- User Since
- May 6 2013, 1:57 PM (447 w, 5 d)
May 10 2016
Very nicely put - it's been a pleasure exchanging opinions.
Hi again DarkWanderer,
Thanks for continuing to follow the conversation - it's good to hear informed views. My own aviation background (flying helis since 1977, instructing since 1984, sadly having to retire on age grounds from operational work (police/ HEMS/ lighthouse ops) next year to concentrate solely on instructing) still hasn't equipped me to answer the co-ax questions from my own experience so I still have to rely on others. Every day's a school day! Fortunately a search through the Tech Log pages on the Professional Pilots Rumour Network site (PPRuNe.org) came up with a gem:
When you read the replies, it's worth bearing in mind that Shawn Coyle has set up test pilot schools in America and the UK, while Nick Lappos is back at Sikorsky (where they're developing the X2 Co-ax...) having previously been the lead on Commanche test flying before moving to Bell for a while. I'd never heard of 2 Ka50s having been lost through "self mid-air collisions" before - it appears that high-G/low-G and even prolonged right-yaw manoeuvres can lead to excessive blade coning in the lower rotor which have had serious consequences. Anyway, back to the autorotation discussion with a quote from Shawn Coyle in the above link:
"Autorotations for co-axial rotors with mechanical flight control systems are a problem because of the way yaw control is mechanized. What works in the proper direction in powered flight for yaw is reversed in autorotation. So all the current co-axial machines use large vertical stabilizers and rudders and need to have forward airspeed to keep yaw working the 'normal' way. The KA-32 Flight Manual has a minimum airspeed in autorotation of about 60 KIAS if memory serves me well. Below this airspeed 'Special Techniques' are necessary."
A minimum of 60 kts Indicated Airspeed is not too restrictive - the EC135 I currently fly has the same minimum IAS in autorotation. The reference to 'current' co-axial machines is intended to differentiate between them and the possible-future rigid-rotor designs like the X2. I'd love to know what the Kamov 'Special Techniques' would be - a bit like the old constant-attitude autorotations we used to do in aircraft like the Whirlwind/ Gazelle, where your sphincter muscles received far more exercise than is good for them(!).
If I may just go back over your latest reply:
- Fixed wing CofG control - especially in an aircraft with active FBW - is a very different kettle of fish from rotary. They can move their CofG over a massive range relative to the Centre of Lift, whereas a heli with just one rotor mast will always hit control stops at extreme 'moments'. The F16's 4 internal tanks is nothing unusual - my old Wessex had 9, all under the cabin.
- I think we can agree to differ here. There is as you say an artistically-deliberate strong resemblance to a Mi28 with Kamov co-axial rotors, but I believe that the aft-tilted rotor mast combined with the SH60/Ka60-style mid-tail-boom undercarriage leg would lead to control difficulties on the ground.
- I don't think I used the word 'instantly'; I mentioned that in autorotation a raised collective 'left uncorrected' would lead to rapid RRPM decay because that's exactly what happens when we flare-check-level at the bottom of an engine-off landing. We trade the energy stored in the head to decelerate the aircraft vertically, knowing that if we mis-time things the landing will get interesting. You are right in saying that the 2 co-ax rotors are linked through the gearbox; the reduction of RRPM in one rotor would only lead to an equivalent reduction in t'other because there is nothing driving that other. In Kamov designs using articulated co-ax systems the yaw pedals are also connected to the vertical control surfaces on the tailplanes to achieve useful autorotative (I stress that word..) yaw control. The fact that they give impressive sideways-flying characteristics under powered flight is a very useful bonus.
But hey, listen to me blather on. What started as aesthetic observations on a minor part of a marvellous PC sim programme has become a bore-fest on helicopter aerodynamics. As Byku pointed out earlier I'm probably whistling Dixie at this late stage in the development of the programme. To add further insult to injury, I rarely use the helis in ArmA/ Op Flashpoint (too much like what we in the UK call a "busman's holiday") - I'm a frustrated wannabe infantryman who's in the game primarily for the ground activity. I really must let the whole Mi48 thing just wash over me, right?
Ta all for reading, and for the feedback. For DarkWanderer, in order of observation:
- If the fuel in the Mi48 isn't under the floor it's behind, ahead of or above the cabin. If one or both of the first 2 there would be considerable CofG changes as fuel was used, if the last then the aircraft would be exceptionally top-heavy and would probably blow over in the slightest wind. Manufacturers generally like to keep motion-lotion as low down in the airframe as possible for these reasons, as well as to stop the troops from complaining when the Iranian-manufactured fuel tanks start to leak through the roof onto their buttie-boxes.
- I agree that this is probably the weakest of my original observations. However, in attack helis like the Mi28 and AH64 the tail-wheel is at the rear of the aircraft. This location would resist rearward movement and would not allow any yaw-roll coupling on the ground. As the Mi48 currently exists, the combination of a short-track undercarriage and aft-leaning rotor head would give a real world aircraft some very real problems.
- Interesting - your first reference describes yaw control when power is applied, using raised collective on one head and lowered collective on t'other to achieve differential torque. Raising collective when the head is under power is fine; the governor or FADEC system will increase power output to that head to maintain RRPM. Raising collective when no power is applied to a rotor will cause a rapid loss of RRPM as drag increases, which will rapidly lead (if uncorrected) to blade stall and extreme blade flapping. My understanding from chatting to the Kamov demo pilot at Redhill in 1998 was that the aircraft still require vertical control surfaces to maintain yaw control in autorotation. However, when I'm back on shift at the Air Ambo on Friday I'll ring one of the 3 ETPS graduates who fly for our company and see what the Test Pilot view is on the subject. I'll try and get back here to post something after the w/end if I've learnt anything apposite.
Byku - I suspect you're right but if you don't ask you don't get, eh!? I hope I don't come across as whingeing about BIS and Arma 3, I think it's a wonderful iteration on a simulation I've been enjoying since 2001. A believable Mi48 would just be the icing on the (already pretty spectacular..) cake.
May 9 2016
Also confirmed using TrackIR 5. My profile for Arma2, which works perfectly in that programme, actually reversed the Z-axis controls in the Arma3 Alpha - I have to lean fully-back to zoom in - slightly weird. Using standard 'smooth' profile reproduces the problem as described by previous reporters.