Hotrod Cherokee

Ian Wright knows this, so to get a quick, reliable, economical and easy-to-fly aeroplane, he started with a completely standard, 6600 hour, 1976 Cherokee 140 Cruiser and fitted every possible drag-reducing modification.

After several years of careful improvement, he now has the smartest, sleekest, swiftest Cherokee in town.

This simple, low power, 30 year-old twoplus- two aeroplane didn’t cost too much to buy, so Ian could afford to invest a little cash on increasing its performance. Today, he has a superb example of what is fast becoming a classic airframe. Better still, his immaculate Cherokee 140 out-performs a standard Cherokee 180, and even rivals the six cylinder Cherokee 235 in the cruise, but at a fraction of that thirsty beast’s running costs.

Of the more than 30,000 members of the PA-28 family eventually produced, over a third

“ This immaculate Cherokee 140 outperforms a standard Cherokee 180, and even rivals the six cylinder Cherokee 235 in the cruise.”

were the original, slab-winged ‘Hershey Bar’ Cherokee 140s. There are probably more than 5000 of these still flying, and second-hand examples should be comparatively cheap and easy to find.

Ian’s 1974 Cruiser 2 Plus 2 (VH-ITW) was a later, deluxe version of the basic 150hp (110kW) Cherokee 140. The short list of extra equipment comprised wheel fairings (spats or pants), a pair of quickly removed rear seats, two additional fresh air vents, an enlarged luggage area and a hat shelf. Otherwise, the only other change from the standard model was a recognisable ‘Palm Beach exterior’, or two-tone paint scheme.

By 1976, even the basic Cherokee had gained a number of improvements over early models, both inside and out. In the cockpit, a neat central power quadrant replaced the original plunger engine controls, a more logical instrument layout became standard, colourcoded plastic rocker switches replaced the earlier simple toggles, and re-settable circuit breakers were substituted for the old onceonly fuses. An autopilot became available, while the pilots’ seats gained more adjustment and inertia-reel harnesses, plus overheadair vents. Even air conditioning came as an option.

Outside, the fin was slightly altered, and the entrance step became standard. Up at the front, bigger rubber inserts in the Dynafocal engine mounts helped reduce the engine vibration being transmitted into the airframe.

Finally, ground handling was improved by increasing the nosewheel’s steering angle and incorporating springs into the system.

Differential brake pedals also became standard for both pilots. You will notice that most of these modifications were small, mostly options that everybody chose anyway, and often little more than cosmetic.

You see, contrary to popular belief, American lightplane companies like Piper and Cessna did not exist to build aeroplanes – their sole purpose was to make money. They did this by churning out as many airframes as the market can take, building them as cheaply as possible, and then selling them for the highest practicable price. Even then, Piper went bust! Because of this purely mercenary motivation, mass-produced aeroplanes are rarely refined or developed as much as they might be in a perfect world. Minor improvements are sometimes incorporated later, but almost never if it will significantly increase production costs.

It is left to smaller after-market companies to devise, develop, test and market improvements and additional equipment under the FAA’s supplemental type certificate (STC) scheme. Each company thus fulfils US certification requirements for its equipment to be installed on individual aircraft on a oneoff basis. Most other countries automatically accept these STCs.

The Cherokee has been popular throughout its 40 year existence, so the list of available airframe modifications for it is long. As well as simple cosmetic enhancements, you might choose from a selection of options to reduce stall speed and improve low-speed handling for short-field operations, or you can choose to concentrate on better cruise speed and economy. Ian researched all the possible options, finally selecting a couple of powerboosting modifications for his engine plus several drag-reducing enhancements for his airframe.

Decorative modifications available include: a one-piece, late Seneca style windscreen without that unsightly central divider, and/or additional windows in the roof. Bob Fields offers improved door seals. R/STOL systems (formerly Robertson STOL) sells leading edge cuffs, stall fences and drooping flapperons for ultra-short strip work. Jim Bradshaw’s Knots 2 U offers drag reducing aileron and flap gap seals that also claim to improve low speed roll control while slightly reducing the stall speed.

Met-Co-Aire makes replacement wingtips, and Laminar Flow Systems markets larger, more fully enclosed and streamlined wheel spats and flap track fairings. I am sure there are many more accessories available, and of course, the range of avionics that could be fitted is almost infinite.

