Flight test: North American NA-64 Yale
- Credit: NA
The Yale was originally meant to train scores of French pilots - but ended up in German hands. Flying the only airworthy example in Europe | Words: Dave Unwin - Photos: Keith Wilson
Eeeeeeeee-urk! The rising whine of the electric inertia starter ends abruptly as I pull the red handle, the prop lurches and then starts to rotate as a couple of cylinders fire, stumble and fire again.
The blades turn faster and faster as the big round engine rumbles into life. There’s a Spitfire in the circuit and a Hurricane taxying past, but any truly knowledgeable warbird aficionado knows that the most interesting aeroplane on the airfield today is the one I’ve just started, as it’s the only NA-64 in Europe.
Unsurprisingly, the North American Yale is very closely related to its stable- (or perhaps that should be school?) mate, the Harvard. The test aircraft was one of a batch of 230 built by North American Aviation at its Dallas, Texas factory in 1940 for the French Air Force and Navy, of which almost half had been delivered by the time France surrendered.
And here’s an interesting fact: the Germans probably operated more Yales than any other allied aircraft, as they captured 93 from France and put them into service with 24 different Luftwaffe units.
I first meet it with owner Ian Jones and instructor Neil Oakman at Fenland on a lovely October day. Ian and Neil know how to make an entrance and the Yale looks magnificent as it comes roaring into the circuit with the sun sparkling off its shiny silver fuselage. I am eager to get airborne but there’s a snag. The boys have got to be back to Duxford by 1700, the instrumentation in the rear cockpit is very limited and Fenland isn’t overly large when compared to Duxford.
After a quick discussion, we decide the prudent thing to do is to get the pictures in the can today and for me to go to Duxford and get checked out in the front seat at a later date.
I make a reasonable job of the take-off, fly a fair formation for the photographs... and totally screw up the landing. I do, however, get the chance to briefly assess the control and stability, and soon discover that low-speed flight and stalls are very interesting. With the flaps up, the wing simply stops flying, with no warning and a wicked wing-drop (almost ninety degrees) to starboard.
With half flap there’s a little warning before it rolls to port, and flaps down, it breaks straight ahead. Not ideal traits in a trainer, as quite a bit of height is lost in the recovery?but it must have impressed on students the importance of monitoring airspeed.
Spiral stability seems slightly negative to port and vaguely positive to starboard. Directional stability feels somewhat ‘soft’ while longitudinal stability is strong.
Several months then pass before the opportunity arises to reacquaint myself with this fascinating flying machine and, frankly, I can’t wait. It’s a beautiful Spring day and the Yale really does look stunning standing out on the grass at Duxford. I’m eager to jump in the front seat and go flying, but Neil is already airborne in his L-4 Cub.
While we wait for him to return, I can do what I didn’t get the chance to at Fenland, a proper walk-around. The first thing to strike me is its size. It stands over ten feet tall and weighs in at nearly two tonnes, while the military paint scheme and pugnacious round nose certainly give it a wilful, belligerent appearance. It’s also in fantastic condition.
The heart of all powered aircraft is the engine, and lurking, half-hidden under the cowling is a Wright R-975-E3 Whirlwind, which turns a Hamilton variable pitch metal two-blade prop (not constant-speed, note).
The engine is fed from a pair of wing tanks with a combined capacity of 394 litres. This air-cooled nine-cylinder radial has a swept volume of almost sixteen litres, and if you’re finding the concept of a sixteen-litre engine a bit difficult, consider this: each one of its nine cylinders has more ccs than the entire Rotax 912 engine of the EuroFox I’d been flying a few days previously. In fact, it takes more oil (36 litres) than my Jodel D9 does fuel!
Also used in tanks
During WWII Continental Motors built tens of thousands of R-975s under licence, but not for aeroplanes, as Pratt & Whitney’s very similar R-985 had cornered that market. Instead, Continental-built R-975s powered a variety of armoured vehicles, including the famous Sherman tank, self-propelled guns, and troop carriers, as well as some early helicopters.
