Aircraft Engine Conversions

The Prime Directive

Few would disagree that Reliability is the primary factor when evaluating an aircraft powerplant. However, once a powerplant has been identified and determined to be acceptably reliable, there are some very different criteria forEVALUATING a conversion using that powerplant.

The INSTALLED WEIGHT of any proposed powerplant conversion is probably the most important consideration in determining the suitability of a particular engine change program, from the standpoints of:

1. the safety of the aircraft,
2. the flying characteristics of the aircraft,
3. the engineering and testing required to obtain an STC and
4. the marketability of the result.

These considerations are especially critical in multi-engined aircraft which have the engines cantilevered off the wings.

If you are considering an engine swap on an experimental aircraft, the arguments presented here are just as valid. The laws of physics apply equally to certified and experimental aircraft. With an experimental aircraft which is under construction, you are not required to obtain an STC. HOWEVER, you, as the designer / builder / A&P of the aircraft, become solely responsible for the safety and soundness of the installation. Those are the very issues which certification addresses.

Here is the short version:

If the proposed new powerplant installation will be significantly heavier than the old one, no matter how much more power it has, the swap is probably not a good idea, and the cost / effort to obtain an STC (if required) is typically not justified.

Here is the reasoning behind that statement.

1. In any program which undertakes to modify an existing aircraft, it is (usually) of paramount importance not to do anything which will detract from the flying qualities and performance of the aircraft. That is simple logic, and applies to any aircraft, but it is most critical in the case of a certified aircraft for which the program goal is to obtain an STC for the modification.

To that end, it is very important that the modification retains the aircraft empty CG in essentially the same location as before, and does so without the addition of ballast, and without moving large amounts of weight toward the tail, thereby increasing the moment of inertia in the pitch and yaw axes.

2. Aircraft range and payload criteria are very important determinants of aircraft value. If the empty weight of the aircraft is increased by an engine change, then either the aircraft payload will decrease by that amount, or substantial additional costs will be incurred to increase the certificated maximum takeoff weight (and probably maximum landing weight) in order to restore the payload to its original value (or to at least reduce the loss of payload the engine weight increase caused.

Generally speaking, it can be very difficult to obtain a gross weight increase in a previously certified aircraft without having significant assistance and support from the holder of the type certificate.

In the experimental aircraft world, it is commonplace to simply declare a gross weight increase. But such a decision flies in the face of logic, reasonableness and sad experience. To arbitrarily declare an increase in gross weight without doing the engineering and testing to validate the declaration is, quite simply, foolish. It is a strong indication that the designer / builder graduated from Wishful Thinking School of Engineering.

It is sometimes claimed that the greater single-engine climb capability provided by a more powerful engine will enable an easy increase in gross weight. However, the single-engine climb criterion is not the only one used to determine gross weight. Other considerations such as wing loads and landing gear loads must be taken into account, as required by FAR's including, but not limited to:

23.333,   23.335,   23.337,   23.341,   23.441,   23.443,   23.479,   23.481,   23.483,   23.485,   23.493,
23.499,   23.507,   23.509,   23.641,   23.723,   23.725,   and   23.726.

The argument that a gross weight increase can be easily obtained by virtue of "similarity to another heavier and / or more powerful model " is flawed as well. If the allegedly similar aircraft has a significantly higher gross weight, chances are it has turbine power. A turbine engine will be MUCH lighter than a liquid-cooled piston engine with similar power.

Further, if the proposed retrofit engine package is much heavier than the original engine, the greater overhung moment of the new installation will lower the resonant frequency of the wing in both torsion and bending.

If the new installation produces a substantially greater overhung moment than the old one, it will likely be necessary to significantly strengthen the aircraft structures to which the engine mounts attach.

In order to rely on similarity, the burden of proof remains on the seeker of the increase to prove that the structures are the same, and that the loads are similar or less. Without access to the type-certificate holder’s engineering drawings and data, that proof could quickly become economically unfeasible.

Further, if the aircraft gross weight increases significantly, it is likely that dive speed (V_d) will be required to increase as well. That has further ramifications, covered later under structural issues.

Evaluating A Conversion

As you read this page, please keep in mind that one of EPI, Inc.'s primary specialties is Aircraft Engine Conversions.

Also keep in mind that two of our primary product lines are Propeller Reduction Gearboxes and Liquid-Cooled V8 Aircraft Engines.