Ian began updating his aeroplane with an interior refurbishment. The glareshield was re-covered, and the seat frames were stripped, re-painted and re-upholstered using fireproof conformal foam covered with blue wool fabric and sporting a dark blue vinyl trim. All the old cardboard backed panelling was replaced with more durable and less flammable aluminium backed panels. New carpet was fitted throughout.

The windows were all replaced, and the whole aeroplane was re-painted. The existing scheme was retained, although a slightly lighter shade of dark blue was used for the highlights, since the original looked nearly black from a distance.

Ian’s selection of airframe mods (in chronological order) started with a complete set of gap seals and wing root fairings from Knots 2 U. This comprised aluminium strip seals for the ailerons, flaps and stabiliser anti-servo tab to reduce airflow leakage, plus enlarged glassfibre root fairings. Large AM R&D vortex generators on the wing root leading edges are designed to smooth out the airflow. Laminar Flow Systems fairings were later added to the flap hinges, and undercut, Hoerner style wingtips incorporating landing lights were fitted. A new slimline tail strobe light completed the ensemble.

Because of this purely mercenary motivation, mass-produced aeroplanes are rarely refined or developed as much as they might be in a perfect world

A large nosewheel spat from Laminar Flow Systems also encloses most of the nose gear’s draggy leg, while the sleek main wheel spats came from Knots 2 U. The undercarriage upgrade was completed with new tyres, brakesand hoses. Other components replaced during this refurbishment included the voltage regulator and an improved modern Concord battery, while copper cables replaced the original aluminium ones.

To counteract the slight weight increases forward of the firewall, the battery was removed from under the back seat to a pre-exsisting compartment in the rear fuselage (as in the Cherokee 180).

When Ian bought his Cherokee, its engine had amassed over 2500 hours – legally running beyond its nominal 2000 hour TBO (time between overhauls) ‘on condition’. By the time he had incorporated all the various airframe improvements and interior refurbishment, those engine hours had risen to 2850, so although it was still running well, it was clearly overdue for overhaul or replacement.

Rather than waste time and money on dismantling, inspecting and rebuilding his old engine, Ian opted to replace it with a less expensive ECI Titan ‘clone’ of the trusty Lycoming O-320. By incorporating high compression pistons, its output was increased from 150hp (112kW) to 160hp (119kW) for no gain in weight, complexity or cost. (He could have fitted a 180hp/134kW) O-360, but that powerplant is heavier, thirstier and more expensive. ) A decade ago, an American company called Power Flow Systems discovered that most manufacturers fitted simple exhaust systems which merely reduced engine noise a bit and kept fumes out of the cabin at minimum weight and cost.

They realised that, with a little care and some tuning expertise, a better-shaped system would release extra power by minimising the backpressure without being significantly heavier. Powerflow now makes exhaust systems for most common production aeroplanes, claiming a measurable performance increase. I have read independent tests that confirm this.

Ian has too, so he opted for a Powerflow short-stack tuned exhaust, which clams to liberate an extra 10 horses (7kW) from his engine – a very worthwhile benefit.

So now his motor might just be producing 170hp (127kW)! A K & N air filter helps even further, by improving breathing, and is claimed to boost performance and increase economy by up to one litre per hour.

To convert all this additional grunt into thrust, Ian had his standard 147cm pitch propeller re-pitched to a coarser 154cm. This gives a higher cruise speed for a given rpm without being so coarse as to spoil the hot weather takeoff and climb performance. He could have gone up to 157cm, and that’s still a possibility.

Although Piper claimed a sea level top speed of 121kt for the Cherokee 140, with a 75 per cent cruise at 7000ft of 115kt, in practice owners rarely saw honest numbers anything like this high.

A selection of Cherokee 140 drivers polled by The Aviation Consumer reckoned the best they ever really got was 95 to 100kt in the cruise. This is not unusual.

Competition between US lightplane manufacturers has always been fierce, and it was long common for marketers to tell be loose with the truth in a country devoid of consumer protection legislation, especially in GA’s boom days of the sixties and seventies.

Inevitably, the after-market manufacturers also make extravagant claims.

