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3 Answers

Pulling G’s in the vertical…

Asked by: 4871 views , ,
Aerodynamics, Flight Instructor

Hey guys,

I'm a CFI myself and a WWII combat sim enthusiast. Sometimes, the love for one overlaps with the other, hence my question.

I'm teaching one of my acquaintances about the basics of flight, when the subject of lift came up. They asked about how WWII fighter pilots could pull up vertical in a propeller aircraft, since max AoA was exceeded.

Touching on regular turn aerodynamics - in a normal turn, drag is increased and lift is reduced unless nose-up is added to compensate for both. This has the net effect of decreasing airspeed.

Similarly, to turn tighter, the pilot must increase the bank angle, therefore decreasing available lift even more, requiring either more power input or a trade of altitude for airspeed to maintain the turn.

HOWEVER...

Let's say that you pull the nose straight up in a WWII-era fighter, after building up a few hundred knots of airspeed, say 400kt. Straight vertical. This is in a prop plane with a thrust-to-weight ratio of less than 1.0. 

Please correct me if I'm wrong here, but I have three separate questions (and answers, if I worked through these correctly).

  •  Once you get past the transition from flying at less than max AoA to more than max AoA, any lift your wings are producing is now a combination of lift being generated by relative wind from the aircraft's momentum moving forward. In other words, it's not sustainable, but lift will be produced while the aircraft has enough momentum to keep moving forward at or above to V-stall. This would be true from when the aircraft surpasses max AoA to just below 90* vertical attitude. Is this correct?
  • Once the aircraft reaches vertical, lift does not matter (or rather, has no effect and therefore does not matter). The only need for relative wind is control surface deflection, correct? The aircraft remaining in the vertical depends on the remaining momentum (what we would usually refer to as thrust), being greater than both weight and drag.
  • Does an aircraft bleed airspeed faster pulling a high-G turn in the vertical, compared to the same aircraft pulling a lesser-G turn in the vertical? Both aircraft pitch nose-up, from 0* (straight and level) to 90*. Provided that both aircraft are below maneuvering speed and both airframe designs allow them to pull (for sake of argument) 10G's max load before stalling, will the aircraft that pulls 8Gs bleed airspeed faster than one that pulls 4Gs? Since we are no longer relying on the wings for 100% lift, and we're not losing any lift in the turns, my work thus far says it shouldn't matter.

I've spent hours reading and cannot find any true technical l discussions of it, as most of the materials I've come across are either GA or aerobatic in nature and don't cover this in enough detail.

 Thank you.

 

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3 Answers



  1. Wes Beard on Jan 14, 2014

    I think we may be missing something here. For all readers, the angle between the relative wind and the chord line of the wing is called the angle of attack. For an airplane to climb vertically, the relative wind vector is pointing in the same direction as gravity (weight) or pretty close to it.

    To answer you three bullet points. (1). Hopefully you never exceed the max AoA or what most pilots call the critical AoA. For an airplane whose thrust to weight ratio is less than one as you go vertical you will be changing kinetic energy (airspeed) for potential energy (altitude) and at some point you are guaranteed to run out of kinetic energy or airspeed. Once you run out of kinetic energy the airplane will turn into a lawn dart and the nose (hopefully) will drop and you will rapidly turn potential energy back into kinetic energy.

    (2). Lift will not play a key part when the airplane is vertical. Thrust must overcome both the weight of the airplane and the drag produced by the airplane just to “hover”. To actually climb, the thrust must be greater than both of those. This brings me to a really great point, the airplane will always climb (steady state) due to excess thrust. Even when you are trading kinetic energy (airspeed) for potential energy (altitude) as in a zoom climb that extra airspeed came from excess thrust from the engine.

    (3). The aircraft will absolutely lose more airspeed when pulling a high G climb versus a low G climb. The reason for this is due to induced drag. You are increasing the coefficient of lift very quickly and the lift vector will tilt aft. One of the factors for the coefficient of lift is the angle between the lift vector and the relative wind. If you pull back to quickly that angle increases sharply and the lift vector tilts backwards. If you were to break up that vector to the vertical and horizontal components of lift, you can see that the horizontal (aft) portion is big. That is what we call induced drag. If we were to pull back on the yoke slowly, the relative wind would have enough time to react to what we are doing and the coefficient of lift would not change very much and that results in lower indcued drag.

    I hope this helps.

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  2. LTCTerry on Jan 16, 2014

    I’m not generally this bold, but a CFI who doesn’t know the difference between deck angle and angle of attack? Yikes. I wonder if he knows you can exceed the critical AoA in a steep dive.

    Maybe some aerobatic training would be in order.

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  3. Brian on Jan 17, 2014

    You need to stop caring about attitude relative to the earth in your analysis’. An analytical analysis of how the airplane remains flying cares only about the direction of the relative wind in relation to the A/C. Pulling a high g maneuver, for instance, pulling vertical from a level flight condition, will only result in a stall if the AOA between relative wind and the wing exceeds critical.

    Rethink your analysis with a strict consideration of the relative wind.

    Gear shift, you asked: Will the aircraft that pulls 8Gs bleed airspeed faster than one that pulls 4Gs.

    Study up some more on induced drag. You were once taught that induced drag is a byproduct of the production of lift. More lift means more induced drag; a misnomer as it were. To strive for accuracy think in terms of more angle of attack results in a higher lift coefficient. It is this higher lift coefficient that increases induced drag.

    With that in mind answer this: Is the aircraft pulling 8g producing the same amount of lift as the one pulling 4g? Or, again for accuracy, is the aircraft pulling 8g flying at a higher angle of attack/lift coefficient or is it the same? If you said higher than you agree drag is higher for the 8g aircraft.

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