Yes, we like doing engine conversions.

     And yes, we like to sell engine and gearbox systems.

when you apply the cold, hard engineering realities, it turns out that most of the time, replacing a Lycoming or Continental air-cooled aircraft engine with a liquid-cooled piston engine is a bad idea.

That being said, there are a few cases where it is an excellent choice. If you are convinced that you have decided on a sufficiently-reliable powerplant to replace the existing one, here are some thoughts to help decide whether to go further.

1. When you do a true, accurate, apples-to-apples comparison of the total, firewall-forward weight of a complete, flying Lycoming or Continental powerplant against that of a complete, flyable liquid-cooled powerplant of the same or greater power (real, measured SAE Horsepower, not "BlantonPower"), the liquid-cooled installation will almost always outweigh the Lyco-Nental by a considerable amount. There is a table at the end of this page showing details supporting this statement.
2. NEXT, the CG of a V8-Gearbox powerplant will typically be quite a bit farther forward than the CG of the air-cooled powerplant.
3. The combination of the greater weight and greater overhung moment means that the engine mount structure of a V8-gearbox powerplant will typically be heavier than that for the air-cooled engine you are replacing, because it has to support (a) a larger weight (b) whose CG further from the firewall. (There is a short discussion of mount structures later on this page, and a complete page dedicated to engine mount design.)
4. The combination of greater weight and greater overhung moment applies proportionately larger tensile, compressive and shear forces to the support structure behind the firewall to which the engine mount attaches. MOST airframes (fuselage / nacelle structures) are not strong enough to support these larger forces. In fact, in most conversions we have done or assisted with, significant reinforcement of the airframe was required.
5. The greater weight of the powerplant and the structural additions to the airframe to support it will increase the empty weight of the aircraft, which will either (a) reduce the payload (if you understand dynamic airframe loading) or significantly reduce the ability of your aircraft to survive in-flight maneuvering loads if you DON'T understand dynamic airframe loading and arbitrarily decide to "DECREE an increase of the gross weight".
6. The combination of greater powerplant weight and further-forward powerplant CG location will move the empty CG of the aircraft further forward. You will have to compensate by moving the wing forward or by moving some other masses rearward to re-establish the proper relationship between the aircraft CG and the wing center of lift.
7. Unless you move the wing forward (not usually a simple task, nor one without other potential pitfalls) the adjustments needed to bring the aircraft empty CG back into range, combined with the more-forward CG of the powerplant, will increase the Mass Moment Of Inertia (mmoi) of the aircraft about the pitch and yaw axes (AND, in the case of a twin, around the roll axis as well). That change alone can have a dramatically undesirable effect on the flying qualities of a previously pleasant-flying aircraft. It reduces the stabilizing influences of the rudder and horizontal stabilizer on the aircraft. It makes the aircraft less responsive to control inputs, (more "sluggish"). It will reduce the resonant frequencies of the aircraft about the pitch and yaw axes (and roll in the case of a twin). That change in resonant frequencies can introduce some nasty dynamic behavior in flight, such as pitch or yaw instability.

Several years ago, we worked on an engine conversion project on a popular piston twin, in which the goal was to STC the replacement of the aged and overtaxed Continental GTSIO-520 engines with a pair of certified, liquid-cooled Orenda-V8 engines, which had substantially greater takeoff and cruise power.

The Experimental-R&D prototype aircraft was impressive to fly. The takeoff, cruise and engine-out performance of the prototype with 1200 HP on tap were stunning, especially if one was accustomed to the performance of the unmodified aircraft. However, the installed powerplant weight was a major engineering flaw which doomed the project.

The total, firewall-forward weight of the original GTSIO-520 ( engine, gearbox, prop, turbo, heat-exchangers, mount, cowling, baffles, ductwork and plumbing ) was about 930 pounds. The comparable firewall-forward weight of the V8 installation (absolutely apples-to-apples comparison) was slightly over 1260 pounds, which added about 660 lb. to the empty weight of the aircraft.

That increase in the aircraft empty weight had a number of undesirable side-effects, including:

1. The wing-structures to which the V8 engine mounts attached were unable to safely support the substantially higher loads imposed by the V8 engines;
2. The large increase in nacelle overhung moment reduced the resonant frequencies of the wings in torsional and flapping modes;
      oh, and, BTW,
3. The resulting aircraft was a functionally-useless "full-fuel, zero-person" aircraft.

