Propellor Thrust

  • #26
russ_watters
Mentor
19,705
6,043
What I dont undersatnd is how the Aircraft gets enough speed in this equation. 14675.72745/34650 = 0.423541918m/s = 1.524kmph. Does the Linear Velocity add? I tried this in my model and It worked too fast. Does rpm come in? If so, then force = 660407.7N, giving it a velcocity of 19m/s = 68.61kmph.
You are using the numbers incorrectly: f=ma. a is acceleration, not speed. And m is mass, not weight. So all the thrust and weight can tell you is how fast the plane accelerates, not what its top speed is. Top speed is determined by drag.

Thos numbers tell us....

a=f/m = 14675/(34650/9.8)=
....that the airplane accelerates at 4.15 m/s/s, which sounds reasonable to me.
 
  • #27
Thx for pointing that out. The aircraft takes off comfortably with an Efficiency of 0.8-0.9 (the FOM is too low).
I have found some graphs on the propeller efficiency/advance ratio with the propeller pitch setting.
http://www.mudpond.org/fs_props.pdf" [Broken]
I could use the graphs, or I could use these equations:
http://www.aa.washington.edu/courses/aa441/07-Propellers.pdf" [Broken]
What should I use?
An easier alternative is to graph the highest prop pitch settings together (as the prop pitch will be automatic anyhow) and use them for the efficiency.
 
Last edited by a moderator:
  • #28
rcgldr
Homework Helper
8,696
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The aircraft takes off comfortably with an efficiency of 0.8-0.9 (the FOM is too low).
At launch, the advance ratio (airspeed x pi / (prop tip speed perpendicular to airspeed)), is zero, and the efficiency is low as shown on the graphs. Also, even though power output is described as thrust versus airspeed, thrust versus induced airspeed at the prop handles the case of zero airspeed.

An easier alternative is to graph the highest prop pitch settings together (as the prop pitch will be automatic anyhow) and use them for the efficiency.
I don't know about this. The throttle setting and air speed affect the prop pitch, with a region of parameters where the prop rpm is nearly constant.
 
  • #29
Thanks alor Mr Reid. My calculations are done. I plotted the efficiency and set it with the advance ratio in my program and now thrust is fine.
This doesn't fit in with the name anymore, but could anyone help me with the roll, pitch and yaw physics now?
And how do the leaving the undercarriage and flaps affect drag? I need this for my equation as well.
 
  • #30
rcgldr
Homework Helper
8,696
528
This doesn't fit in with the name anymore, but could anyone help me with the roll, pitch and yaw physics now? How do the leaving the undercarriage and flaps affect drag? I need this for my equation as well.
I don't know how the math is done for this. The control surfaces generate a torque, and you have a combination of aerodynamic drag (large effect) and angular inertia (small effect) affecting the response rate. The prop on a P51 also generates a signifcant amount of roll torque, especially at low speeds during take offs. The "rolling" prop wash can also affect the vertical stabilizer, adding a yaw related response.

Landing gear results in slight drag and downwards pitch torque. Flaps have a lot of drag and increase downwards pitching torque. I would assume tail control surfaces have suffiencient authority that these aren't issues, but they shouldn't be used at high speed. The standard P51 didn't have dive brakes, but there were variations that did.
 
  • #31
2,985
15
Thanks alor Mr Reid. My calculations are done. I plotted the efficiency and set it with the advance ratio in my program and now thrust is fine.
This doesn't fit in with the name anymore, but could anyone help me with the roll, pitch and yaw physics now?
And how do the leaving the undercarriage and flaps affect drag? I need this for my equation as well.
I'm currently making a flight dynamics model for two aircraft. In a nutshell, you're way over your head.
 

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