Building an Airdrome Aeroplanes Fokker D.VII

Part 3: the lows and highs of phase one.

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The pinnacle of aircraft construction is realized when a builder can finally post to a “first flights” section in an aviation magazine or internet forum. These proclamations announcing the conclusion of an often years-long process usually include statements of gratitude to loved ones and others who assisted the builder and too often the worn out phrase, “It flew like a homesick angel.” A builder who has persevered through the challenges of creating a flying machine in his/her garage certainly has earned the privilege of sharing an exciting moment in the culmination of their aircraft construction project and often the climax of a long-held dream.

But how do we respond when first flight reveals a new aircraft that “flies like a sick angel”? While much less common than the successful first flights, occasionally initial tests reveal an aircraft that is in need of serious help. How a builder responds to the challenge of a new aircraft that does not meet expectations, and perhaps even prompts safety concerns, speaks volumes about the determination of the aviator to see the project to a satisfactory conclusion. Such was the case with me and my new Fokker D.VII.

The previous installments of this series of articles have detailed the decisions that brought me to selecting this aircraft as my fifth project and how I constructed N1918Q with refinements intended to match my mission profile. The construction phase was completed in an uncommonly brief amount of time, and I eagerly anticipated seeing my locale from the cockpit of my own WW-I fighter.

Both N1918Q and its builder suffered the indignity of taking apart a new aircraft. Fortunately, enough panels could be removed to allow limited access to the rear spar carrythrough, which needed to be raised to achieve proper wing incidence.

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An Unexpected Surprise

Phase One for an Experimental aircraft is often relegated to “burning off hours” because the builder isn’t interested in a rigorous test phase or the aircraft is a well-mannered iteration of a common design. But within ten seconds of lifting off the runway on the first takeoff of N1918Q, I knew my plane had a major problem. Regardless of airspeed, nearly full-down elevator was necessary to prevent the D.VII from rearing up into an aggressive climb. Something was seriously out of whack.

The plane had been carefully weighed on calibrated scales and the CG was located at the forward end of the recommended range. The engine mount was fabricated with zero vertical and lateral thrust, and all flying surfaces were straight and square and set up per plans. But something was making this Fokker want to climb straight up. Otherwise, the plane flew quite nicely; aileron response was crisp, and the first landing was a feather-like arrival at what felt like jogging speed.

Jigs similar to what was used during initial rigging of the D.VII were constructed for correction of wing incidence. Considerable time was spent carefully leveling all structures to guarantee a straight airframe.

A conversation with Robert Baslee, designer of the Airdrome Aeroplanes D.VII, produced suggestions to raise the horizontal stab leading edge a small amount and reflex the ailerons up a few degrees in order to inhibit lift. But these changes had no effect. I then shimmed the horizontal stab up as far as possible (+1.5 degrees referenced to the fuselage top longeron) and added 1.9 degrees down-thrust to the engine mount. The aggressive climb tendency was still present. I next attached a huge trim wedge to the elevator, but the insistent climb persisted. The plane could not be trimmed for level flight at any speed.

It is one thing to have an aircraft that does not fly as expected, but a much more serious matter to have an aircraft that you feel is unsafe to fly. My safety concerns were based on how a scenario that involved failure of the elevator control circuit would eventually conclude. Conventional aircraft are designed to allow a pilot with failed elevator controls to make a survivable landing using only throttle and pitch trim. But if I was faced with that scenario, the Fokker’s inherent aggressive climb would result in an unrecoverable stall at best or overstressed airframe at worst. Either conclusion is unacceptable.

Fabric on the aft fuse went under the knife to gain access for relocating the top longerons. They were dropped at the tail post, so the horizontal stab incidence could be increased.

So…what do we do? Consultation with the aircraft designer had occurred and there was still no resolution. In an act of desperation and an attempt to shake loose information from the fleet, I posted videos of my test flights on the Airdrome Aeroplanes email list showing the constant down elevator required to hold the D.VII level at all speeds. Off-the-record communication revealed all of the customer-built Fokker D.VII aircraft initially had flight characteristics similar to mine. Various remedies had been tried in an attempt to tame the beast including adding large amounts of engine down-thrust and jacking up the leading edge of the horizontal stab to an extreme.

