Pilot report - Bölkow Bo-207
[Reprinted from Today's Pilot Magazine]
Although it is a 40-year old design, the bölkow 207 is as good an aircraft as any of its contemporaries and better than most. Dave unwin tests this rare teutonic thoroughbred.
BÖLKOW 207 HISTORY
The Bölkow 207 is a development of the Klemm KL 107C, which was actually a strengthened KL 105 with an improved flap system. Consequently, it can trace its lineage right back to 1938, which was when the prototype KL 105 first flew. This aircraft was an interesting departure from previous Klemm designs, which had tended to feature rather angular fuselages, while the 105's fuselage had an oval shape. Despite being a very efficient light aircraft, only six KL 107s were built before production ceased at the start of World War Two.
In the late 1950s an agreement was reached between the supervisory board of Hanns Klemm Flugzeugbau, Klemm Technik GmbH and Bölkow GmbH to produce the Klemm KL 107 under licence. In 1959 Bölkow began producing the KL 107C, which featured a number of improvements over previous 107s. However, in December 1960, the prototype of a further upgrade, the Bölkow 207, made its first flight. This aircraft was substantially different to the KL 107, having four seats, a modified wing that incorporated the fuel tanks, a streamlined windscreen and a more powerful American engine. Series production began in August of the following year and Bölkow GmbH eventually built 91 examples, with production ending in 1966.
Despite the rather dull light so typical of a gloomy September Sunday, something about the big red-and-white taildragger definitely caught my fancy. A quick chat with the Bölkow's owner, Trevor Loveday, promptly set up a meeting and barely two weeks later, this very handsome machine was sinking purposefully towards Sibson's runway. Even before Trevor had disembarked, I was hurrying across the grass to have a closer look at his rather rare aircraft. You are immediately struck by this aircraft's size for a four-seater it really does look rather large. This could be because of its rather stalky undercarriage, which does tend to make the Bölkow look, well, big.
The next surprise was that it is constructed predominantly from wood. The reason I say 'surprise' is that the fuselage is so curvaceous that it almost looks as if it is constructed from composites. In fact, the wings, tailplane and fuselage are all built from organic materials. The fuselage is a wooden semi-monocoque structure covered with plywood, while the wings and tailplane are of the cantilever design. The wings are covered with plywood and fabric and feature plywood-covered Frise-type ailerons. The tailplane is constructed along similar lines, although the elevators and rudder are fabric covered, with pitch trim provided by a variable-incidence tailplane. I also noted that the split flaps lurking under the mainplane seemed uncommonly large.
Stepping back to take in the aircraft as a whole, I was extremely impressed by its looks. Indeed, the Bölkow has a symmetry of line that really is pleasing and is only slightly marred by the over-tall undercarriage. Initially, I thought that this might be for propeller clearance, but closer inspection revealed that several inches could be removed from the legs before prop clearance was really compromised. I was also intrigued to note that it was fitted with a Tost-type release hook for glider or banner-towing duties. Trevor does not use it for either purpose, and while I'm sure that it could easily tow any sailplane produced in the early 1960s I suspect that it would probably struggle with a heavy modern two-seater, particularly from a soft surface.
Access to the cockpit is nothing short of excellent. There is a small step just aft of the wing and also a handle to assist you in stepping up onto the wingroot walkway. The gull-wing doors are enormous and open wide, making it very easy to gain access to either the front or back seats.
The instrument panel and control layout is clearly from an age when the science of ergonomics was still not fully understood. To be fair, it's not that anything is particularly difficult to reach, it's just that it really isn't clear exactly which control you're reaching for! As the photograph of the single row of controls below the instrument panel shows, with the exception of the prop control (which is white and should be blue) every other control is topped with a black knob! While this point is of little interest to the private owner, for someone like myself, who might fly up to four different types in as many days, having the various controls readily identifiable by both shape and colour would certainly make life easier. I was also fascinated to note that the throttle is of the vernier type.
Apart from this, the rest of the cockpit is excellent, with comfortable seats, superb visibility and incredibly light controls. Indeed, the ailerons are so light that they feel almost frictionless. The control stick is also worthy of a mention, being of an unusual semi-circular design set atop a gracefully-curved stick. The biggest surprise awaiting me in the cockpit was the pair of vertically-mounted wheels, located between the front seats. These control the flaps and pitch trim, with the trim wheel situated nearest to the pilot. The trim wheel is pretty big, but the wheel that drives the flaps is nothing short of massive, and probably wouldn't look out of place on the flight deck of a jumbo jet. Located coincident with the trim wheels are a pair of linear position indicators. As it is possible to extend the flaps to a whopping 60 degrees, once the pointer travels past the 45 degree setting, the colour of the numbers changes from white to red. To further remind the pilot that he is carrying a lot of flap, a red warning light illuminates when more than 45 degrees has been set. This is probably a most prudent addition to the instrument panel, as a go-round with 60 degrees of flap set would probably be a manoeuvre fraught with danger. Indeed, much past 45 degrees I doubt if much, if any extra lift is being generated, just a tremendous amount of drag.