Laminar Flow alleges you will gain up to 17 extra knots from its kit, while Knots 2 U offers an improvement of between four and seven knots for its modifications.

In reality, it is impossible to assess any increase in cruise speed, since there are so many variables. Apart from the weather (temperature, atmospheric pressure and humidity all affect performance), inevitably there will be variations in airframe weight and state, ASI and tachometer accuracy, propeller diameter, pitch and condition and pilot technique.

The only true way to assess a performance improvement is to find two airframes with identical full-throttle top speeds at a given pressure altitude and ambient temperature, modify one of them, and compare their performance again afterwards. Using full throttle eliminates most propeller, tachometer, and ASI variables. To get an actual true airspeed means towing a remote pitot/static bomb some distance behind the aeroplane, distancing it from the airframe’s influence on the surrounding air, and then calculating this assembly’s drag and making an allowance for that.

Naturally AA does not have these facilities, so we will do our best to give accurate performance readings while warning that you will have to make your own assessment before incorporating any of these modifications yourself.

From a distance, Ian’s aeroplane looks like any other Cherokee – albeit rather shinier, for he keeps it in gleaming condition.

But, as you come closer, you begin to spot the differences. The first thing to catch your eye is that big front spat with its long upper fairing covering the noseleg’s oleo. Then you realise the other spats are non-standard too. Again, larger than usual, they are carefully blended to the airflow, rather than merely being By incorporating high compression pistons, the ECI Titan’s was increased from 150hp (112kW) to 160hp (119kW).

The new fin top with slimline tail strobe. cosmetically styled.

Beside the nosewheel protrudes the Powerflow exhaust’s tailpipe – very shiny, pointing rearwards, and noticeably bigger than the standard outlet. To see the other engine improvements means opening the cowlings – always an easy job on the PA-28. Inside is probably the cleanest, neatest engine compartment I have ever seen; testament to Ian’s meticulous nature.

Next, you notice the vortex generators – four of them on the upper forward surface of each inboard wing. Quite big and not particularly pretty, nevertheless the vortices they generate should help to smooth the inboard wing’s turbulent airflow at both high and low speeds.

Down at the roots, you can also see the enlarged wing root fairings, which similarly smooth out the root flow to reduce interference drag.

Looking under the wings’ ends you can see the undercut wing-tips. These claim to minimise the draggy trailing vortex. Bending down, you can now inspect the gaps seals. Aluminium strips stretching from tip to root along the aileron and flap gaps, they stop highpressure under-wing air from escaping to the upper surface and reducing the lift/drag ratio, again both at high and low speeds. They should also improve roll response.

I have five aeroplanes, and fitting gap seals has markedly improved their control effectiveness in all axes. Elevator gap seals have also reduced the trim drag on two of them, although I cannot personally measure a speed benefit from any of the aileron seals.

Also below the wings are neat flap hinge fairings. Again, these should reduce drag, although such small hinges set so far aft are likely to be in turbulent flow, so smoothing out that airflow might not show much benefit except as part of an overall drag-reduction program. The last gap seal is under the stabilator, where it closes the gap between that and its trim/anti-servo tab with similar claimed results to the aileron seals. Finally, the fin’s top is new, with a smaller, neater strobe light.

Otherwise, it is a standard Cherokee.

Few pilots haven’t flown one, and those that don’t know the type should take the time to become acquainted. Cherokees are comparatively cheap to hire, easy to fly and perform reasonably well provided you don’t overload them. The little Cherokee 140 helps you avoid this by having no external baggage door and a parcel shelf limit of 45kg.

One snag the whole PA-28 family shares is that single door, so rear seat passengers have to board first, by climbing on to the right wing root walkway via a convenient fixed step. Folding the right front seat’s back forwards allows them to clamber down into the rear footwell.

Then the pilot boards, followed by the right seat’s occupant, who has to slam the door and then clip its upper catch to secure it.

Ian’s interior is immediately impressive – the aeroplane looks brand new, and better than when it left the Vero Beach factory 30 odd years ago. The panel’s gleaming black plastic trim contains all the right instruments exactly where you would expect them. I particularly like the power quadrant with its neat, colour and shape coded levers and adjustable friction. The tachometer is sensibly positioned immediately beside the throttle, with secondary engine and fuelinstruments just above that quadrant, and a row of clearly captioned plastic rocker switches extending away to their right. My only criticism is of the suction gauge, across on the panel’s far right, and not close beside the gyro instruments it serves.

