Unfortunately, I’m not very current on the theory of flight mechanics. I’m wondering whether somebody has experience in rough performance estimates of airplanes for a few relatively simple questions.
I’m trying to estimate the expected cruise speed of an aircraft at 4000’, when the cruise figures at 8000’ are given. For the airplane in question (150 hp O-320 engine, fixed-pitch prop), the manufacturer specify two true airspeed figures at 8000’:
(The figures are commonly accepted to be real-world accurate and not overstated.)
My questions are:
1. What would be roughly the expected true airspeed at 4000’ PA at 55% (and 75%) power?
2. Is there a simple relationship for the variation of indicated airspeed with pressure altitude at same % power setting (fixed-pitch prop)?
3. Is there a simple relationship for the engine fuel flow over density altitude, or do I have to resort to the Lycoming Operator’s Manual?
For question 1, using the rule of thumb v_TAS = v_IAS * k where k = 1.02 * PA / 1000 (pressure altitude PA), I can come up with 176 mph * 1.04 / 1.08 = 169 mph. However, would that account for all density effects, thus both aerodynamic (airframe / propeller), and combustion (engine)?
Thanks,
Michael
The drag is proportional to v_IAS squared. The power is proportional to drag * v_TAS.
So for a constant power P = k * v_IAS^3 = v_TAS^3 / k^2. Or v_TAS is proportional to k^(2/3).
That doesn’t account for the difficulty of determining what is “75% power” and whether it is actually constant. But 75% should be 112.5 hp, however obtained.
