Why must VTOL engines be larger than normal engines?

  • Context: Undergrad 
  • Thread starter Thread starter E'lir Kramer
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Discussion Overview

The discussion revolves around the reasons why Vertical Takeoff and Landing (VTOL) engines are considered to be larger and heavier than traditional engines used for conventional takeoff and landing. Participants explore the mechanics of thrust, lift, drag, and the operational differences between VTOL and conventional aircraft.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that VTOL engines must produce more thrust to counteract gravity during vertical takeoff, while questioning the necessity of larger engines compared to traditional engines.
  • Another participant compares the work required for a VTOL aircraft to take off vertically versus a conventional aircraft that uses a runway, suggesting that the latter benefits from ground support until lift-off.
  • A participant expresses confusion about how aircraft with a thrust-to-weight ratio less than one can take off, using the example of a 747-400 with a T/W of 0.27.
  • Some participants discuss the efficiency of different engine types at various speeds, indicating that larger, low-velocity fans can produce necessary thrust for takeoff at lower power compared to smaller jets, which may not be as effective at high speeds.
  • There is a contention regarding the relationship between thrust, lift, and drag, with some arguing that the thrust must exceed drag for conventional flight, while others clarify that lift is primarily generated by the wings during horizontal flight.
  • One participant emphasizes that during VTOL takeoff, thrust must continuously counteract gravity until sufficient altitude is gained, after which thrust can be redirected for forward motion.

Areas of Agreement / Disagreement

Participants express differing views on the mechanics of thrust and lift in VTOL versus conventional aircraft, with no consensus reached on the necessity of larger engines or the implications of thrust-to-weight ratios during takeoff.

Contextual Notes

Some participants highlight the complexity of thrust and drag dynamics, noting that the efficiency of engines varies with speed and that the lift-to-drag ratio plays a significant role in flight mechanics. There are unresolved questions regarding the assumptions made about thrust requirements in different flight modes.

  • #61
It's simple physics. F = ma. In a steady climb, acceleration = 0 so F_net = 0.
 
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  • #62
cjl said:
It's simple physics. F = ma. In a steady climb, acceleration = 0 so F_net = 0.

if you were climbing in zero gravity ?

you are going to have to do a better job of explaining
 
  • #63
davenn said:
if you were climbing in zero gravity ?
No, gravity is part of the F in F=ma.

davenn said:
you are going to have to do a better job of explaining
How about you explain what is wrong about it?
 
  • #64
Davenn, I understand your skepticism. According to everyday experience, it may appear that a force needs to be applied to an object in order to make it move -- and that's partly correct. In reality, however, a force only needs to be applied temporarily in order to get it moving. After that, you take away the force and it keeps on moving by itself -- indefinitely. The reason this might be counter-intuitive is that friction and air resistance tend to obscure the underlying law.
 

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