The C172 is a well-designed aircraft ubiquitous to aviation. Its superior handling gives it wide appeal, but its range is limited to just four and half hours.
Extending that range is possible as the Cessna 172 is not just a stable platform, but one that is tried and tested having been around since the fifties. There are numerous STCs and strategies to legally increase its range.
As a corporate pilot and flight instructor, I constantly see, first hand how well it does in the instructional environment. But to take it on long flights, there are upgrades and modifications that can be done.
Cessna Modifications for Range
We pushed the limit to see what could be done to use legal modifications to increase the amount of fuel the aircraft could carry, increase the efficiency of the engine, and improve the aerodynamics of the aircraft.
The key here is to increase the amount of fuel that could be carried in the C172 while keeping the payload as close to the original as possible.
As a benchmark, using Best Power settings, the fuel flow on the Cessna 172 is 9 gallons per hour, giving it an endurance of (40 gallons / 9 gph) 4 hours and 24 minutes.
The first modification that needs to be contemplated in increasing the range is the amount of fuel that the aircraft can carry. A standard Cessna 172 can carry 40 gallons of Avgas. Using STC modifications, a C172 can add tip tanks and inboard tanks to increase its fuel capacity.
A pair of tip tanks can add 23 gallons of usable fuel without altering the CG envelope. However, the increase in fuel weight will reduce payload weight since the MTOW of the aircraft remains unchanged.
While It is also possible to add fuel tanks to the fuselage, you would have to think about structural issues. Adding weight to the wings does not cause the spar to flex as much when loaded up.
Lift from the wings pushes up while gravity causes the weight of the fuselage to pull down. Adding more weight to the fuselage causes additional stress to the airframe. It could also cause complications with the CG moving out of its envelope during flight.
Well, assuming no other modifications are added, the extra 23 gallons of usable fuel will give you a total of 63 gallons of usable fuel. At MTOW this will give you 7 hours of endurance at 4,000 feet, leaned for best power.
Exhaust Manifold Modifications
STC exhaust modifications are also a way to increase the efficiency of the stock Cessna 172 engine. The idea of the modification is to increase the evacuation of the exhaust gas from the cylinder so that more fresh fuel and air enter.
By reducing the amount of spent gasses in the cylinder, fresh combustant will provide better combustion and thus more power, requiring less fuel. On a Cessna 172, this has been proven to reduce between 1 and 2 gallons per hour of fuel flow.
With a reduction from 9 gph per hour, set for Best Power, and the added manifold savings of an average of 1.5 gph, the C172 can now burn 7.5 gph with usable fuel of 63 gallons. This gives it a new endurance figure of (63/7.5) 8 hours and 24 minutes.
Without any other changes, the Cessna 172 has now almost doubled its endurance.
Vortex generators are little protrusions on the surface of the wing and fuselage to energize the boundary layer so that the airflow over the wing remains laminar. This has the ability to increase MTOW while reducing stall speed.
The inclusion of the vortex generators helps with the stability of the plane and the ability to carry the added weight without any loss of control surface effectiveness.
It does not directly change fuel flow rates. It does, however, allow the aircraft to fly at much lower speeds without loss of control. This could make a difference with power settings and, therefore, fuel flow rates. Just remember, that going too slow and getting behind the power curve will actually use up more fuel.
Wheel fairings have a significant impact on drag reduction in Cessna 172s. There is a mild weight increase penalty that would have to be taken into consideration. Most wheel fairings will weigh approximately fifteen pounds and increase the aircraft's empty weight. While the MTOW will remain the same, resulting in a lower payload.
However, the benefit outweighs the penalty. For the same Best Performance setting the C 172 will fly 10 knots faster. Adjusting the power setting back to keep it at the same 122 ktas. Fuel flow will now be at 6.5 gph.
The reduced drag results in lower power needed to fly at the same speed. The fuel flow of 6.5 gph results in (63 gallons/6.5 gph) endurance of 9 hours and 42 minutes. Flying at 122 ktas in no wind conditions, the Cessna 172 will be able to fly 1183 nautical miles.
Air Filter Upgrade
Standard air filters serve to scrub the ambient air of dust particles and introduce cleaner air into the combustion chamber. Without them it is likely the cylinder wall could experience damage over time. But that air filter is also the cause of reduced air flowing into the carburetor.
Changing the air filter to a high-flow carburetor would increase the airflow enough to boost the power giving you at least an additional 1% increase in airspeed. This will allow you to throttle back by about 2% of fuel flow.
If your fuel flow with the fairings was down to 6.5 gph, that means you are now at 6.435 gph. While the change seems minuscule, it translates to (63 gallons/6.435 gph) 9 hours and 47 minutes.
High Compression Pistons
High-compression pistons are not a modification, but rather can be done during your next top overhaul. It does not require an STC or an IA sign-off. OS there is no added cost to it if your aircraft is already due for an overhaul.
Increasing the compression can increase BHP output by at least ten horsepower, allowing you to throttle back which will reduce the fuel flow rate by 1 gph.
Launching from the fuel flow rate after the high flow air filters we are now at 6.435 gph. Reducing that by 1 gph gives us 5.435 which corresponds to (63/5.435) 11 hours and 30 minutes of endurance at 122 ktas.
In a no-wind condition that translates to 1400 nautical miles.
Re-Pitch Constant Speed Prop
The standard Cessna 172 comes with a fixed pitch prop that is set for an optimal balance of climb versus cruise. Your POH should tell you the diameter of your prop. On a C172 it is typically a McCauley 76-inch, 53-degree prop.
It may not be perceptible to new pilots or in the winter, but when you are in the middle of summer and the density altitude at your field is soaring, a plane with a cruise prop will have marked difficulty in getting off the runway.
On the other hand, a prop optimized for climb, will not be able to reach its full cruise potential as the props average angle of attack with relative wind is now not as optimal and therefore won’t go as fast.
Re-pitching your prop is not an expensive proposition, but it will allow you to alter its setting to one more suitable for cruise. Doing this will give you a five percent saving in fuel flow rate to achieve the same airspeed.
A 5% saving in fuel flow from 5.435 gph gives you 5.2 gph. This corresponds to (63/5.2) 12 hours of endurance. At 122 ktas, that corresponds to 1460 nautical miles.
EGT and CHT Monitors
Individual cylinder probes to give you a readout of all the cylinders can allow you to set the mixture to better improve fuel consumption and power.
The Cessna 172 can be flown for best performance or best economy. You should check your POH to determine those settings.
The EGT and CHT monitors that you upgrade to, however, will allow you to optimize the fuel flow. This will allow you to possibly save an additional 0.15 gph. If range is the question then every attempt to stay aloft longer, especially with a strong tailwind will give you the additional distance.
About THE AUTHOR
After spending years watching every video I could find about flying, I finally scratched the itch and got my pilots license. Now I fly every chance I get, and share the information I learn, here.Read More About Joe Haygood