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EUROPEAN AERONAUTICAL LABORATORIES — ZAHM 



19 



the same amount of air. The adjustment once made assures, we were 

 told, an air stream uniform in velocity at all points of a cross-section 

 and at all speeds. One would think that a considerable change of 

 speed would require a new adjustment of the cells to maintain uni- 

 formity. Emerging from the tirst honeycomb, the air passes through 

 vertical sheet metal guide blades, each a double sheet and of tttrbin 

 blade form, which turn the stream 90°, without eddies; thence 

 through similar blades giving- 90° more turn ; thence through a 

 much finer honeycomb to remove minor eddies. This last comb, 

 placed just before the test part of the tunnel where the models are 

 inserted, is made of sheet metal strips 10 centimeters wide reaching" 

 from floor to ceiling of the tunnel, and held in position by their 



Fig. I. — Prandtl's Honeycomb in Wind-tunnel 



mutual pressure, comprising among them 90,000 cells. The stream of 

 air issuing from the last honeycomb is said to be uniform, and has 

 a speed ranging up to 10 meters per second. 



The measuring instruments employed are numerous ; but as 

 several of them resemble the ones already described, they need not 

 be noticed. One favorite method used by Prandtl to measure the 

 resistance of a model, say of balloon form, is to suspend it in the 

 current by fine wires, and hold it against stream by horizontal moor- 

 ing wires whose tension is measured in the adjoining room by means 

 of a bell crank and sliding weight. Very accurate measurements 

 can be made without the mooring wires, if the weight and dis- 

 placement of the model along stream be observed, as in my experi- 

 ments of 1902. This method, as extended by Mr. Mattullath, has been 



