This is probably the cheapest dyno to make and the most useful.
Not only will it measure your actual HP in the air on a
continuous basis but it will help greatly when optimizing
your propeller. It works the same way as a torque meter on
a PT6 turbo prop engine. It measures the un screwing force on a
helical gear set. It works best with a pressure
sensor and a digital read out but a 360 degree mechanical
pressure gage will also work. It is based on an invention
of mine 35 years ago that is now widely used through out
the world as a manual transmission clutch throw out system.
You are probably driving a car that has one in it. Consequently
The annular hydraulic cylinder could be scavenged from a junkyard.
Look for GM cars with V8's. Ideal if your building you own
PSRU.