The trouble is that the R-975 only puts out a maximum of 420hp, which is not exactly a lot for an aeroplane that weighs a couple of tonnes, and has a fixed undercarriage and a relatively crude two-position prop. Ian observes that “the Yale basically has two gears?first and fifth”.
The cantilever mainplane is of aluminium-alloy stressed skin construction, with a dual-spar centre-section and single-spar outer sections, while the ailerons are metal frames covered in fabric, and the flaps all metal. These are split flaps and extend inboard from the ailerons and across the belly.
The fuselage is constructed in two main parts. Aft of the firewall to just behind the rear cockpit is a welded chrome-moly steel tube structure, covered by aluminium-alloy panels which can be removed easily for maintenance. Behind the rear cockpit, the fuselage is made of aluminium-alloy and is essentially the same type of stressed-skin construction as the wings.
Its training role meant that when in service this aircraft would have to endure its fair share of ground loops: consequently, the wingtips are designed to be easily removed and replaced.
Talking of ground handling, the undercarriage is blessed with a reasonably wide track, and it was particularly interesting to note that the wheel brakes are hydraulic (and even better, toe-operated).
The Yale also has a fully enclosed cockpit, suction-driven blind flying instruments (there are two Venturi tubes on the starboard side) and ample electrical power, enabling it to be fitted with internal and external lighting, radio communication and navigation equipment comparable with contemporary operational aircraft. For an aircraft designed in the 1930s these must have been real innovations.
The tailplane is also of the cantilever type and follows the same design philosophy as the wings in that the moveable surfaces (elevator and rudder) are fabric-covered metal frames while the fin and tailplane are of aluminium-alloy stressed skin construction.
A clever design feature is that the elevators can be removed quickly and are interchangeable. The tailwheel does not retract, but is steerable fifteen degrees either side of neutral through the rudder pedals, beyond which point it ‘breaks out’ and castors.
Preflight complete, it is time to saddle up. It’s a bit of a climb up to the front cockpit but the well-designed handholds and steps ensure that it is not too onerous; although I’m sure it must have been a lot harder with a heavy old seat-type parachute.
The rudder pedals and seat offer plenty of adjustment, and once I’ve set them and strapped myself in, I note that the seat really is quite high up. The tall stick is topped with a pistol grip and is (along with the throttle quadrant) nicely located. In addition, since I find operating some of the controls (such as the carb heat) a bit of a stretch the inertia reel shoulder harness is quite useful.
As I begin to study the instrument panel more closely I realise that it bears little resemblance to the panel of a Harvard or a T-6. When the Armée de l’Air ordered the aircraft, the purchasing mission ‘Franglicised’ the cockpit and installed French instruments and controls.
As a result, the panel of a Yale is quite different to US Standard. For example, the central panel of the Harvard has the classic ‘sacred six’ configuration for the flight instruments, whereas on the panel in front of me, they appeared to have been installed almost at random. The airspeed indicator is in mph, which is the unit favoured by USAAF aircraft of WWII.
Apart from the somewhat unusual layout of the flight instruments, I soon begin to feel quite comfortable in the cockpit, although a real gotcha is the non-standard power quadrant. This has the mixture control between the prop and throttle levers which isn’t ideal, but it could’ve been much worse: when the Armée de l’Air placed their order, it would’ve specified pulling the throttle back to increase power, and pushing the mixture control forward to lean.
Even now there are a few reminders of the aircraft’s original owner, such as the carb heat lever being labelled ‘Froid’ and ‘Chaude’ or the parking brake ‘Blocage Frein’.
All in all, at first glance the cockpit does look a bit cluttered, but closer inspection reveals that it is reasonably well-designed, the various systems and sub-systems all being laid out quite logically?with one glaring exception. The fuel tank gauges are mounted in the floor, making them not very usable.