At this point I must confess I was conjuring images of a hangar queen that would never fill my desire to fly my own WW-I replica. A friend of mine is building a Fly Baby…perhaps he would be interested in the O-200 on the nose of the D.VII. Maybe an aviation-themed restaurant would enjoy a magnificent piece of WW-I art to hang from their ceiling. Yes, I was seriously considering abandoning N1918Q.

After a period of introspection, I stubbornly pushed aside those thoughts and returned to a mental checklist of airframe defects that could prevent the plane from being properly trimmed. CG was within limits, and the airframe was carefully built to plans, and all flying surfaces were straight. The defect that persistently ascended to the top of the list was incorrect wing incidence.

The Fokker D.VII at its best: relaxed, low ‘n slow aviation on a beautiful evening.

Looking Backward

I began researching old biplane technical papers and an interesting specification popped up. Most biplanes have wings with less than 1.5 degrees incidence from neutral, sometimes positive, other times negative. But the D.VII plans clearly called for +2.5 degrees for both wings. This reinforced my suspicions that the plane had too much positive incidence, which made landings very docile but created an aggressive climb. However, the D.VII designer insisted +2.5 degrees is how his prototype was constructed and it flew “hands off.”

A few days after my flight-test videos created considerable discussion on the Airdrome Aeroplanes email list, I received a remarkable call from Robert Baslee. In an apologetic manner he stated an error had been discovered in the D.VII drawings. Another look at his prototype revealed that, yes, it had 2.5 degrees wing incidence, but it was negative 2.5 degrees, even though he never intended for his plane to have that much negative incidence. The reason his plane was flying smoothly was the 2.5 degrees difference in incidence between the wings and the horizontal stab. A plane built to plans had 2.5 degrees difference in the other direction, a huge deviation of 5 degrees. No wonder the poor Fokker was clawing for the clouds…downwash from the top wing was pushing the tail down beyond its ability to maintain stable level flight.

Robert stated he wanted to make this situation right with builders and offered to assist with labor and/or parts to rebuild D.VIIs to achieve an acceptable wing incidence. Because my plane was unsuited for continued flight as it was, I volunteered N1918Q as a test article so resolution of the pitch trim issue could be achieved. We settled on modification of wing cabanes and N-struts to raise the trailing edges of the wings to achieve an incidence of 0.0 degrees. The goal was to squelch the aggressive climb so the plane could be trimmed normally.

Changing the Wing Incidence

Re-rigging a biplane is a huge job, especially on a plane with non-adjustable struts, and the difficulty was compounded because N1918Q was freshly covered and painted. In preparation of the rebuild, the interior of the cockpit and fuselage side panels were removed, and all wings and struts were removed and hung on the hangar wall. Little did I know re-rigging the wings was only the first chapter of a tedious process.

Installing the lower wing rear spar carrythrough is the most daunting step in D.VII construction. This establishes wing incidence, which must be matched in the upper center section, and the location of this part between fuselage stations makes it difficult to align and drill. Executing this operation in a covered aircraft compounds the difficulty immensely. With the Fokker stripped of fuselage panels and interior components, I could, with great difficulty, wriggle my upper body into the rear portion of the cockpit to access the carrythrough attachments. Drilling new locating holes to raise the rear spar carrythrough to neutral incidence was a test of patience and physical dexterity.

The cabanes supporting the upper wing center section were modified by replacing the aft tubes with longer ones, so the upper wing center section rear spar could be raised to achieve 0.0 degrees incidence. While it was necessary to only replace the aft tube in each cabane, the only component in the N-struts that could be left untouched was the forward tubes. Both center and aft tubes were replaced with longer ones. Each of these operations required the wings and fuselage to be secured in jigs and fixtures to hold the entire structure in alignment as was the case when the airframe was initially rigged in my shop.

