What differences would a helicopter designed to withstand many negative Gs have compared with a normal one?

Question

There are a few helicopters that are able to take negative G's, and to this day only one can actually perform aerobatics (ie. Chuck Aaron's BO-105). The BO 105 is a rigid rotor helicopter that has been modified to perform aerobatics, and sustains loads of +3.0/-1.0 G's.

How would a fully articulated helo be different if designed to withstand negative G's? Maybe -3 or -4 G's. What if the helo is a coaxial counter-rotating one?

Answer 1

What differences would a helicopter designed to withstand many negative Gs have compared with a normal one? Maybe -3 or -4 G's

Off the top of my head I'd see the following limitations/problems:

• G-Force resistance: "Resistance to "negative" or "downward" g, which drives blood to the head, is much lower. This limit is typically in the −2 to −3 g0 (−20 to −29 m/s2) range... Negative g is generally unpleasant and can cause damage. Blood vessels in the eyes or brain may swell or burst under the increased blood pressure, resulting in degraded sight or even blindness".¹
• fuel system: more complex and heavy since it has to be able to pump fuel out of the tanks and deliver it to the engines also at negative Gs i.e. when the fuel is floating to the top of the tanks and of the pipes.
• oil and cooling system: the same is valid also for these two systems.
• structure: all the structural parts which normally work "one-way", i.e. mainly in tension or in compression, now can work both ways and have to be designed accordingly, for example the struts connecting the gearbox with the fuselage.
• aeroelastic: the high negative aerodynamic load which has to be created by the blades would bend them downward so much that they would cut through the tailboom and possibly the fuselage; to avoid this a very high mast and/or very stiff blades would be required, increasing weight, drag and vibrations.
• aerodynamic: blades should have to be built around an airfoil possessing good aerodynamic characteristics both at very high positive and very high negative AoA; this might perhaps be obtained but only accepting other aerodynamic limitations like higher drag, vibrations and control loads.

---

¹ https://en.wikipedia.org/wiki/G-force#Vertical

Answer 2

This performance isn't quite that rare. Most heavy combat helicopters (as opposed to lightly-armed recon helos) are also capable of aerobatic maneuvering.

The AH-64 Apache, the Mi-28, and the Eurocopter Tiger are all capable of +3 to +3.5 positive and -0.5 negative g. For the Ka-50, it's also +3.5 g; the negative load isn't stated in any reliable source, but might be in the -0.5 to -1 g range. Coaxials fail catastrophically if their load limits are exceeded, but perform well up to that limit.

If designing a helicopter specifically for aerobatics, one would want less weight and a smaller rotor diameter. So you'd have many shorter blades, to reduce blade flex. The tail rotor would have to be larger, to provide more control. The reason the Bo 105 only has 4 blades is cost.

A coaxial rotor helicopter would also need even more rotor separation than a combat or utility one, since they bend in opposite directions under load. Coaxials are potentially capable of more maneuvers, but their problem is also design cost and complexity - the transmission and the hub are elaborate and very highly loaded.