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18 Answers

Load factors in level flight

Asked by: 13620 views ,
Aerodynamics

I have read that an increase in airspeed increases the load factor of an airplane in level, straight flight. Why is this?

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18 Answers



  1. Bob Watson on Aug 20, 2012

    An increase in airspeed would, at the very least, increase the drag forces on the airframe. If the angle of attack remained constant, it would also increase the lift (and cause the plane to climb, if it was in equilibrium before).

    If this still isn’t making sense, perhaps you could provide some more context or the reference in which you read this.

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  2. John D. Collins on Aug 20, 2012

    Dave,

    Are you referring to the load factor that may be imposed on the airframe if you make an abrupt pitch input? If so, the stall speed of the aircraft limits the maximum amount of load that may be imposed on the airframe and aircraft structure. The stall speed is related to the load factor by the square root of load factor, so if you are flying at 2 times the power off stall speed, you can impose a load factor of four G’s before the aircraft will stall. The establishment of the maneuvering speed VA is based on this relationship.

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  3. Lucas on Aug 20, 2012

    I Think I already posted this video but just in case here it is again.

    http://www.youtube.com/watch?v=oEHJXTlu6B8

    It discusses maneuvering speed and might give you a better understanding of load factor.

    Cheers Lucas

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  4. Dave Sandidge on Aug 20, 2012

    I think maybe we are doing a dis-service to our students by not fully explaining what a ‘load factor’ really is. The L/W = load factor equation is misleading, I believe. One university explains load factor this way: “The load factor is defined as the component of aerodynamic force perpendicular to the longitudinal axis divided by the aircraft weight. Assuming the angle of attack is not large, n = L/W. This is the effective perpendicular acceleration of the airplane in units of g, the acceleration due to gravity.” With this explanation I can understand that if you simply increase the airplane’s airspeed (in straight, level flight) you also increase the load factor. Essentially it is saying this: “If you increase lift, whether or not in a turn or bank, you also increase the load factor.” I welcome comments from each of you. Am I going off somewhere in an uncoordinated bank?

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  5. Dan Bernard on Aug 21, 2012

    It does not increase the load factor. Straight and level flight implies that lift = weight, regardless of any airspeed changes (though note that airspeed changes will necessitate/are caused by a change in AoA). Load factor = Lift/weight = weight/weight = 1.

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  6. Dave Sandidge on Aug 21, 2012

    Dan,
    I want to agree with you; however, there is one aspect of this equation that is still not clear to me. If an airplane’s speed is increased in level, straight flight to a specific new value the center of pressure moves further aft to a specific location (assuming you have an airplane with a normally cambered wing, not elliptical). The nose naturally wants to pivot downward around this aerodynamic center. To counter this downward moment the tail needs to create additional downward lift to balance the new equation. (Even with an increase in the down wash off the wings onto the tail you still need some nose down trim). This new downward (negative) lift adds algebraically to the overall lift that is being created and puts an additional load on the wings. So, does not any increase in the load on the wings increase the load factor?

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  7. Kris Kortokrax on Aug 21, 2012

    What is the disservice to our students? Do they really need to have the same understanding of aerodynamics as an aeronautical engineer?

    I regularly fly a Luscombe which has an accelerometer installed. Never noticed any change in load factor (G) between takeoff, cruise and landing due to change in speed.

    Page 15-7 of the Airplane Flying Handbook discusses change in COP and shows in Figure 15-10 that the COP moves forward between Mach .5 and Mach .7 and then moves significantly rearward. Of what use might this be to my student flying a 172 or Cherokee? I would much rather he understand the increase in load factor that occurs as a result of maneuvering flight (turns, pullouts).

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  8. Dave Sandidge on Aug 21, 2012

    Kris,

    As a matter fact, I am attempting to teach two engineers how to fly. They work together, and they will not be satisfied until they understand theoretically and mathematically every nuance of information that may or may not pertain to what they are actually doing with the airplane. All I can tell them is that I do not know the answer, but that I will try to find out.

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  9. Dan Bernard on Aug 21, 2012

    Dave,

    I see what you’re thinking regarding tail down force. However, as stated by Wikipedia,

    “In the definition of load factor, the lift is not simply that one generated by the aircraft’s wing, instead it is the vector sum of the lift generated by the wing, by the fuselage and by the tailplane[7], or in other words it is the component perpendicular to the airflow of the sum of all aerodynamic forces acting on the aircraft.”

    So, even though there is a tail down force, in straight and level flight the NET lift is still equal to the weight, hence the load factor remains constant at unity, even as the airspeed changes. And to reiterate, you would have to instantaneously change AoA as the speed changes to keep perfectly straight and level and avoid increasing the load factor.

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  10. Brian on Aug 22, 2012

    “So, does not any increase in the load on the wings increase the load factor?”

    What’s of particular importance that’s been missed is how you define weight. Lift equaling weight is predicated on weight being defined as the sum of all forces acting down (toward earths center).

    Now, in a turn, total lift is greater than the sum of all downward acting forces and your engineer students should understand this with the simple vector diagram shown in virtually all private pilot material.