The 207 is powered by the ubiquitous Lycoming O-360, which really is one of the 'all-time greats' of the GA world. Indeed, while I was writing this report I received a press release that announced the delivery of the 300,000th horizontally-opposed Lycoming engine and, you've guessed it, it was of the 360 family. Lycoming's first aero-engine, the 200hp Model R-680 of 1928, was an air-cooled nine-cylinder radial that powered a number of military and civilian aeroplanes. In 1938 it began producing horizontally-opposed engines specifically designed for light aircraft. Initially these early engines were only either 50 or 75hp and were both of the 'flat-four' configuration. Today, Lycoming's 'flat' piston engines produce from 115hp to 400hp, and are available as fours, sixes or eights.
The still-warm engine started readily and we were soon trundling towards the active runway. For a taildragger, the view while taxiing is excellent, with only the area immediately under the nose obscured by the cowling. The pre-take off checks are pretty standard, with perhaps the only slightly unusual action on an aircraft of this size being to lock the tailwheel prior to applying full power. Trevor recommended raising the tailwheel slightly as soon as the airspeed started to build, which seemed to rather obviate the need to lock the tailwheel. Upon reflection, the tailwheel lock is probably more important during the landing roll, when the lack of propwash over the fin and rudder makes directional control more difficult. A combination of short, dry grass, the wind on the nose and the fact that we were some way below gross weight, saw the Bölkow off the ground after a commendably short ground roll, and rapidly climbing away with the VSI indicating 900 feet/min. I was immediately struck by two different aspects the aircraft's superb handling and outstanding visibility.
I've already mentioned how light and frictionless the ailerons had felt on the ground, and the application of aerodynamic loads did not compromise this impression. Frise ailerons invariably have a nicer 'feel' than simple 'flat-plate' ailerons, and the Bölkow's are both extremely authoritative and wonderfully light. The elevator is equally light and well-balanced, while the rudder is powerful without being heavy. Control harmony is also very pleasant, with the rudder being the heaviest control and the ailerons the lightest... which is as it should be.
Now, on some aircraft, describing the controls as 'light' can actually be construed as a more kindly way of saying 'twitchy'. However, this is most definitely not the case with this aircraft, in fact, I may have to stick my neck out here and say that the 207 is quite possibly the sweetest handling vintage four-seater I've flown. The roll rate is also surprisingly crisp, and although a hint of rudder was required to keep the slip ball centred in a steep turn, reversing a turn required only a few seconds from 45 degrees to 45 degrees. I was therefore not surprised to learn that Bölkow also produced a dedicated trainer version, the 207T. This model was certificated in the utility category, as being suitable for aerobatics, with the principal design differences over the touring version tested being the installation of a four-point harness, a quick release canopy and an enlarged fin and rudder.
Stability around all three axes was also extremely good, and I think that this aircraft would be very easy to fly on a long cross-country or on instruments. While exploring the stick-free stability I instigated a 10-knot excursion from a 110-knot cruise. This saw the 207 return to the trimmed speed after a single long wavelength low amplitude phugoid. Cutting the speed to explore slow flight revealed equally benign handling, with either flaps up or down. Indeed, with 35 degrees of flap and a smidgen of power, the 207 was distinctly reluctant to stall, preferring to simply 'mush' with a high sink rate. A more energetic stall did produce a proper 'G' break and a very gentle wing drop that was easily corrected with coarse rudder.
Cruising back towards Sibson at Trevor's recommended setting of 2000 rpm and 0.69atm (65% power according to the P.O.H.) saw the Bölkow indicating 110 knots at 3,000 feet for a fuel flow of around 6.5 gph. The view out of the cockpit is nothing less than exceptional, and, unusually, the back seat passengers are also treated to an excellent vista. This is due to the huge doors, which are fitted with a single piece of Perspex and have no framework to obscure the view. Back in the circuit, the easy handling and outstanding visibility was once again very apparent. Trevor recommended using 70 knots on final approach, bleeding back to a 'last look' speed of 65 as we passed over the fence. Having opened the cowl flap, turned on the electric fuel pump and pushed the prop control to fully fine, I asked Trevor to wind out some flap for me. As the flaps were extended there was a noticeable change in pitch, which was easily trimmed out, although I did notice that I ran out of aft trim on final. However, the stick forces remained tolerably light and it is also relevant that our C of G would have been towards the forward limit. Trevor said that with four on board at max weight, a fully-aft trim setting is spot on. Speed control all around the circuit was very easy, and on final, the Bölkow slid smoothly earthwards as if it was on rails. The relatively high seating position caught me out, with the result that the wheels touched fractionally earlier than I was expecting, and produced a low hop. On my second attempt I had adjusted my mind to the slightly unusual (for a four-seater) high sight line, and was rewarded with a smooth three-pointer. At the end of the ground roll it was simply a matter of unlocking the tailwheel and retracting the flaps before taxying back to the parking area and shutting the engine down.
As I'm sure you will have gathered by now, the Bölkow 207 impressed me big time! Not only is it aesthetically pleasing, but it has the ability to carry four people and their baggage over a respectable distance at reasonable speeds, with good fuel economy and a high degree of comfort. It also has the ability to get into and out of most farm strips. Indeed, with those massive flaps, I should imagine that with a bit of practice, it would be possible to land it practically anywhere, although getting it back out again might prove more difficult! Finally, it is an absolute delight to fly.
I asked Trevor if there were any aspects of his aircraft that he didn't like, and after giving the matter considerable thought he did concede that it could be a bit heavy to move by yourself. That's a pretty small price to pay for owning such a beautiful, practical aeroplane.
[END OF REPORT]
Note: PlaneCheck often has some nice Bölkow Bo-207s for sale