A Question for the Experts in Helicopter Design

[Richard E]

Gentlemen

I'm contemplating a possible WHIF project based on an in service helicopter which has raised a couple of credibility questions:

If I extend the length of a helicopter's cabin would I need to insert two separate plugs, one either side of the main rotor blade, to ensure that they remained on the aircraft's centre of gravity?

And, if I increased the aircraft's overall weight, would it need longer rotor blades to cope with the additional weight or possible increase in engine power?

[rotorheadtx]

As for changing the length evenly, that is not necessarily true. IIRC, Carson helicopters modified the Sikorsky S-61 to the "shortsky" configuration by removing a 1.3 meter section behind the cockpit, but did nothing to the aft fuselage.

When Sikorsky lengthened the Sea King airframe to the S-61N, the fuselage was stretched both fore and aft of the main rotor, but I do not believe the added sections were of equal length. (I'd have to go digging through my references to back that up... but I'm fairly sure I have it correct.)

As for lengthening the rotors, that may just come down to aesthetics of the model... if it looks goofy with the original blades, make 'em longer.

In most cases where real helicopters have gained stronger power-plants, adding additional blades is the de rigueur solution (Bell 206 -> 407, 212 -> 412, 222/230 -> 430; Hughes 500/OH-6 4 blades, then 5 on MD-530, now 6 on AH-6M, etc.)

Edited October 18, 2012 by rotorheadtx

[Danny]

Alternatively, depending on the size of the helicopter, why not replace the blades with something like a Carson blade (which I think is fitted to Apache's and possibly Sea Kings) or BERP blades found on Merlins and Lynx's?

Danny.

[Bell47man]

Hi,

If we take a typical helicopter design which happens to be my all time favourite the Bell 47G as an example. In its standard config it had the battery in the boom, when the engine was uprated and longer blades fitted the boom had to be extended to miss the tail rotor, the battery then moved into the cabin in front of the pedestal to balance things out.

I fly RC helicopters and balance is very important.

Normally helicopters balance on the mast, hope this helps.

Tish

[Test Graham]

Remember that if you are balancing by adding distance between the main rotor and tail rotor, you are increasing the effect of the tail rotor. Given that a larger helicopter will be heavier and thus require more powerful engines to do the same job, then this may be desirable anyway.

[Piltdown Man]

I helicopter has to near enough be in balance when lifted by its rotor head. Bear in mind that what you can't see in the real thing is the location of the engines, batteries, avionics etc. which the designer uses to keep the CofG within tolerance. And yes, a bigger helicopter means more power, bigger gearbox and bigger and/or more rotor blades to absorb the increased power. Increasing the length of a blade is often quite difficult because of tip speeds (both of advancing and retreating blades).

PM

[pigsty]

Increasing the length of a blade is often quite difficult because of tip speeds (both of advancing and retreating blades).

Just what I was thinking. The other route - increasing chord - isn't perfect but it actually reduces speed-related problems, by decreasing the thickness/chord ratio. But the most used solution does seem to be just to add more blades - as in the UH-1, the CH-53 and the Mi-6/26.

[Flankerman]

A couple of years ago, I built two stretched Mi-8 Hips - using the Hobby Boss kit....

I wrote an article describing the conversion.......

With the Mi-8MT helicopter successfully employed in operational service, the Kazan branch office of the Mil OKB was tasked with looking at ways of improving the design.

In 1978, two Mi-8MT airframes were set aside for conversion – the first stage of which consisted of increasing the cabin volume by the simple expedient of a fuselage stretch.

This was achieved by the insertion of two plugs fore and aft of the CG – one of 500mm (1ft 7in) in the forward fuselage and another of 680mm (2ft 3in) at the rear.

This cabin stretch increased seating capacity from 28 to 36.

The helicopter’s flight instrumentation and avionics were upgraded at the same time – although the engines, power train and rotor system remained unaltered.

Designated as a Mi-18, the first prototype took to the air in 1979, but as soon as armament testing with external outriggers and stores pylons began, it soon became clear that the ‘quick fix’ approach had not worked – the Mi-18s structural rigidity had been reduced and its vibration characteristics had increased to an unacceptable level.

Note that the despite the stretch, the engines, power train and rotors remained the same.

Mil then had another go at stretching the Hip.....

Following the failure of the first prototype Mi-18 stretched Hip; the Kazan branch of the Mil OKB took a second Mi-8MT airframe (c/n 93114) and designed an improved airframe to eliminate the structural rigidity and vibration problems of the first prototype.

This new design had no forward fuselage stretch, but did incorporate a longer aft fuselage stretch – of 1,020mm (3ft 4in).

At the same time, the fuselage was deepened and the fixed undercarriage was replaced by a retractable type – with the nosewheel retracting into the deeper front fuselage, the mainwheels into sponsons attached to the rear fuselage.

The external tanks of the Mi-8 were re-located under the cabin floor – as was the cabin heater.

The new, structurally stiffer fuselage allowed the Kazan engineers to incorporate a sliding door in the starboard fuselage side, reducing disembarkation time.

The prominent Doppler box under the tailboom was deleted – the antennas were mounted flush inside the tailboom.

The new Mi-18 made a successful first flight on 28 April 1984 – wearing a two-tone green camouflage. It was later given the tactical code ’84 Red’ and showed a marked improvement over the standard Mi-8MT and it was proposed to launch production at the Kazan plant in 1985.

It's a bit of a long-winded answer - but, essentially, Mil produced two stretched Hips - without increasing the rotor size.

The first stretched Mi-8 ended up in Afghanistan - as a Presidential VIP Mi-17..... (bottom model)

The second was re-designated Mi-18 and still exists in the museum at Torzhok..... (bottom model)

Ken