The project manager DECREED that this huge weight increase was not a problem, because, he claimed, he could easily obtain, as part of the STC, a gross-weight increase based on the claimed "similarity" of this model of the aircraft to a turbine-powered model which has a certified gross weight approximately 1000 lb. greater, and which has substantially lighter (TPE-331) engines.

An unfortunate snag to that decree was the fact that the allegedly "similar" aircraft was very DISSIMILAR in the wing structure (to which the engine mounts attach). And there were other serious snags, including the ones described in (a) and (b) above.

This was a prime example of a "don't confuse me with facts - my mind is already made up" project.

COLD HARD NUMBERS

Admittedly, the conversion described above was an extreme example. But look at the following table, which shows representative comparisons of the installation weights of several Lycoming engines against the lightweight (for a Liquid-Cooled-V8), normally-aspirated, 505-HP EPI Gen-1 Aircraft Engine.

NOTE: A powerplant based on the GM Gen-4 engines ( LS-2 / LS-7 ) on an apples-to-apples comparison, weighs about 25 pounds less. 

The weights listed in the table are generally from measured components being used on actual aircraft. The Lycoming weights are direct from Lycoming documentation, verified from hands-on experience. AND, this is not an exhaustive list. It shows only the major items.

PLEASE NOTE: The Gen-1 V8 in the chart below DOES NOT include any turbocharging considerations. To estimate a turbocharged V8, add 60-90 pounds to account for the weight of (a) the turbocharger and wastegate, (b) turbo support structure, (c) intercooler, (d) inlet plumbing and miscellaneous other stuff.

Item	500 HP EPI Gen-1-V8	200 HP Lyc IO-360-C1C6	260 HP Lyc IO-540-D4A5	300 HP Lyc IO-540-K1E5	425 HP Lyc TIGO-541-E1A
Engine (complete)	424.0	293.0	402.0	447.0	702.0
Reduction Gearbox	86.0				included
Air Cleaner	4.9	3.0	3.0	3.0	3.0
Inlet Air Duct	2.1	2.0	2.0	2.0	2.0
Alternate Air Mechanism	3.5	3.5	3.5	3.5	3.5
Exhaust Headers	16.2	7.0	9.0	9.0	14.0
Exh pipes to Muffler	3.3	2.0	2.0	2.0	included
Muffler	14.2	5.0	6.0	6.0	n/a
Exh pipes from Muffler	2.9	3.0	3.0	3.0	3.0
Oil Filter + 1 qt oil	3.9	included	included	included	included
Oil Heat Exchanger + 1 qt oil	10.9	9.9	10.2	10.2	11.7
Oil Lines	5.6	1.7	2.3	2.3	4.9
Engine Oil	30.7	11.5	15.3	15.3	19.2
Coolant Heat Exchanger	32.3
Expansion Tank	3.1
Overflow Tank	1.9
Coolant Lines	6.2
Coolant (14 qt)	29.2
Fuel Filter	2.2	2.2	2.2	2.2	2.2
Prop Governor	3.4	3.4	3.4	3.4	3.4
Vacuum Pump	2.5	2.5	2.5	2.5	2.5
Baffles / Ductwork	9.3	4.2	5.2	5.2	6.2
Propeller	125.0	55.0	79.0	79.0	125.0
Spinner	9.1	7.0	8.0	9.0	9.1
Cowling	35.0	22.0	27.0	27.0	33.0
Engine Mount	47.0	13.5	19.2	19.2	31.0
TOTAL	914.5	451.5	604.9	649.9	975.7
DIFFERENCE		463.0	309.6	264.6	-61.3

These numbers can be made to vary a bit by altering the specific configuration. Composite propellers can be substituted for metal ones.  Cowling, ductwork, and engine mount numbers can vary.  Things can be omitted. But the table is a good representation of reality, taken from actual flying aircraft.

Note that the only engine in the table which makes sense to replace with a V8 is the Lycoming TIGO-541 (the engines on a Piper Navajo). There are a few others which make sense too, including Pratt & Whitney radials (R-985 and R-1340) and most installations using the GTSIO-520 / 550. But we have had people come to us wanting to replace all the other engines listed in the table with some form of liquid-cooled V8.

