Quote:
Originally Posted by Holtzauge
Drgondog, You seem to have problems understanding the propeller thrust formula Harri is using and it's limitations. In the context that has been discussed here (max speed) it is most certainly valid. However, when the plane just starts moving at very low speeds as in your example it is NOT valid. Since you claim to have an Ms in aeronautical engineereing I'm frankly surprised you do not know this and imply the formula is wrong since the thrust would be infinite at zero speed.
The question of Infinite Thrust at zero velocity was the question I posed to help you understand the difference between Net Thrust in the THp equations and Total Thrust (max) available as measured, at rest, for the system. Net Thrust will then decrease as velocity increases from zero up to max V.
So, answer the question - when does the equation for Thp relative to Thrust and Velocity become valid in your workd Holtzauge? Relative to total positive Force in the horizontal plane for level flight?
10kts? 30kts? 100kts? - and why doesn't it apply in deriving a Force vector on the system at zero to low speeds? What law of physics makes it valid at higher speeds? (The correct answer for industry standard practice is 105 kts) but remember it is for Net Thrust.
When do you arrive at a Force value to equate to Force Available - Force Reguired to overcome Drag and accelerate?
What is your value for Force at equilibrium with brakes on, zero velocity
Using the thrust formula Harri posted above in the analysis has a clear advantage over the power method: For a WW2 figther, there is a significant part of the thrust (especially at high speed) that is derived from exhaust thrust. The exhaust thrust is close to constant with speed. It would be interesting to hear how you account for this in the "thrust horsepower" model.
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It is more interesting for you and Hari to answer the repeated and specific questions I Posed -
However since you asked, for a turbo prop in equilibrium at maximum Hp in level flight - Equivalent Shaft Hp =
ESHp = Direct SHP + T/2.5 noting that 2.5 is Strictly empirical, and T is the Net Trust (real) from the exhaust gas.. and BTW this equation along with Shp = (Net Thrust x V)/325x.8 is an approximation due to the uncertainty of calculating true thrust for any propeller/engine combo.
The exhaust Thrust is a real Force to be added to Total max Thrust available at rest - minus the prop drag, the induced drag and the parasite drag
You guys are treating
Net Thrust as The Total positive Force acting in the Horizontal system when in fact it is the Force (max) at rest minus the drag on the propeller system at any specific velocity... the drag on the propeller system does decrease as velocity decreases bringing the Net Thrust value higher at the lower speed until it approaches Maximum Thrust.
Having said that, the Maximum Thrust, and the Total Force acting in the positive direction are one and the same.
BTW, Crumpp and I have been back and forth over modelling the manuever performance of various Fighters at different altitudes and Bhp profiles.
One of the reasons for this debate is that solving a free body equation when parasite drag in NOT known or clearly accessible for many ships REQUIRES that one assume that Thp is converted to true thrust so that Dparasite can be solved when the velocity at Vmax is known.
But when True Thrust(Max) is offset by an unknown Propeller drag to achieve Net Thrust which is calculated by the methods we have been debating, one more drag component (unknown) is intruduced to the (often) unknow Parasite Drag
At different speeds this positive Total Force is offset by the increased drag on the prop, the induced drag and the parasite drag. As the speeds increase to max V, the Net Thrust is sufficient to overcome the Induced and Parasite Drag - but it is Not the Total Thrust Force available from that engine/prop combo..
At the end of the day, however there is no other easy way to get 'close' other than to assume that Net Trust does indeed cancel out the Prop drag and exhaust thrust and will yield a 'close enough' approximation for parasite drag
The reason that an a/c can dive faster than horizontal speed is that the Weight vector is added to the Max Force of Prop Vector until the increased drag on the prop, plus the induced drag and parasite drag are again in balance - resulting in terminal velocity. It can't go any faster for that flight profile.