Unfortunately, flight tests following re-rigging of the wings revealed N1918Q was still severely out of pitch trim. It was obvious further adjustments were needed to increase the relative incidence between the wings and horizontal stabilizer. The practical limit of raising the stab on spacers only yielded +1.5 degrees, and flight tests indicated at least twice this angle was needed. Because there was no realistic means of raising the leading edge of the stab this much, I decided to modify the aft portion of the fuselage to accommodate the stab at +3.0 degrees. This step was not taken lightly because fabric had to be cut away from the tail of the fuse. The plan was to drop the aft attachment of the top longerons at the tailpost 2.25 inches, which would allow the horizontal stabilizer to be set at +3.0 degrees.

After this modification was complete, the vertical fin no longer fit as it should. A large gap between the stab and fin rendered the fin practically useless for contributing yaw stability to the D.VII. The rebuild of N1918Q was completed by the construction of a new vertical fin and application of new fabric and paint to the aft portion of the fuselage.

To state the obvious, this was an extensive invasion of my new aircraft. A builder faced with a similar situation will experience emotions ranging from exhilaration upon approaching first flight to despair when the scope of remediation is contemplated. I was fortunate to have the experience of previous projects upon which to draw, but an airframe defect of this magnitude might sink the dreams of a novice builder. Support of the kit vendor is a major factor determining the outcome when addressing design issues. I was fortunate Robert Baslee was willing to be a stakeholder in resolution of the problem. The importance of researching the history and ethics of kit vendors when selecting a project cannot be overstated.

Success at Last

After an extensive process of wing, empennage, and fuselage reconstruction, N1918Q was airworthy. It is unfortunate the need for modification wasn’t discovered until after the plane was complete, but I am grateful successful results were obtained. Six months after N1918Q received its airworthiness certificate, Phase One test flights could be safely resumed. To my relief I had a Fokker with flying characteristics suitable for both plane and pilot.

The re-rigging of N1918Q resulted in a Fokker that flew hands-off at 60 mph and needed only a light push on the stick when cruising at 80 mph. I was confident my safety concerns were alleviated because it would be possible to gradually fly down to a long runway solely with throttle if something failed in the elevator control. Testing revealed 60 mph is the sweet spot for climb and descent in the pattern as well. I designed a simple adjustable bungee trim assist that attaches to the control stick so all stick pressure at cruise speed can be neutralized without the need for an elevator trim tab.

Let’s take the D.VII for a low ‘n slow flight around the neighborhood and see how this replica behaves. After setting the toe brakes and turning on the fuel pump to verify good pressure, the engine fires promptly when the Sky-Tec starter is engaged. The O-200 settles into its lumpy idle as it barks through the open Aeronca Champ exhaust stacks while the oil pressure gauge comes alive. The view of the taxiway is completely obstructed by the cowl rearing up in front of the cockpit. Releasing the brakes allows the D.VII to roll smoothly down the taxiway under positive control of the RV-sourced tailwheel. It is necessary to use S-turns as we proceed to the runway because of poor forward vision over the nose.

The small drum brakes will only hold the D.VII for a 1300-rpm runup as mags and carb heat are checked. After turning onto the runway, the throttle is smoothly advanced, and within three or four seconds the tailwheel is off the ground. Slight right rudder holds us on the centerline, and the Continental quickly has the Fokker up to 50 mph and ready for flight. If the D.VII hasn’t already committed aviation on its own accord, a slight tug on the stick has the D.VII climbing smartly at 60 mph at 2200 rpm. The ailerons are firm and react quickly when called upon to counter turbulence. Once pattern altitude has been achieved, airspeed climbs to 80 mph with 2150 showing on the tachometer.

You are probably wondering why the O-200 wasn’t spinning any faster than 2200 rpm during full-power climb. The engine is rated for 100 hp at 2700 rpm, but the Culver 74×42-inch prop limits it to 2200 rpm. I didn’t feel the need to run the Continental at full power in an aircraft with an empty weight of 625 pounds, so I ordered a prop that would cruise nicely at 2400 rpm. Apparently the 74×42 is a bit large to allow more than 2200 rpm, but this does not prevent the D.VII from performing very nicely. I like the relaxed cruise rpm and exhaust note—the lower rpm no doubt will lend longevity to the high-time engine. I discussed ordering a new prop with less diameter and/or pitch with Alaina Lewis at Culver Props, but we came to the conclusion that increased engine speed probably would not add significant performance due to the smaller prop disc. I suspect the massive drag of the D.VII airframe inhibits airspeed to about 80 mph.