The engine mount numbers in the table above are quite different. Why is that? Because Lycoming engines are cantilevered off the back face, which allows the mount to be quite short and stiff, therefore light. Pictured below is a 19-pound mount for a 300 HP Lycoming IO-540 engine, and it includes the nosegear support structure as well.

The picture here (below) shows the engine mount (still in the fixture) for one of our cropduster conversions. That mount weighs 53 pounds, and supports all the FAA combined loads from a 750 HP, 1235-pound V8 powerplant with a comfortable margin. (The little hose at the front is for purging the moisture-and-oxygen-bearing air inside the tubes and replacing it with the inert gas argon.)

Additional Conversion Considerations

In addition to the installation weight (covered HERE), there are several other important criteria which should be considered during the design phase of an engine conversion. Although many of these considerations may seemto apply only to certified installations, it is prudent to keep in mind that they apply equally well to experimental installations.

In the experimental world, the FAA isn't looking over your shoulder to assure compliance, but keep in mind that the laws of physics tend to be quite unforgiving, and are not the least bit dependent on whether or not a project is certified by the Government.

1. When designing an engine retrofit, it should be a high priority target that the empty CG of the aircraft with the new powerplant is very close to that of the original aircraft. That simplifies the certification structural issues, and helps to preserve the flying qualities with which the airplane was certified.
2. The engine mount structure should be designed and tested to meet FAR-Part 23 specified criteria for all loading conditions. In addition, it is prudent to consider whirl mode effects, even though that analysis is not required for piston engine installations.
3. Even if the weight of the retrofit installation is similar to that of the original, there is still the potential issue of greater overhung moment due to the typically-greater length of the liquid-cooled installation. If there is a substantial increase in the overhung moment of the installation, it will be necessary to analyze (and perhaps strengthen) the aircraft structures to which the engine mount is attached.
4. One of the most challenging aspects of the retrofit of a liquid-cooled powerplant is to achieve suitable cooling capacity to meet the FAR hot day climb performance requirements, as well as the necessary margins to assure cooling at high altitudes and high power settings.
         There are typically two heat exchangers to be considered: engine coolant and oil. If the engine is supercharged, a third heat exchanger ( the aftercooler, aka "intercooler") will probably be necessary.
         The inlet and exhaust airflow ductwork for these heat exchangers will require skilled design and implementation work, especially if good pressure recovery and low cooling drag are important. Sometimes the cooling drag can be reduced by the use of an oil-to-coolant heat exchanger instead of another air-to-liquid heat exchanger.
5. If the conversion includes a turbocharger, there will be significant challenges with the location and insulation of the turbocharger, wastegate, controller, and exhaust plumbing. There must be well-designed accommodations for exhaust plumbing flexibility to allow for the differential expansion of components in direct contact with the exhaust gas. If the turbocharger is not mounted directly on the engine, a separate mount attachment and suitable support for the g-loads and gyroscopic loads must be incorporated into the mount structure.
6. If the location of the turbocharger does not provide sufficient vertical height above the oil sump, then a scavenge pump will be needed. Special provisions are necessary to prevent the scavenge pump from overpowering the turbocharger internal seals and sucking large amounts of oil into the compressor, and of course, then onward to the engine.
7. Many fuel, oil and coolant plumbing components such as lines and hoses (fireproof for oil and fuel), fillers, expansion tanks, filters, air-oil separators, tanks, etc will need to be designed, drawn, fabricated and tested.
8. There will be a design and fabrication effort required to implement a suitable alternate air system for the converted aircraft.
9. New cowlings must be designed, fabricated and tested. Mathematical Lofting and Computational Fluid Dynamics (CFD) capabilities will aid significantly in designing low-drag cowlings with efficient diffuser and nozzle ducts for the ingress and egress of cooling airflow. Cooling air must also be provided under-cowl for accessories such as dry vacuum pumps and alternators.

BOTTOM LINE: An engine swap on an aircraft is a complex effort, and necessarily involves alterations to several major systems, including the structural characteristics and various resonant frequencies on the aircraft.

If it is intended to certify (STC) the retrofit, especially if the retrofit is on a multi-engine aircraft, be prepared (both technically and financially) for the FAA to require, as part of the STC program, a ground vibration test (GVT) on theconformed prototype.