This design feature has probably caused more than a few accidents, because although the maximum fuel capacity is 394 litres, the rate at which it is consumed varies widely. For example, with the power at METO (Maximum Except Take Off – 32.5in mp and 2,200rpm) it’s a very thirsty 140 litres per hour.
However, pull the power right back to the best economy setting of 20in mp and 1,600rpm, and the fuel consumption drops to around 90 lph. A typical cruise power setting is 26in and 1,950rpm, which gives an IAS of 120mph while burning 110 lph. One aspect of the cockpit that I really like, however, is its ruggedness.
Having spent the previous year flight-testing a variety of lightweight ULAs, VLAs and LSAs, I really appreciate the way that all the knobs, levers and switches (and indeed every aspect of the cockpit) feel very robust, and all the switches click on with a very positive feel.
Forward of the large rotary fuel cock on the port sidewall is a large hand wheel, a red handle and what appears to be a window winder handle. These operate the elevator trim, wobble pump and flaps respectively. The elevator trim wheel has a small notch cut into it, and the trim is at ‘neutral’ with the notch in the eleven o’ clock position.
Wonderful, liquid rumble
Time to ‘reap the Whirlwind’. Neil stands on wing to help as I need to operate the wobble pump, electric inertia starter, mags and primer correctly. As the propeller slowly begins to revolve, a ribbon of smoke curls lazily backwards from the exhaust and with a series of coughs and grunts, the engine growls into life.
As the pulse of the motor begins to throb through the airframe it really does feel like a huge animal is waking up. Starting an engine like the Whirlwind is an experience in itself, and if you have any mechanical soul at all I guarantee you’ll revel in the wonderful, liquid rumble of a big round radial. At one point the engine starts to fade, but Neil has anticipated this and ensured I’ve left the primer plunger ‘out’.
I give it a deft squirt and it rumbles on, so with the motor grumbling quietly to itself Neil straps in and we wait patiently for the oil temperature to rise to at least 40°C. This takes a surprisingly long time, even though it has an oil cooler shutter.
Finally, the temperatures and pressures are all satisfactory, it’s time to taxi out. Although there’s a strong temptation to wave (there’s bound to be a few folks watching) it’s much more important to weave.
That curvaceous cowling blocks a significant amount of the view forward, and it is imperative to slide the canopy all the way back and proceed along the taxiway in a series of sedate S-turns.
Out at the run-up point, I slide the canopy closed and Neil talks me through the pre-takeoff checks. These included cycling the prop at 1,600rpm, and checking the mags, mixture and manifold pressure at 2,000rpm. Yes, I almost pulled the mixture to ICO instead of cycling the prop?but Neil was ready for me and was already holding it.
I suspect it catches quite a few people out, and felt some colour coding might be helpful?but then it wouldn’t be so original.
An important check is to ensure that the fuel is on either ‘Reserve’ or ‘Right’. The ‘Left’ setting is not used for takeoff as it draws from a stand-pipe in the left tank which only allows access to 135 of the 197 litres. The ‘Left reserve’ setting feeds from the bottom of the tank, which gives you access to the entire tank capacity. The ‘right’ feeds from the bottom of the right tank.
The idea was for students to use up the right and left first, and if the engine then quit, they still had enough in the reserve (62 litres) to make it home. Harvards and T-6s have the same system which, unsurprisingly, has caused a lot of confusion and accidents over the years, since the ‘left’ selection offers less than the full content. Interestingly, the South African Air Force modified its T-6s and simply had ‘on’ and ‘off’ settings, where the engine drew from both tanks at the same time.
The flap lever is wound six turns anti-clockwise to show half flaps on the indicator, mounted in front of the flap handle on the left-hand side. The trim wheel is set to the eleven o’ clock position.
Neil reminds me that “it’s basically a Harvard but with 200hp less, while the fixed undercarriage makes it very draggy,” and we’re good to go. It really is a beautiful day, and as I turn onto the grass runway note that the wind is about ten knots at ninety degrees to the runway?but at least it’s from starboard, which will alleviate any tendency to swing.