Flight Characteristics

Once in level flight the cowl drops sufficiently to allow a satisfactory view forward, and the view to the side between the wings is better than I anticipated. I added a small windscreen even though D.VII pilots didn’t have that luxury. The small screen is efficient enough to make goggles unnecessary unless flying to a breakfast in full war paint. The elevator and ailerons are nicely harmonized, but yaw stability is weak. No, it is nearly non-existent. The rudder has satisfactory authority, but if you take your feet off the pedals the Fokker instantly yaws to the left as the slip ball runs all the way to the end of the tube. This lack of yaw stability took me by surprise the first few flights, but I quickly learned to maintain steady pressure on the rudder pedals while keeping the slip ball in my scan. This works well until turbulence arrives, and the tail starts wagging in concert with the wings rocking. Fortunately, I don’t have to worry about preventing a passenger from turning green as we rock, roll, and skid through the bumps while trying to keep up with the elusive slip ball.

This behavior should come as no surprise when we consider the flight characteristics of the original WW-I planes and the minimal surface area of the vertical “stabilizer.” The Fokker is a plane you fly all the time, and I’m OK with that. It doesn’t have unpredictable tendencies, and you learn to work with its design that originated just 15 years after the Wrights’ first flight. If the D.VII was intended for cross-country flights, this would be an issue. But this warbird is best suited for local flights as the perfect sunset is stalked. Besides, 11 gallons of fuel minus reserve at 80 mph won’t allow a patrol and safe return much past the local area.

Steep turns, while keeping in mind this aircraft is not intended for aerobatics, also encourage the pilot to maintain coordination with the rudder. There is an overbanking tendency that can be easily managed with proper combination of rudder and aileron, and when flown properly the D.VII arcs smoothly into and out of turns. The pilot sits far enough behind the top wing trailing edge to allow scanning the sky for traffic.

Landing

Once the perfect sunset has been vanquished, a tug on the carb-heat control and reduced power sets up a 60-mph approach to the aerodrome. I’ve settled on two landing procedures depending on whether a long runway with a shallow approach is available or a steeper, shorter pattern is desired. The shallow approach is flown at 60 mph and when less than three feet above the runway, power is gradually reduced and the D.VII will settle beautifully into a wheel landing at less than 40 mph. Pulling power completely off brings the tail to the ground, and rollout is easily controlled with no nasty tendencies to swap ends as long as the pilot is paying attention. The steeper approach requires holding cruise power until short final, then reducing power to nearly idle. Maintaining 55 mph will result in a very steep descent as all the cables and struts hanging in the breeze, combined with the low-mass airframe, create startling deceleration. The windscreen will be filled with runway, and if the roundout to a three-point arrival is timed properly, an abbreviated landing roll completes the flight. The prudent D.VII pilot will practice this approach in case the need ever arises to execute a landing under unplanned circumstances.

If this description of flying the D.VII sounds similar to what is experienced in most low-mass, high-drag aircraft, that is by design. The pilot with time in a Champ, J-3, or similar Experimental aircraft will possess skills that translate nicely to the D.VII. The biggest difference will be the necessity to pay more attention to the rudder in order to maintain coordinated flight. The Fokker is easy to fly, but demanding enough to fly precisely that the experienced pilot won’t become bored.

I find the Airdrome Aeroplanes Fokker D.VII a satisfying complement to the RV-6. One aircraft is suited for promptly transporting me and a guest to any corner of the country…the other is not. One aircraft is perfect for sampling aviation as it was long ago, while savoring local ambience in an open cockpit…the other is not. The journey to achieve this balance of aviation was not without frustration, but all hurdles were eventually cleared. The Fokker D.VII provides not only a passage into the realm of low ‘n slow aviation, but also a window into the past and how far we have come in the first century of flight.

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