    Do remember that the L/W equation refers to total lift. Which, not coincidentally, is equal to weight in a level flight condition. Where as, in a turn, total lift increases to keep the vertical lift vector equal to weight. But weight, for all intensive purposes, can be considered unchanging in either condition.

    —

    Hopefully that helps, now for a few subtle corrections.

    Increasing speed decreases your AOA, which results in CP moving aft as you mentioned. The result is an increase in the arm (remember CG is ahead of CP for a stable airplane) between CG and CP, therefore increasing the pitching moment, required a greater tail down force, and total lift requirements. HOWEVER, weight is the sum of aircraft weight and this new amount of tail down force AND is equal to airplane lift. Load factor remains one.

    Finally, the aircraft rotates on all axis about the CG, not the CP.

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  11. Brian on Aug 22, 2012

    Ooops you said it pivots around the AC, not CP. Sorry. Either way, the correct answer is it pivots around the CG.

    However, AC is the term you should stick to when considering flight dynamics. Reason being, it is not effected by changes in AOA.

    Personally I have no clue why the PHAK chose to spend 2-3 pages talking about CP shifting with AOA and how it changes tail down force. It’s damn confusing and down right useless. Aft CG gives better cruise performance, improved pitch control, and reduced pitch stability. That’s all you need to know.

    Even engineers generally avoid working with the CP because it’s a mathematical nightmare. It’s for this reason that AC is used in virtually all number crunch and for conceptual understanding. Errors caused by using this fixed, verses variable, point of reference, are assumed negligible.

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  12. Dave Sandidge on Aug 25, 2012

    Brian,
    Thanks for your input. You make a lot of sense; you present a good argument. Let me ask you this: Imagine a B-52 in level flight with those long wings. If he increases his speed in level flight do the wings bend upward more, or do they stay at the same degree/angle of upward bending curve?

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  13. Brian on Aug 25, 2012

    They would bend upward slightly more as the main wing will produce more lift. However, keep in mind that this is due to an aerodynamic increase in your weight component (tail down force). So the L/W relationship remains one because both lift and weight have increased by the same amount.

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  14. Dave Sandidge on Aug 25, 2012

    Brian,

    Okay, I think we’re right on the cusp of understanding – for me, that is. So this increase in aerodynamic weight and lift – which do equal themselves out – do not then increase the load factor on the wings? Even though the wings are bending upward more due to the strain – the increase in lift – they are, in fact, not enduring anymore of a load than before?

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  15. Brian on Aug 25, 2012

    I wonder, are you confusing structural load, such as that on the wings, with load factor? Load on the wings has increased slightly. Load factor is what the pilot is concerned with however, and is still one. Structural loads are what the designers are concerned with and vary with virtually any change in flight condition.

    Again though, CP is not used to calculate or discuss structural loads in any design book in my library. AC along with historical data is used during the design process. Finalizations are made for structural loads during the flight test process, if needed.

    I suggest taking a look at page 331 in Aerodynamics for Naval Aviators, the section on Aircraft Loads and Operating Limitations might offer a bit more insight. You’ll notice there is no mention of what we are talking about here in the design considerations. Here is a link:

    http://www.faa.gov/library/manuals/aviation/media/00-80T-80.pdf

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  16. Dave Sandidge on Aug 27, 2012

    Brian,
    Thanks for the input. At this point I’ll have to say that I believe that the load factor itself will not increase by simply increasing airspeed. Of course, some slight disturbance in the air (turbulence) will cause momentary increases and decreases in such, but they will be minimal. If in a constant-banked, level, coordinated turn you increase the airspeed, the radius of the turn will increase without an additional increase in load factor. So I’ll have to agree with you in that the load factor will not increase while in level, constant heading, increasing airspeed flight. But, of course, additional air loads are put upon the airframe. I’m still awaiting info from other sources. Thanks again.

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  17. Brian on Aug 27, 2012

    “Iā€™m still awaiting info from other sources.”

    Without specifics on what info you’re looking for I’m not sure I can help you beyond what’s already been given. I would suggest, if the sourced book that’s free online is not sufficient enough for you, purchasing the book “The Illustrated Guide to Aerodynamics”. The author does a wonderful job accurately explaining flight dynamics while still keeping it simple enough to understand. (HS senior level writing) Whatever you do, don’t use the PHAK to learn aerodynamics. šŸ™‚

    Glad I could help, your latest analysis is spot on. Load factor is independent of airspeed. Now how much the pilot can manipulate load factor before stall is encountered, that is dependent on speed. (Va..)

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  18. Nathan Parker on Sep 01, 2012

    First, there is no such thing as “aerodynamic weight”. Weight is a separate force from the aerodynamic forces on the airplane.

    Second, it should be obvious that in straight, level flight, the load factor must be 1.0 because otherwise, the aircraft would be experiencing a vertical acceleration. End of story.

    Now, it’s possible that the lift on the main wing exceeds weight, but the combination of main wing lift, tail down force, and fuselage lift would have to equal weight. (And that’s ignoring a whole host of things with generate forces that have a slight vertical component.)

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