I slowly open the throttle to 30in mp, check the rpm has stabilised at 2,000, then concentrate on keeping straight while gently picking up the tailwheel. We don’t use maximum power, both to conserve engine life and keep the propeller speed down, and the takeoff is quite a leisurely affair as the Yale doesn’t so much accelerate as ‘gather speed’. There’s a slight swing as the tailwheel comes up but it’s easily held with rudder, the mainwheels skip once or twice and the Yale slides into the sky.
As the airspeed slowly builds, Neil prompts me to keep gently pitching up to maintain 85mph. This will keep the prop at 2,000rpm and the noise down!
I wind the flaps up at 500ft, and then at 1,000ft I pull the prop back and turn crosswind and then left hand downwind for Runway 24R. Usually I’d like to do some upper airwork before circuits, but photographer Keith is keen to get some takeoff and landing shots while the light is good.
To be honest, the first circuit isn’t great.
As Neil had hinted?and more than once?the Yale is quite draggy and bleeds energy rapidly, so I watch the speed, sink rate and aim point like a hawk, in fact to the detriment of basic cockpit discipline. Neil has to prompt me about the flaps, prop and carb heat, and I’d have probably planted it, if he hadn’t given the stick an extra tweak in the flare.
Taxi back for another go, and although the second landing is an improvement on the first, we’ve already ascertained that the bar was set quite low! As Noel Coward might’ve observed “my dear chap, good isn’t the word”.
Powerful and well-harmonised controls
We then depart the circuit and head off to the north-west for some upper airwork, and this soon confirms what I’d learned on the previous flight?the stall is quite abrupt! Apart from that, my initial impressions of the Yale as a fine flying machine are soon confirmed. All the primary controls are powerful and well-harmonised, and any loads on the stick easily trimmed out.
Visibility is excellent and the engine responsive, but perhaps most of all, this is an aircraft that really ‘talks to you’. If you fly slowly, all the cues are there that the aircraft really isn’t happy, and if you fly fast the controls soon firm up, while the noise of the airflow almost makes the ASI superfluous. I know it’s a bit of a cliché, but the Yale is a real pilot’s aeroplane.
Neil is an excellent instructor, and by the time we start heading home I feel much more in tune with the Yale than I’d done thirty minutes previously.
Back at Duxford we join downwind right hand for the ‘warbird’ circuit. This is a much tighter circuit, which suits both me and the Yale. (If you’re unfamiliar with Duxford, the LH circuit sends you a fair way downwind).
It’s a hoary old aviation axiom that good landings come from good approaches, and this is a much better, curved ‘constant aspect’ approach which simply seems more natural. I’m determined to get it right, and my hands dance around the cockpit more confidently: rolling back trim, winding down flap, pushing the prop ‘up’ and the carb heat off while maintaining 80mph and a steady sink rate.
Neil says nothing. Over the numbers and I slowly and simultaneously draw stick and throttle back together, and hold off. After a brief float, all three wheels sink softly onto the grass, there’s a slight skip then the Yale subsides fully onto its undercarriage and rolls out straight and true.
I pull up alongside Neil’s D-Day veteran L-4, run through the pre-shutdown checks and then pull the mixture back to idle/cut-off. As the big prop slows to a stop and the engine huffs over the last few compressions I turn the last few switches off, undo my harness and try to impress a few details onto my memory.
It’s been another memorable flight that I’ve really enjoyed, and it’s not hard to see why the Armée de l’Air and Aeronavale ordered so many Yales. The fully enclosed cockpit, full IFR panel (albeit suction-driven), modern radio and electrics and enough power to run them satisfactorily would’ve made the Yale’s systems comparable with contemporary frontline aircraft.
Finally, and perhaps most importantly, the relatively high-wing loading meant that it had similar handling and stalling characteristics to the Dewoitine D.520 and Bloch MB.152 fighters that students would go on to fly operationally.