Instrumentation was improved too.

The wet compass was overhauled, a reconditioned Sigmatec artificial horizon was installed and a new Sigmatec DG with heading bug was fitted. A JPI FS450 fuel flow computer and an EDM 700 engine analyser were added beside the tacho. As well as the usual readings, the latter usefully supplies OAT and carburettor temperature.

The last phase of Ian’s upgrade programme was the avionics. He wanted to retain both VHF comm sets plus an ILS, since only a glideslope allows safe operation below VFR minima. He also wanted a GPS with moving map display, and he wished to retain the ADF (much more useful than a VOR in Australia).

However, he needed to cram all these navaids into a single column so as not to spoil his newly renovated panel.

With careful research, Ian was able to find a suite of radios to fit into the existing single stack rather than be spread all over the place. His final selection comprised a KA134 audio panel, Garmin GPS155XL GPS (certified for IFR en route navigation), dual Icom IC-A200 navcoms, King KR87 ADF and a Garmin GTX 327 transponder. They fit exactly into the stack, with no room to spare.

The sole disadvantage of such a thorough renovation is that it adds weight. More paint (consider the weight of a single tin) sturdier fabrics, denser cushion foam and improved interior trim all carry a penalty, as does copper cabling and any additional equipment. Even new tyres add several kilos. Each of Ian’s mods taken alone is quite light, but add up the lot, and you get a significant weight increase, from 630kg – a fairly normal empty weight, despite Piper’s ridiculous 578kg ‘standard weight’) to 670kg.

So, with a maximum takeoff weight of 975kg, this aeroplane will lift two males, full fuel (189 litres) and eight kilos of bits and pieces. Limiting the fuel to ‘tabs fuel’ (up to bent metal strips inside the tanks, which Piper always called ‘standard fuel’) of 136 litres allows 45 kilos of payload. However, the 160hp (119kW) Cherokee was allowed to fly at 998 kilos, so presumably Ian’s new 160hp engine allows this extra 23kg, effectively enabling the carriage of another adult.

Put into perspective, the Cherokee 140 never was a true four-seater, and could only safely take a family of four if they weighed 77, 68, 54 and 45 kg respectively, with ‘tabs’ fuel and nothing else.

Realistically any Cherokee 140 is a good, go-places aeroplane with two adults, full fuel and a pair of 30kg suitcases on the back seats. This one carries Ian and his wife Carolyn with a generous 45 kilos of luggage over a 700nm non-stop leg. For our flight, with ‘tabs’ fuel, we weighed 940kg, or 94 per cent of the maximum.

After a single prime, a couple of switch selections, and a twist of the ignition key, the new Titan motor burst into instant life and settled down to a surprisingly smooth idle, courtesy of careful dynamic propeller balancing and those enlarged engine mount rubbers. I have always admired Piper’s parking brake handle, much neater than the ‘pumpand- pull’ valves of other manufacturers.

Releasing that soon had us rolling along the taxiway. I knew about the springs in this late model’s steering, but I can’t say I noticed them. Taxying was just simplicity itself. Taxi checks, run-up and pre takeoff checks were all standard Piper, but the takeoff roll was anything but.

Most of my flying training was on slab-winged Cherokees like this one, but our aeroplanes had 180 horses (134kW) under their bonnets. On occasion, I rented lower powered examples to fly my family and friends, but I was always wary of their performance. I regarded the Cherokee 140 as a severely limited aeroplane, with sluggish performance at high weights, and strictly a two-seater on a hot day.

