«What a drag it is getting old» (The Rolling Stones, Mother’s little helper)
It is time to indulge. Flying is fun. Even for work, flying is fun. Why not make the best of it?
Every one having owned an aircraft can attest to the fact that ownership of a certified machine provides no financial relief. It does allow access to the sky anytime and hopefully can get you to places you would not think of going by ground, simply because time is always scarce and cars quite frankly do not do well over large bodies of water.
By now, you may have deduced that the personnel acquisition project of a Piper Twin Comanche (PA-30) developed itself from an aircraft to fly and travel to an aircraft optimized for the same purpose.
Optimizing comfort (read improving) became a priority following the many comments voiced by my close VIP (spouse) and fellow pilots. Most of those comments related to this older aircraft’s singular smell (see previous article) and to blood flow within the interface that lies between the front seat and its control wheel. So the interior was recently redone with agreeable success.
Remaining on the optimizing subject, an important aspect aside from flying the aircraft, was flying the aircraft as intelligently as possible. First, by intelligently, I refer to being current (easy) and sticking to flight discipline. Second, I also wished to make the PA-30 as efficient as one could, applying all that flight aerodynamic stuff we were thought years back.
Compromise in designs
I have tons of pleasure just thinking I could have gone with a tail dragging classic to hop around short fields or sand bars on river banks. Optimizing in this case would be merrily to scrub off mud and big bugs from the airframe. All aircraft are a compromise in design. In order to obtain a specific goal, an engineer will have to abandon certain desirable features. Since owning more than one aircraft was not an option at that time, I «settled» for an aircraft with appreciable range and speed. For the record: speed like beauty is always in the beholder’s eyes.
The PA-30 was designed to go faster with relative economy than most aircraft of the day. We are talking 1960s here. To obtain a faster aircraft, hence optimizing, Piper Aircraft design team worked quite a bit on drag reducing observable items:
- No passenger foot step
- « Rakish » windshield
- Snug cabin
- Laminar flow airfoil
- Flush rivets where it mattered
- Retractable landing gear
- Some fairings
The aircraft was conceived in a time when slide rules, large plan paper on drawing boards, pencils and rulers were the norm… 60 years later, when someone desires to improve on the concept, a consummate amount of capital will be expanded to produce perhaps 10% improvement in performance with computer assisted design and composites.
Pampering without going overboard
Happy owners of PA-30 are pretty keen to keep their aircraft efficient if not to improve on this efficiency. I discovered over the last couple of years that there exists quite a market out there for various STCd modifications that will improve the aerodynamics of any aircraft for that matter.
The glider pilot in me was quick to pick up on the subject.
I will not dwell on soporific drag theory. Let’s entertain that most drag issues were optimized in relation to cost during conception. I had figured out as many STC owners, that time could be spent working mostly on parasite drag.
In order to make a reality check prior dwelling on the matter, it must be remembered that drag increases along to the square of speed. The gains in speed or fuel economy will eventually taper off for the same amount of horsepower behind the propellers. One can spend so much, to gain at the end so little. The trick here is to optimize.
This complete « project », not a big deal by the way, had to be measured in order to stay away from anecdotal results or perceptions. The best way was to compile cruising data for every smooth flight as it is accomplished by professionals aboard «bigger» equipment. Once stabilized in cruise, I merely take down IAS, TAS, pressure altitude, OAT, manifold pressure, RPM and fuel flow. When log entries are made a few extra minutes are taken to update the spreadsheet created along with concurrent improvements when they are added to the airframe.
First, this PA-30 was acquired with a few mods already installed. Of course not to be categorized with parasite drag, wings tips fuel tanks were added. The obvious use is for improved autonomy. The real performance enhancement comes from the reduction in induced drag. Think winglets here. The measurable gain in performance from those «tuna tanks» on the PA-30 is improved climb rate and improved payload.
The original engine intakes created quite a bit of parasite drag. So an STC was installed improving the aerodynamics. The standard retractable landing gear for Piper models had them retract almost completely in the wings. Keep in mind that there is no covering door per se over the tires. An STC was installed creating more room to tuck the wheels deeper within.
