Angle of Incidence

10 Answers

1. 
   

   
   

   Brian on Jan 16, 2012

   “Whereas angle of incidence of jet planes can be 0. <<— what for???”

   Can be, but not always. Two typical design criteria:

   1) To produce no lift drag (induced drag) during the takeoff roll.
   2) More comonly from my understanding, to make the fuselage level during cruise flight for passenger comfort. This, and by my understanding is more comon, often leads to a minor angle of incidence for transport jet aircraft.

   “How does Angle of incidence affect GA angle of attack during cruise flight?”

   It doesn’t. The angle of attack is still the angle between chord and relative wind. As mentioned for the transport aircraft, though, the angle of incidence is often set to make the fuselage level during cruise flight. You wouldn’t want to cruise around 3 degrees nose high all the time, it would be awkward. 🙂

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2. 
   

   
   

   Bill Trussell on Jan 16, 2012

   It is also important to note that this AOI is often not consistent across the entire wing span.  Recall the terms “wash in” and “wash out” refer to increasing or decreasing AOI as you move out from the wing root toward the wing tips.  This is one of several methods to ensure that the wing stalls from the root first, delaying loss of aileron control until the last possbile moment.

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3. 
   

   
   

   Jack Dander on Jan 16, 2012

   I am new here, so maybe this suggested website is all wet, or not.  But try av8n.com and tell me if this guy’s “book”  there answers your question.  The site seems genuine and helpful, at least to me.

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4. 
   

   
   

   BC on Feb 03, 2012

   AOI is basically the angle that the wing is mounted to the fuselage (chord line and fuselage axis), and is usually set so the fuselage is level in cruise flight, or whatever angle results in the least drag (usually level).
   The B-52 has an exagerated AOI becuase of the desire to have the fuselage exactly level at high altitudes for bomb runs. At sea level, it will fly nose down and climb in a level attitude. With all that AOI there is still plenty of AOA to create excess lift for the climb.
   Basically, the designer workd backwards and puts the wing at an AOA in the environment (altitude and speed) the plane will spend most of it’s time (or mission). Then the fuselage it mounted, usually, close to level. Then the difference between the root chord line and fuselage axis is your AOI. Yes, there is nearly an airfoil transition and twist out to the tip to insure the stall happens at the wing root first.
   A similar, but confusing angle is the”decalage angle”. This is the angle between the chord line of the wing and the horizontal stab. It’s sort of like “longitudinal dihedral angle” in that it determines static pitch stability. The more “up” declage angle, the more “nose heavy” the plane is, and the more pitch (longitudinal) stability is has. It that case it will want to fly at a trimmed airspeed, or return to that speed if a deviation happens.
    
   Capt BC
   Airline Capt
   USAF pilot
    

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5. 
   

   
   

   BC on Feb 03, 2012

   Sorry for the above typos.
    
   By the way, most jets don’t cruise exactly level. I usually fly a B-777 and it cruises about 2 to 2.5 degrees nose high (as do most airliners I’ve flown) depending on altitude and cruise mach. This is likely the lowest drag attitude for the fuselage and might be due to that particular attitude providing a little profile lift to offset drag. Climbs at altitude will be a couple degrees more and descents will be a bit less. Basically, at altitude, the nose (attitude indicator) should always be between approx. 0 and 5 degrees. Air France at 15 degrees nose high? That topic has already been beaten to death, so i won’t comment further. Sad.
    
   This cruise angle becomes annoying to some pilots when trying to sleep in the bunk beds. The bunks are set up “head aft” (where the temp and entertainment controlls are). Some pilots will lay “head forward” because they are sensitive to laying “downslope”. Hopefully, the 787 will have the bunks at a 2 deg angle to make them level in cruise (we’ll see how sharp those Boeing boffins are). For me, I sleep well either way. I love the plane. Anyway, this long example is to show you that we do fly a little nose high in cruise. 
    
   Capt BC
    
    

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6. 
   

   
   

   Brian on Feb 04, 2012

   “A similar, but confusing angle is the”decalage angle”. This is the angle between the chord line of the wing and the horizontal stab.”

   From: NACA Technical Report 269

   “The angle of decalage is the acute angle between the chords of the wings of a biplane.”

   I’ve never read it in reference to the wing and tail; though this sure doesn’t mean it cannot be used as such a reference.

   —

   ” I usually fly a B-777 and it cruises about 2 to 2.5 degrees nose high (as do most airliners I’ve flown) depending on altitude and cruise mach.”

   Where might one source specific design information like this? I apologize for my ignorance, aircraft design is a facinating field for me. Unfortunately, my library only includes books dealing with conceptual design or specific facets of more detailed design. I have no reference that gives detailed specifics like you’ve presented, and I’d like such a reference. Thank you in advance Capt.

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7. 
   

