def tw2pw(thrusttoweight, speed, etap):
    """Converts thrust to weight to power to weight (propeller-driven aircraft)

    **Parameters**

    thrusttoweight
        thrust to weight ratio (non-dimensional)

    speed
        ground speed (in m/s if output in Watts / Newton is required)

    etap
        propeller efficiency (non-dimensional), float

    **Returns**

        power to weight ratio (in W/N if speed is in m/s)

    **See also** ``powerrequired``

    **Notes**

    1. A note on units. If the input speed is in m/s, the other two inputs being
    non-dimensional, the output product is also in m/s, which is equal to W/N
    (W / N = (J/s) / N = (Nm/s) / N = m/s).

    2. The speed input is a kinematic quantity, not an airspeed, so it is generally
    a ground speed (GS) or a true airspeed (TAS) if we are assuming zero wind.

    3. The inputs to the function are scalars or a mix of scalars and `numpy`
    arrays.

    **Example**::

        from ADRpy import constraintanalysis as ca
        from ADRpy import atmospheres as at
        from ADRpy import unitconversions as co

        designbrief = {'stloadfactor': 2, 'turnalt_m': 3050, 'turnspeed_ktas': 140}

        etap = {'turn': 0.85}

        designperformance = {'CLmaxclean': 1.45, 'CDminclean': 0.02541,
                                'etaprop': etap}

        designdef = {'aspectratio': 10, 'sweep_le_deg': 2,
                        'sweep_mt_deg': 0, 'bpr': -1}

        TOW_kg = 1500

        designatm = at.Atmosphere()
        concept = ca.AircraftConcept(designbrief, designdef,
                                        designperformance, designatm)

        wingloading_pa = 1000

        twreq, _, _ = concept.twrequired_trn(wingloading_pa)

        turnspeed_mpstas = co.kts2mps(designbrief['turnspeed_ktas'])

        pw_trn_wpn = ca.tw2pw(twreq, turnspeed_mpstas, etap['turn'])
        pw_trn_hpkg = co.wn2hpkg(pw_trn_wpn)
        p_trn_hp = pw_trn_hpkg * TOW_kg

        print(p_trn_hp)

    Output::

        318.691213406

    """
    return thrusttoweight * speed / etap
