High speed rotorcraft / Derschmidt rotor system

Helicopters have a problem; they're slow.
We wouldn't accept that problem if there wasn't one strength; the ability to take-off and land vertically (VTOL) and to hover.

The practical limit for dominant layout helicopters is 300 km/h, and decades of experiments didn't really change that.

The V-22 Osprey tilt-rotor is slightly faster, but its cruise speed is still below 500 km/h and it's extremely complex and expensive. It's pretty much a niche design that cannot beat comparably complex and expensive heavy-lift helicopters in practical transport capability.

The helicopter's problem is an extension of the problem that almost doomed propeller aircraft. The propellers of WW2 aircraft had a small radius, but many revolutions per minute - the tips approached Mach 1 and that caused such great aerodynamic troubles that propellers were and are impractical for aircraft faster than 800km/h.
The V-22's rotors have less revolutions per minute but bigger radius, thus limiting its speed as well - even below 700-800 km/h because of the requirement for good lift at take-off and landing. Today's propeller aircraft aren't much faster than 600 km/h anyway.

The helicopter with its horizontal rotor that tilts only slightly to produce forward thrust has an additional problem; its indicated air speed needs to be added to the relative rotor tip speed because the rotor tip moves forward half of the time.

Some of the proposed solutions for the problem had the rotor turned into vertical (tilt-wing, tilt-rotor, tail-sitter) while others reduced the rotor to a take-off and landing component; the most extreme proposals had pusher propellers and stub wings and had the rotor folded in flight to reduce drag.

All these proposals weren't exactly a good solution to the problem, but VTOL is still considered to be extremely desirable and thus we keep researching for solutions.

There's one quite obvious solution left that gets usually little attention in English-language writings about the subject; a third most obvious approach:

Do the same as with the wings; swept wing geometry solved the Mach problem for wings, and it can theoretically do the same for rotors.

I don't know exactly why we don't simply use sickle-shaped rotors, but there's another approach that lends from another solution used by fighters; variable geometry. Variable geometry wings were very fashionable in the 60's and solved some requirement conflicts for aircraft like the Panavia MRCA Tornado.

The corresponding helicopter rotor design was the Derschmidt rotor.
The Derschmidt rotor system was tested in 1964-1966 in the experimental helicopter Bölkow Bo 46 and this system uses rotor angles of up to +/- 40°.
Estimated speed potential of the Bo 46 was 500 km/h, it was tested with up to 615 km/h in a wind tunnel and total potential with additional thrust and wings was estimated at 700 km/h.
A helicopter with such a rotor system might today have the same speed range as a V-22 due to the forward thrust of the gas turbines and retractable landing gear.

The lesson was pretty much that the technology of the 60's couldn't handle the challenge. Both control and materials challenges were too great.

It's a recurring topic in German aviation literature that today's or future technology might be up to the challenge and create a high-speed helicopter.

The present helicopter research is more oriented towards ongoing military helicopter programs and near-term solutions for Eurocopter's success, though.
The political interest in renewed research seems to be small.

Nevertheless, it deserves some attention next to the usual suspects that are being preferred by the Americans like Piasecki's compound helicopters, tilt-rotors, ABC rotors and alike.

Hat tip to aerokurier 2/2009 and The DEW line blog, who both made me finally write about this topic.


  1. Sven,

    I did take your advice and posted information about what I am doing at http://www.secretprojects.co.uk/forum/index.php?topic=6450

    As you predicted this has been a nice place to show this new concept to interested amateurs. Thanks for the tip.


  2. > I don't know exactly why we don't simply
    > use sickle-shaped rotors

    These work great on turboprops because the entire blade disk is moving at the same relative airspeed. That means there is a single angle of attack that is appropriate for any given airspeed. That angle might appear to be quite "steep" from the point of view of a passenger, but remains pretty much constant, in terms of relative airflow, throughout the major flight regime.

    The same is not true on a helicopter, as you note in the article. In this case the forward-moving blades are at a much higher airspeed than the backward-moving ones. In order to balance the lift on the two sides of the fuselage, the angle of attack is dramatically different on either side - as I'm sure you're aware. In general terms, the rearward-moving blades have much higher angles of attack.

    That works fine for a long and thin blade, but if you imagine the situation with a sickle-shaped blade you can see the problem - the blade is being pushed through the air edge-on, and as it is much wider than a normal blade, the drag goes up enormously. Any advantage in lowered wave drag on the forward side is wiped out, and more, by the huge induced drag on the backward one.