   
   

   BC on Feb 04, 2012

   I guess decalage angleis the difference between two airfoil chords, so using it to define the differences between the two wings of a biplane would be appropriate. I’m used to the term being used to describe the angle between the wing and stab, since this is an important factor in static pitch stability. I think you see it more in sailplane design where they strive for max L/D by reducing stab drag caused by too much “up” elevator trim (or up chord angle in a full moving stabilator) which is caused by being nose heavy. Sailplane racers put their CG as far aft as possible to reduce this “trim drag”. They even have battery boxes and water tanks in the vert fin to get the CG back (to the aft CG limit specified in the ops manual. Further back than than and you’re a test pilot). The fin ballast is usually to offset the effects of a pilot being…..fat.
    
   The MD-11 has fuel tanks in the tail. During cruise, the fuel system would automatically pump fuel into the aft tanks to move the CG back to reduce trim drag. I understand the MD-11 was a piece of junk, so we sold them to FedEx. The DC-10, in contrast, handled like a Cadillac and was a dream to fly.
    
   Regarding the pich angles during cruise, you can find them in Performance sections of aircraft operating manuals as well as the Emergency or Abnormal sections. You use those numbers when things go wrong on the instrument panel on a dark stormy night and you should have a ballpark figure in mind without looking it up (2.5 deg and 86% N1). That will keep you on an even keel till you get things get sorted out. You don’t yank on the stick and get 15 deg nose high and wonder why you’re dropping like a piano (my Air France pet peeve, again). If you were in level flight, you probably don’t have to do anything (certainly nothing radical like holding that stupid little WallMart joystick against it’s aft stop).
    
   Capt BC

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8. 
   

   
   

   Brian on Feb 04, 2012

   You say 2.5 degrees at 86% N1 makes me think you’re referring to the main wing AOA. Am I mistaken?
    
   PS You didn’t know that little stick makes you go up and down? It controls altitude! I think the airbus’ altitude stick was broken, that’s my recount of the issue. The guy held it back like he should have. For some reason it wouldn’t make him go up anymore. 🙁 *sarcasm*

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9. 
   

   
   

   BC on Feb 05, 2012

   Usual cruise number is 2.5 deg nose up on the ADI and 86% N1 with the throttles. Should keep everything fine till you figure things out on a 777. We normally use EPR’s, but it the electronic engine controls go TU (or the EPR probes also get iced up), then they revert to N1.
    
   2.5 deg on the ADI has nothing to do with AOA. If the AOI was 5 deg, you could guess the AOA to be 7.5 deg, but that would depend on the relative wind. If you’re dropping like a piano, the AOA would be much higher due to the relative wind vector (and obviously higher than the stalling AOA).
    
   Since the wing has a twist (washout), it doesn’t all stall at the same time. Most of the “twist” is actually an aifoil transition (tip airfoil is a lot different than the root airfoil). First, the stick shaker goes off (I guess it’s a stick wiggler on an Airbus :)), then as you get slower, the root stalls and the plane mushes. Then a sink rate develops and AOA increases even if you don’t raise the nose (Air France).More of the wing is stalled. Depending on the AOA, the outer part of the wings may still be flying…lets you roll it to 60+ deg of bank so you can get the nose down….unless you’re in an Airbus, which stops at 30. Sometimes you have to roll it to get the nose down because elevators are not very effective on airliners. You have to run the stab trim (the entire horizontal stab moves) to get any radical pitch response (can’t really do that on an Airbus either since I think it’s automatic)
    
   I haven’t flown an Airbus, so I can’t verify all my Airbus comments. But I am familiar with the design goal of Boeing and Airbus fly-by-wire sytems. “If it ain’t a Boeing, I ain’t going” is a saying that comes to mind. If I hold the stick full over in an Airbus, it will stop at around 30 deg. If I hold the yoke hard over in a 777, it will do an aileron roll (a barrel roll with a little positive G is better on the fuel and CSD systems :)) Yes, it’ll remind me that I’m not supposed to be doing that, and try to fight me. But the plane will do what the pilot wants. If I turn the flight computers off, it won’t even try to fight me. On a really bad day, it still has a couple cables going from the yoke to a couple spoilers and the tail, to control pitch and roll when everything else goes TU. A well thought out plane (it’s still a pilots plane).
    
   Sorry, I have to stop with these long posts.
    
   Capt BC

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10. 
    

    
    

    BC on Feb 05, 2012

    By the way, 2.5 deg AOA would be too inefficient for cruise. Needs to be closer to max L/D AOA. Too much to explain….it would take a book.
     
    Get:
    “Glider Polars and Speed to Fly made easy” by Bob Wander. Google Bob Wander books or the Soaring Soc of America website bookstore. It’s the single best book on what aerodynamics are really all about. It’s only the size of a magazine and easy to read and understand. Read it and you’ll know more about how a wing flies and behaves than even most airline pilots. Why? Because the FAA gears training around airspeed and not AOA. And it’s all about AOA. Get the book.
     
    Capt BC

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