Horten Ho IV

After the invasion of the U-K was canceled, Reimar Horten had the opportunity to start on the next project, the Ho IV. Work was started with no official approval in December 1940 at the glider pilot school in Braunschweig, until the school was moved to Koenigsberg in December 1941. The building of the Ho IV continued in Koenigsberg, and the aircraft was completed in just eight months.

The first Ho IV flew in May 1941. After a few low level winch tows, Horten's test pilot Scheidhauer took an aerotow to 10.000 feet and returned after more than one hour in the air. Without official approval, the flight tests continued quietly, with up to 50 flying hours per month.

I had wanted to build a high aspect ratio 20 meter sailplane for a year and a half, when the opportunity came in December 1940.

I arrived in Konigsberg-Neuhausen, and found a large number of soldiers waiting there for their orders, and with very little to do. They were immediately converted to enthusiastic sailplane builders, and soon the first Ho IV was finished, without official approval or knowledge.

Each wing had triple elevons; the outer serving mainly as ailerons, while the middle and inner were rigged like those on Ho III.

Plans for the Ho Vl were made at the same time. This would be a similar aircraft with somewhat higher span and aspect ratio. the main difference would be the T-4 line, which went through the Ho IV center section in the form of a parabola. On the Ho Vl it would bend aft to a point, giving the aircraft its characteristic pointed tail. We would use the same geometry on the Ho IX later for higher aerodynamic efficiency. (The T-4 line is an imaginary line through the maximum thickness point of all the airfoils, from wingtip towing tip, ideally located at 25% of the chord. We had found that the "middle-effect" was determined by the shape of this line rather than the leading edge.)

The first Ho IV flew in May 1941. After a few low level winch tows, Scheidhauer took an aerotow to 10000 feet and returned after more than one hour in the air. Still without official approval, the flight tests continued quietly, with up to 50 flying hours per month.

Stories have been published that my brother Walter was severely reprimanded for secretly building the Ho IV prototype, and later cleared of all charges, when Northrop's flying wing program became known. These stories are untrue, as Walter was flying Me 109s in France at the time. He later became the Administrative Director of our operations.

Both the Ho IV and the Ho Vl utilized the semi-prone pilot position. The pilot's body lay in a 30 angle to the horizontal, and his knees were in a well inside the aircraft keel.

The tandem wheel landing gear used on the Ho II and some Ho IIIs did not endure landings in rough terrain, so a spring suspended main skid was used on the Ho IV instead, along with a retractable nose skid. A droppable wheel was attached to the nose skid for takeoff.

The "praying mantis" pilot position proved to be very comfortable on long flights. The pilot could rest his chin on a soft cushion, the safety harness was buckled on his back, but could be released along with the cockpit cover in an emergency.

Miniature instruments were used to maximize visibility on the Ho IV. The Ho VI had the instruments mounted inside the wing's D-tube. A mirror arrangement was used to read them.

The stick was replaced by a handlebar on both aircraft. The ram's horn-shaped bar would rotate for aileron control, and slide fore-and-aft to move the elevators. Control forces were light at all speeds, since ball bearings were used throughout.

The natural oscillation frequency of the prototype wing was 60/min., 80/min. on the Ho VI, and 110/min. on the later Ho IVs. The wing flutter would reach these numbers at about 80 MPH, when the flight path diversions would match the wing oscillations. One could dampen the flutter by depressing both rudder pedals, and "fly through" this critical speed range.

To establish the performance of the Ho IV, we used a "Kranich" two-place sailplane, which Walter flew, while I snapped pictures. The "Kranich's" performance was quite inferior, so we obtained the use of a "Weibe". Again the results were inconclusive, due to the Horten's superior glide-ratio. Finally we were able to test it against the famous Darmstadt D-30, flown by Hans Zacher. The comparison flights showed a 32:1 glide-ratio at 73 km /h. Under less than ideal flying conditions.

The three additional H IVs were built in Gottingen, and completed in 1943. Two survived the war, and were flown by R. A. F. personnel in Germany and England until 1950. The story of one of these, LA-AC, serial z25, can be traced right to the present.

It was concealed in Gottingen, when Robert Kronfeld approached us in 1945 and obtained a "Lend -Lease" on it, with the understanding that it would be returned to us when soaring was again allowed in Germany. We were surprised to see him ignore the trailer, and load the Ho IV into a waiting DC-3, to be transported to England, instead of his native Austria! The aircraft went through several owners after Kronfeld's death, and ended up in the USA in 1951. The following year, Rudolf Opitz won two major soaring contests with it, and placed seventh in the Nationals, after losing points through a navigational error.

After the contest season, the aircraft was turned over to the Mississippi State University, where it underwent a comprehensive study under the direction of Dr. August Raspet. The results were published at the OSTIV congress in 1960.

The LA-AC has been restored and is at the Planes of Fame museum in Chino, California.

Serial number 33 made several flights in 1945, while number 34 was hidden in Bad Hersfeld, where it was found by the American troops, and shipped to Northrop for studies. Its remains are now in the Smithsonian Collection. It has been shipped to Germany for restoration.

Specifications - Ho IVa

Usage	High Performance Sailplane
Fuselage Construction	Steel tube
Wing Construction	Wood (Metal Wingtip)
Capacity	Pilot
Span	20.3 m
Sweep Angle	20 degrees
Taper Ratio	6.0
Wing Root Thickness	16% chord
Wing Root Depth	1.55 m
Rib Spacing	0.20 m (0.10 at the leading edge)
Wing Area	18.9 m2
Aspect Ratio	21.8
Pilot position	Prone
Mid-section width	1.6 m
Cockpit width	0.8 m
Cockpit height (from seat)	0.55 m
Empty weight	250 kg
Ballast	water
Additional payload	80 kg
Maximum weight	330 kg
Wing loading	17.5 kg/m2
Stall speed	55 km/h