Hirth Acrostar details questions

[HoHun]

Hi everyone,

OK, it's not a WW2 aircraft, but the forum seems to cover other topics so well that I thought I'd give it a try: Has anyone details on the Hirth Acrostar?

Specifically, I'd like to know:

What are the control throws for the
- elevator?
- ailerons?
- flaps?
- rudder?

What is the pitch range of the propeller?

What I have concluded from various popular books is that the ailerons and flaps both are moved by the elevator control, with the ailerons making only half of the travel of the flaps. From a photograph, it's also obvious that the flaps can extend above the "zero" position to give the wing inverted camber. Another conclusion from the books is that the flaps also serve as ailerons, and I'd think that the "half of the aileron travel" also applies here.

What I'm not sure of is if there is a way to extend the flaps other than by pulling back on the stick.

Details of the Eppler 467 airfoil used on the Hirth Acrostar would also be welcome - maximum lift coefficient or best lift-to-drag ratio for example would be helpful, but of course a polar plot would be appreciated as well! :-) (If you have the coordinates, that would be almost as good.)

The concept of the Hirth Acrostar was to have an almost symmetrical aircraft that would fly upside down as good as right side up. The ailerons and flaps cambered the wing according to the elevator inputs so that the angle-of-attack of the main wing would change only very little in comparison to conventional aircraft.

I'm in the process of building an X-Plane model of the Hirth Acrostar (or rather converting one that was built for an older version - unfortunately without any knowledge of the special features of the Acrostar), and I've tried to make some educated guesses on the required values, but the resulting aircraft has some rather funny characteristics.

For example, as the main wing does not change the angle of attack much as lift is increased by increasing camber, my Acrostar model cannot effectively be stalled. This seems not right for an aerobatic aircraft.

Another effect is that due to the flat angle-of-attack coupled with the high idle speed of the Franklin engine used by the Acrostar, it's rather hard to slow down for landing, and elevator can not sensibly used to keep down the tail when using the wheel brakes as this causes the flaps to extend and the Acrostar to float up again.

The flying-tail style elevator has to be limited strictly in its travel as obviously, if the elevator stalls at low speed the pilot is going to lose control - if that happens during the landing, he'll lawn-dart immediately. (Don't ask how I noticed ;-)

Any information on the Hirth Acrostar would be most welcome! (I was surprised how little information there is on the internet, considering that it was a highly innovative and groundbreaking type!)

Regards,

Henning (HoHun)

[Graeme]

Hi Henning,

Sorry mate, can't help you. I've a number of early 70's Jane's entries for the Acrostar, but none go into the detail that would answer your questions. In fact I'd have to answer your question with a question. Was there ever a Mk I Acrostar? Everywhere I look it I see the Mk II.

[HoHun]

Hi Graeme,

>Sorry mate, can't help you. I've a number of early 70's Jane's entries for the Acrostar, but none go into the detail that would answer your questions.

Thanks for the picture anyway! It shows the original paintjob of the machine I photographed, which is the same one the X-Plane model I'm converting features but apparently in an older (I presume) variation. Your picture shows the Schempp-Hirth logo on the rudder, while the version I'm familiar with has two different logos: a fairly large one that looks like that of some aerobatic competition, and a smaller one that looks like a white plane (or bird) on blue ground which I couldn't identify. (For lack of bitmap resolution.)

>In fact I'd have to answer your question with a question. Was there ever a Mk I Acrostar? Everywhere I look it I see the Mk II.

This has been puzzling me, too. A friend found a book yesterday that described that Wagner had flown a Kramme and Zeuthen KZ VIII, a small aerobatic aircraft with a conventional asymmetric airfoil which he converted to the Acrostar setup with camber-changing flaps and control mixer. Maybe this proof-of-concept modification was the "Acrostar Mk I", to speculate a bit.

(Attached a photograph of a KZ VIII my friend shot in Denmark this year. Only two or three were built, but the type can be considered the original inspiration for the modern crop of aerobatic aircraft like the Sukhoi and the Extra.)

Regards,

Henning (HoHun)

[Graeme]

Originally Posted by HoHun

Maybe this proof-of-concept modification was the "Acrostar Mk I", to speculate a bit.

Evening Henning!

Quite possibly.

Jane's talks about a "prototype", then a production machine called the Mk II. A later edition talks about an improved MK III. After 1979 Hirth and the Acrostar disappears from any further editions.

Very nice looking aeroplanes, thanks for making me look!

[HoHun]

Hi Graeme,

>Jane's talks about a "prototype", then a production machine called the Mk II.

Hm, it seems that the aircraft I photographed, D-EMKB, is the same aircraft that flew in the aerobatics world championship of 1970, which would mean that it was the first Hirth Acrostar ever produced. It seems to be listed as "Mk II" everywhere, so perhaps the modified KZ VIII was in fact the Acrostar I ...

Arnold Wagner, the Swiss chief designer at Hirth who developed the Acrostar, is currently trying to make a "ball engine" work that is quite an amazing bit of technology. Here is an animation:

http://kugelmotor.peraves.ch/Bilder/Kugelmotor_en.wmv

It is a reciprocating four-stroke engine with a rotating piston. It claims twice the power per crankshaft revolution (like a Wankel engine), but as far as I can tell, this is a bit misleading as the parts of the "ball" piston reciprocate twice per revolution. Still, it's a beautiful machine ... apparently first patented in 1961 by an American engineer called Frank Berry, then re-invented by the Swiss Herbert Hüttlin, from whom Wagner licensed the engine before discovering that Berry's patent pre-empted Hüttlin's. Now Wagner and Hüttlin are both developing their own engines in direct competition.

With regard to my X-Plane model, I made up various wing section polars which all failed to make the Acrostar fly differently from the strange way it does. Of course, it was built for strange behaviour, but I'd still think an aerobatic aircraft should stall more easily ... right now, it's next to impossible as the ingenious control mixer keeps the main wing at almost constant angle of attack.

By the way, if you're interested in aerobatics, here is an extremely neat little program to generate Aresti charts:

Olan - One Letter Aerobatic Notation

Attached a screenshot of a sequence I just assembled :-)

(I really believe X-Plane is a great simulator, but when it comes to aerobatics, I really miss the old Flight Unlimited "1.0" ... no simulator ever came close with regard to post-departure modelling.)

Regards,

Henning (HoHun)

[ROMEO41]

Hola, quisiera aportar los siguientes paginas del manual por si pueden servir.

Un saludo

Antonio

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