The Mu-15

This was a continuance of the Akaflieg Munich's tradition of cheap to build, practical and light sailplanes with welded steel tube fabric covered fuselage and wooden fabric-covered flying surfaces, which had performances comparable, it was hoped, with the best sailplanes in production at the time. 




Its lightness is demonstrated by its empty weight of 250kg (5501b) and its load carrying capability of 200kg (4401b). This was creditable for a 19m span two-seater which achieved its best L/D of 1129 at 80k/ph, and its min. sink of 0.7m/s at 65k/ph. This was a considerable speed performance at the time, considering its lightness. However, because of its 4401b load, its wing loading was relatively high, being 23.95kg/m2 (4.911b/sq.ft). At the time wing loadings for high performance cross country sailplanes were about 411b/sq.ft. The Kranich's wing loading was only 3.91b/sq.ft, and this was considered a fast glider then. 

Practicality was shown by its quick and easy rigging with no loose pins and automatic coupling of ailerons and airbrakes. Unusually, its wing trailing edges were in four sections. Whereas the outer section was uniquely an aileron, and the inner section uniquely a landing flap, both midsections could be used as ailerons and landing flaps. The fuselage was fitted with a retractable landing wheel. 

When was it designed and builtu As the Mu-17 was designed in 1938 and took part in the February 1939 Olympic sailplane trials in Italy, it is reasonable to assume that Ludwig Karch, father of the Mu-17, took a considerable hand in its design. The Mu-15's fuselage is similar in many ways to that of the still flying Mu-23 motorglider, and also the view from the back seat, whilst not good forwards, is certainly adequate sideways. 

How many Mu-15s were builtu The Mu-15's tail surfaces were later very much altered, its tailplane resembling very much that of the Mu-17. Was this a second prototype... or the first, much modified? 

'Das Schiebrollmoment" by Heinrich Schumacher (Schuster). Translated by Chris Wills from "60 Jahre Akademische Fliegergruppe Munchen" 

In the Autumn of 1941 I was ordered to go from the DVL (Deutsche Versuchsanstalt fur Luttfahrt - German Experimental Institute for Aviation) at Berlin Adlershof, where I was assistant at the Akaflieg, to Munich. Although the group was not very large, they regularly managed to fly from Prien when the weather was fine. 

Benno Kullarzt Fritz Schloegel, Paul Hackert ... names that come to me still, and others, pushed our machines to the start with the enthusiasm of old, which still happens now. So it was on that clear, frosty winter's day. The airfield was covered with a mantle of snow. We had as a visitor an Italian, an assistant at Milan University, who wanted to learn about our Akaflieg, and wished to fly our Mu-15 himself, and I was to fly with him as ballast in the rear seat. 

The Heinkel Kadett towed us off in the direction of the Chiemsee across crackling ice, and soon the lake was beneath us. My pilot seemed to be doing well. Only when we turned back, climbing towards land did he become more and more nervous. He ruddered the aircraft left and right, the cable hanging loose one moment and then becoming taut with a jerk the next, and then ... and then before the cable broke, I released it. I called out that I would now fly. At some distance, dead ahead, lay the village of Rimsting, and our height was sufficient, with the following wind, to get back to our airfield. Therefore I initiated a left turn. 

At this point I must mention something about the flying properties of the Mu-15. Aileron effectiveness was as good as nil!! However, the over-large all-flying rudder with its excellent push-roll moment enabled elegant turns to be achieved.

 
Mü-15

Out of habit, one also used aileron. Therefore, rudder É. Nothing happened. Rimsting came ever nearer. Still we kept going straight on. Donnerwetter! ... what nowu Rimsting within gliding distance dead ahead. Would use of full aileron in a stall bring about a turnu That didn't work either! This damned foolproof Mu-15 profile! There was only strong sink straight on... height 50 metres. Where can I emergency landu On the house roofs or pancake in front of the church toweru 'Drill for petrol!" is an old pilots' saying, so shove the nose down. Above was the thick canopy of a tree. Pull out over an orchard. Little trees gave way before us. A thicker tree came up and bored in crackling beside the fuselage and the wing, bringing us to an abrupt halt ... three metres from the first village house. At last, for the first time I drew a deep breath. "Hello, what about you in frontu" Gentle groans and sounds of undoing of straps. With some haste , the canopy was removed. Out of the house rushed the inhabitants from their lunch, serviettes at their throats. That was something! Superficially, there was nothing wrong with us. Our Italian had lost a few buttons from his winter coat, which we sewed on. That evening, of course, it was necessary to celebrate our "birthdays". 

But what had happened to the Mu-15u The leading edge D-box was considerably damaged. The port wing had come adrift from its rear fuselage fitting and was therefore swept forward. The fuselage was undamaged, but behind, the rudder had gone ... which had been obvious during the last minutes of the flight. 

Meanwhile, calling loudly, our team came from the airfield and were amazed at the Mu-15 and our undamaged state. They had observed the rudder failing from the aircraft and were able that afternoon to fish it out of the lake with a motorboat. The rudder was then hung up in the Akaflieg's rest room for some years. (Was this the end of the Mu-15u C. W.) 

And now my explanation. Only a single safety pin secured the all-flying rudder on two studs. During the takeoff, possibly due to the shattering ice on the ground, the safety pin came undone, and in flight it worked loose, and during the slight pilot induced turbulence, it came out altogether. And so I was left without that rudder and its fine push-roll moment." 

It is obvious that the Mu -15 was intended as a possible Kranich replacement. The Kranich 2 was then in general use for training and high-performance flying, but it had very limited visibility from the rear seat. 

It is worth comparing their statistics. The figures in Table 1 should not be taken as gospel, since most Kranich 2s weighed considerably more than 561lb empty. 

The Mu-15 was in theory a larger, faster, higher performance sailplane than the Kranich 2. The Mu offered more visibility from the rear seat. Visibility from the rear seat was the Kranich's failing. However, the Kranich 2 had been well-developed since 1935 by DFS and was in large scale production. It was popular, a delight to fly, and had broken international records. 

The Mu-15 may have been a good attempt to better the Kranich 2, as the Mu profile has proved to be better at higher speeds when used in the Mu-23 and SF25 Falke motorgliders. However, the Mu-15 ailerons were ineffective. One wonders whether the long wing was torsionally stiff enough to have stood more of the trailing edge being used as ailerons. One can understand why on such a relatively fast ship of the time there was a wish to improve its low-speed performance with flaps. 

As it was, the Mu-15's aileron response was so poor, that when threatened with a landing on a village, an attempt was made to turn with opposite aileron, which should have stalled the wing and caused it to drop in the direction of the intended turn. But the foolproof, almost unstallable Mu profile did not stall. With the large rudder missing aft, the C. of G. would have been further forward than normal. 

It is clear that the war stopped further development, but we believe that something of the Mu-15 lingers on in the Mu-23.

 

 

Mu -15

Kranich 2

Span

19m

18m

Aspect ratio

19.3

14.3

Length

8.2m

7.7m

Empty weight

260kg (572lb)

255kg (561lb)

Load

190kg (418lb)

180kg (396lb)

Wing loading

24kg/sq.m (4.92lb/sq.ft)

19.15kg/sq.m (3.93lb/sq.ft)

Max L/D

1/29 at 83km/h

1/23.6 at 70km/h

Min sink

0.7m/s at 72km/h

0.69m/s at 45km/h