Ian’s Cherokee 140 is nothing like the tired old examples I had previously encountered. The moment I opened the throttle it pulled us steadily forward. We hit the 55kt rotate speed within seconds, and after a ground roll of little more than a hundred metres – and that was despite a warm temperature of 27°C! A brand new Cherokee 140, with a finepitched propeller, in the hands of Piper’s finest test pilot, and on the best of many attempts, managed an 244m still air takeoff roll. This example flew in half that distance! Then, the moment we got into the air, several more things struck me. To begin with, this Cherokee was surprisingly smooth and quiet, quite unlike those I had previously flown. Moreover, its ailerons were crisp and responsive – something you could never say about a standard Cherokee’s roll controls. Finally, we were soaring skywards at a timed 840ft/min from 500ft, easily trouncing Piper’s claim of 660ft/min for the aforementioned brand-new Cherokee 140 in ISA at sea level, while also beating both a Cherokee 180’s 750ft/min and a Cherokee 235’s 825 at max weight.

Ian’s aeroplane has so much power that, even with its coarser, re-pitched prop, at full throttle the engine just over-revs in level flight at low altitudes, preventing measurement of a meaning-ful top speed. Even at 4500ft and 116kt (just three knots below Vno), we had to throttle back an inch or two to avoid exceeding the 2700rpm red line, so the propeller probably needs an even coarser pitch yet. Our speed trued out at 126kt TAS, with an indicated fuel consumption of 46 litres per hour.

To get a true indication of this aeroplane’s performance advantage over a standard model 140 or a Cherokee 180, we would have to arrange a fly-off against a representative example, but first that propeller would need further re-pitching to optimise it for the engine’s significantly increased power output. If the opportunity arises, I will let you all know the results.

Meanwhile, reducing power to 2500rpm still gave us 102kt indicated (110kt true) but at a much better fuel burn of 33 l/hr. Ian generally calculates on an average fuel consumption of 30 to 33 l/hr.

The interior sound level was very low, and the vibration was negligible – side benefits of the new windows, improved streamlining and careful propeller balancing. Of course, improved aerodynamics do not just give you a higher top speed. This aeroplane will now have less drag and be more economical at all airspeeds, but because dynamic pressure follows a ‘square rule’, the benefit will show up most towards the upper end of the scale.

If you have a Cherokee 140, by all means compare these numbers with your usual figures, but be aware that tachometer and ASI variations can be surprisingly large, and often greater than the increase provided by such aerodynamic mods.

As well as that top speed increase, Ian’s Cherokee shows a useful reduction in stall speeds. Clean, the warning horn sounded at 52kt, preceding a gentle wings level stall at 47kt. With 10 degrees of flaps set, these speeds were reduced by one knot. Flaps 25 dropped them further to 47 and 43kt, while full flap lowered them still further to 46 and 41kt.

Compare this with Piper’s quoted fullflap stall speed of 48kt and you can see Ian’s modifications give a full 7kt benefit.

Perhaps the most noticeable result of Ian’s work was not the simple performance increase, but the tremendous improvement in roll handling. I well remember occasions in Cherokees during an approach, especially in a strong crosswind, when I have had to use full aileron plus a huge bootful of rudder to right a low level, gust induced wing drop. This aeroplane’s ailerons were very noticeably crisper and more effective than a standard Cherokee’s, even at low airspeeds. I reckon this improvement in low speed roll control alone would make it worthwhile incorporating those gap seals.

Back in the circuit, on a gusty, bumpy, thermally day, that opinion was reinforced by the ease of positioning the aeroplane. While all the flying was generally PA-28 easy, this slab-winged example responded in roll at least as well as the later, taper-winged models. Using a 70kt approach speed and reducing to 65 on short final, and as seen in the accompanying photos, the aeroplane was extremely controllable, right through the flare and hold-off to a nose high, mainwheel landing, despite a gusting, swinging, 15kt wind.

In conclusion, it is much more difficult to assess the benefits of speed mods to an aeroplane than to a road vehicle.

A car is driven directly by its wheels, and its speedometer merely counts those wheels’ revolutions, whereas an aeroplane is hauled through the slippery air by its flailing propeller. Its ASI merely measures dynamic pressure, and is subject to a variety of imperfections, including instrument and position errors, while its readings are dependent upon altitude, temperature and QNH.

Nevertheless, Ian’s Cherokee is certainly faster than it used to be, and quicker than most other 140s. It also clearly takes off shorter and climbs much better, and its roll handling is demonstrably improved. Whether the results were worth the expense, only you can judge.

Whatever your conclusions, Ian is delighted with the result of his efforts and expenditure, and justly proud of his immaculate aeroplane. o