When the PA-30 arrived in the hangar for the US to Canada importation process, we went through the aircraft in many ways. For the purpose of «optimization» obsolete items were removed.
- Old Loran C and its antenna. Weight and drag: out!
- ADF receiver with its immense football (american style) shaped antenna. Weight and big drag: out!
- Old abandoned electrical system cables. Weight: out!
- Countless old antenna coaxial cables. Weight: out!
- Old hardware for ADF clothesline antenna hanging on the fin. Drag: out!
- Very old belly non-functioning anti-collision light. Drag and weight: out!
By the time the Canadian certificate of airworthiness was issued 13 pounds were recovered on the empty weight, never mind the reduction in drag. It is worth to remind ourselves that a heavier aircraft will indeed be slower in cruise. Those wing tip tanks are an immense feature for this owner. But flying full gross weight allowed, regardless of what the book says, cost 4 kts for the first 2 hours out the 7 available. Fuel consumption is about 100 pounds per hour.
The grandfather for airspeed quest is cleanliness and smoothness. A polished aircraft is a strict minimum! Dead bugs adorning leading edges are a nogo. So much that competition gliders are often equipped with wing leading edge bug scrubbers. Deceased bugs on the aft (black) side of propellers are also important thrust robbing (drag producing) items.
On a normal day, I cannot be seen around any type of ground transportation means, waxing the eventual rust bucket. Aircraft are the timeless exception. Beside providing ultraviolet and oxidation protection to the paint job, wax or equivalents make an aircraft ever so more slippery. Right off the top, one can rehabilitate 1 to 2 KTAS to match a little more closely the AFM section 5 figures. This applies to any aircraft, the bigger they get, the better the improvement. Facing it, a polished aircraft looks great!
An eye-opener for me, lets call it an «observed anecdote» some years ago, occurred aboard my first glider (PIK 20B). The full composite aircraft was then polished to remove any oxidation and every season waxed for performance. At that time a friend of mine had acquired a sister ship, 2 serial numbers ahead. He did not yet had time to apply elbow grease to his PIK. Yet it was very much airworthy, of course. On a late afternoon with no residual «lift» in the air mass, we joined formation to return home. Same aircraft, same speed, on a long 15 nm final glide to the airfield. Wing to wing, his aircraft was distinctly descending in relation to mine: drag.
Make no mistake, a polished aircraft performs better.
Fairings and gaps
Now let’s get back to the Twin Comanche. As the aircraft matured regardless of the fact that it was always hangared, plastic and fibreglass fairings (ahhh composites… Sorry, fun to mention) took their beating. They gradually distorted away from their original streamline. Many flight control hinges were also exposed by design. All these items scattered across the airframe were producing airflow disturbance. Then I asked myself : «Why not tape everything over exactly like we do for gliders»? Believe it or not, a clean 2 KTAS was generated. « Granted taped, over as it is, this Twin Comanche does have a non-conventional, unique look, to say the least. Often when making tech stops away from base I am told by informed observers that by the look of the airplane, I must be a glider pilot.
I also eventually installed landing «gear lobes». They are probably the best bang for the buck in this department of flight operations. These are streamlined devices installed aft from the main landing gear. This reduces buffet creation behind the tire portion and the outboard gear fork sticking out in the air stream.
Gain and effort to a limit
Following many hours and as many cruise data lines, I can safely attest to the fact that this Twin Comanche is solidly cruising 6 KTAS faster than the manual states.
Removing older equipment is not an easy operation. It takes time. «Accessibility» is an oxymoron when dealing with antennas within a narrowing fuselage. Cabling removal and who knows what else can only be done when an interior is being removed. This is not an impossible job and not so complicated by the way. On a 60 years old aircraft treasures abound.
Obviously, this process of reducing drag cannot go on infinitely. At cost of repeating myself, drag does increase to the squared speed. Every humble little knot added creates in very measurable quantity its own drag. Hey, If I wanted to really fly faster, I could get myself a jet. This would be another story!