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  Last Update: 19 Jun 2011

- The Prime Directive -

Why Total Powerplant Installed Weight Is So Critical

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 for EVALUATING 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 (Vd) will be required to increase as well. That has further ramifications, covered later under structural issues.


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