**3.2 Specific weight of an engine**

The specific weight of an engine indicates the weight efficiency of an engine, or in other words, how much engine weight is required to produce a given level of thrust or power. The equation for specific weight is

• for turbojet bypass or turbofan engines.

$$
\gamma\_{\rm en} = \mathbf{m}\_{\rm en} \,\mathbf{g}/\mathbf{P},\tag{3}
$$

where men is the engine weight; g is gravity acceleration;

• for piston and turboprop engines, [daN/kw] or [kg/h.p.]

$$
\gamma\_{\rm en} = \mathbf{m}\_{\rm en} \,\mathbf{g}/\mathbf{N}.\tag{4}
$$

So, the minimum specific weight of the power unit is required. Modern time, the lowest specific weight for different types of engines are


For an example, Prof. Mozhaisky airplane steam engines had γen = 10.7, Brothers Writes airplane with piston engine had γen = 8.4 and now, the most powerful turbojet in the world GE90-115B has γen = 0.167.

## **3.3 Specific weight of a power unit and fuel**

The weight of a power unit is determined not only by the engines but also by all the subcomponents and subsystems, including the fuel. And here we can see the collision. On one side as smaller weight then better, but on the other side as more higher weight of fuel than more longer range and duration of a flight. The required weight of fuel is determined not only by the required range and duration of a flight but also by fuel consumption, which will be discussed below.

The minimum specific weight of an engine and fuel is determined by the eqs. [7].

$$
\overline{m}\_{p.u.} = \frac{m\_{p.u.}}{m\_{t.o}} = n\_{en}k\_{p.u}\frac{m\_{en}}{m\_{t.o}} = n\_{en}k\_{p.u} \left(\frac{m\_{en}\mathbf{g}}{P\_0}\right)\left(\frac{P\_0}{m\_{t.\mathbf{g}}\mathbf{g}}\right) = n\_{en}k\_{p.u}\gamma\_{en}t\_0,\tag{5}
$$

$$
\overline{m}\_f = \frac{m\_f}{m\_{to}},
\tag{6}
$$

where mp.u is the weight of a power unit; mt.o is the take-off weight of an aircraft; nen is the number of engines; kp.u is an empirical factor for the additional weight of a power unit of subcomponents and subsystems for an engine, engines (approximately kp.u = 1.2–2.2); t0 is the specific weight to thrust ratio of an aircraft, t0 = P0/mt.og; mf is weight of the fuel; *mf* is the specific weight of the fuel.

Some values of *mp:<sup>u</sup>* and *mf* based on statistical data for different types of aircraft are presented in **Table 1**.

### **3.4 Specific fuel consumption**

The specific fuel consumption indicates the efficiency of the engine because it shows how much fuel the engine needs under given flight conditions to produce 1 N of thrust (or 1 W of power) in 1 hour.

Minimal specific fuel consumption can be achieved in several ways: good aerodynamics of an airframe, rational choice of flight parameters (altitude, speed, etc.), rational operation of subsystems of a power unit, and, of course, it depends on the engine performance. **Table 2** shows some examples of the fuel consumption for the most powerful engines.

Specific cruise fuel consumption is the primary determinant of an aircraft's economics and has a significant impact on the amount of emission emitted into the atmosphere.


**Table 1.**

*Examples of mp:<sup>u</sup> and mf based on statistical data.*


**Table 2.**

*Examples of the fuel consumption for the most powerful engines.*
