Keith Williams and barit1:
“When the aircraft is stationary on the ground with engine running all of the propeller power output is being wasted in accelerating the propwash rearwards. So none of the propeller power output is being delivered to the airframe. That means that there is no THP, because as Barit has said "THP is the power delivered to the airframe".
In theory if the aircraft were to fly at its own propwash speed no power would be wasted in the propwash, so all of the propeller power output would be being delivered to the airframe. So it would all be THP. This is of course only true in theory because if the aircraft was flying at its own propwash speed, no air would be being accelerated rearwards by the propeller, so there would be no thrust.
At any speed between the zero and the propwash speed some of the propeller power output would be being wasted in the propwash and the rest would be being delivered to the airframe as THP.
If you think about it this means that the propeller power output is the THP plus the power wasted in the propwash. So we cannot say that propeller power output is THP.
BUT. If you look at any fixed wing diagram of power available against power required you would see that the power available is the THP. It would perhaps be better to say that this is the USEFUL power available.
Beacuse THP is the power being delivered to the airframe, it is thrust x aircraft speed.”
You say THP is thrust x velocity. I agree. But I say that only works for a specific set of assumptions. Static thrust is not the only scenario that doesn’t conform to the assumptions. Here is another:
Aircraft flying straight and level at speed Va, pilot sets new power to accelerate to
Vb. Assume constant propeller efficiency between Va and
Vb due variable prop. Final THP = thrust x
Vb, we agree that much. Using the hypothesis you outlined above we can draw some conclusions:
The new power is set, the BHP has gone to the prop but THP doesn’t catch up until the plane reaches
Vb?!
Thrust horsepower during the acceleration is less than final THP when you get to
Vb?!
Thrust must have increased, but this didn’t reflect an increase in THP, this was because power was wasted in the propwash?!
It’s all a bit odd, if that’s what you believe. Personally I simply accept that the increase in power results in an increase in THP in accordance with the equation THP = BHP x PE. And that work done on the propwash isn’t wasted if it’s giving thrust.
You say there are two types of power output from the prop. The two types you identify are “wasted propwash power” and “THP”. You both favour this over the common aircraft performance terminology which considers power available to be simply: ‘THP’.
To be clear, nobody is saying that no power is lost in the propwash otherwise THP would simply equal BHP. But that is different from what you are implying – that all the work done on the air is a waste and only work done on the aircraft counts toward THP:
If you think about it this means that the propeller power output is the THP plus the power wasted in the propwash. So we cannot say that propeller power output is THP. BUT. If you look at any fixed wing diagram of power available against power required you would see that the power available is the THP. It would perhaps be better to say that this is the USEFUL power available.
According to Hubert C Smith, Associate Professor Emeritus, Penn State, in his “Illustrated Guide to Aerodynamics”:
“actual power available for thrust is obtained by multiplying BHP times propeller efficiency”
“The [amount of power] available is the thp, which is the bhp times propeller efficiency”
… the same equation you both keep ignoring that appears in numerous sources including the USN training doc already referenced in this thread:
THP = BHP x Propeller Efficiency
…and this quote from the same USN reference: “Thrust horsepower (THP) is propeller output, or the power that is converted to usable thrust by the propeller. The ability of the propeller to turn engine output into thrust is given by its propeller efficiency (p.e.).”
…which directly contradicts the position you both hold:
“you think that if thrust is not zero then propeller efficiency cannot be zero. If you do believe this, then you are wrong (once again).”
In fact, you have both made it clear that you believe a prop with zero propeller efficiency can still produce thrust! And please don’t repeat the equation for propulsive efficiency because it is not valid unless you observe the assumption it depends upon; namely, equilibrium flight. Therefore I have to say the assertions you are making are not credible opinions to hold, but such is the level of cognitive dissonance required to believe in this meme of zero THP for a hovering aircraft. Let’s see it again:
...zero propeller efficiency, zero thrust horsepower, and zero useful work on propwash. I don't think so.