Announcement

Collapse
No announcement yet.

FAO: Gabriel: Flaps and Drag (Evan shall not comment)

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • FAO: Gabriel: Flaps and Drag (Evan shall not comment)

    Flaps can cause significant drag, but more so at higher amounts of deployment.

    Cessna 150 rapid descent with flaps 40 and an aggressive slip to drop the plane in. Sideways slip to slow plane over runway and kick straight to land. Exer...


    Try that in your 747 Boeing Bobby!

    Is this an unstabilized cowboy approach, or just routine fundamental airmanship?

    Important note: this post is not for Evan nor flame fanning.

    It is for Gabriel and others who find the subject of flaps and drag to be cool AND because it is off topic relative to the Dash-8 follow-me affair.
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

  • #2
    Originally posted by 3WE View Post
    Flaps can cause significant drag, but more so at higher amounts of deployment.

    Cessna 150 rapid descent with flaps 40 and an aggressive slip to drop the plane in. Sideways slip to slow plane over runway and kick straight to land. Exer...


    Try that in your 747 Boeing Bobby!

    Is this an unstabilized cowboy approach, or just routine fundamental airmanship?

    Important note: this post is not for Evan nor flame fanning.

    It is for Gabriel and others who find the subject of flaps and drag to be cool AND because it is off topic relative to the Dash-8 follow-me affair.
    I have a few hours in the Cessna 152, but it had only flaps 30.
    I have even fewer hours (maybe 2) in a C-150 that had flaps 40. Man, the change from 30 to 40... I don't know what it did with the lift, but it was like hitting the brakes.

    As you know, I have most hours in the Piper Tomahawk. Unlike the flaps of the Cessna, which are wide chord, slotted and a bit fowler, the Tommy ones are narrow and simply hinged in the lower skin. They do not "extend" at all, they don't let air flow around them, and they leave a gap in the upper side. I believe that the upper side of the flap is stalled even in the 1st position. Take-off flaps is zero, except for very soft / mussy runways (I used zero in dirt and grass strips). For short runways, they reduce just a bit the take-off run but increase quite a bit the distance over the 50ft obstacle. They are really useful just a speedbrakes (and they are good at at) and I always wondered why they even bothered to put flaps on it, if this is what they were going to put. My conclusion was that, because this is a plane designed specifically for training, the flaps were there just for procedural training. They may as well have put just a flip knob labeled "flaps" that did nothing.

    They have 3 positions, 0, half and full, and they reduce stall speed from 49 to 47 kts. But oh boy they do add drag.

    I did a lot of landings where I was high on final, and idling the throttle, having full flaps, and doing full rudder slips looked very much like this video. And it was a lot of fun too!!!! Actually, that was my typical landing, since I always wanted to be able to reach the field id the engine quit at any point while in the pattern.

    Click image for larger version

Name:	CeLkZ5sWAAE452_.jpg
Views:	1
Size:	40.9 KB
ID:	1015214

    --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
    --- Defend what you say with arguments, not by imposing your credentials ---

    Comment


    • #3
      Did you catch my comment asking if a plane, slowed down and using 'one notch' of flaps, might have more fuel efficiency than at cruise...

      Probable or improbable, Mr. Hell-better Aeroengineer?
      Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

      Comment


      • #4
        Originally posted by 3WE View Post
        Did you catch my comment asking if a plane, slowed down and using 'one notch' of flaps, might have more fuel efficiency than at cruise...

        Probable or improbable, Mr. Hell-better Aeroengineer?
        Yes I caught, but it was in the other thread. Unlikely.

        There is a legend that the 727 burned less fuel with 2 deg of flaps, that needed to be applied using the electric alternate extension because the first position in the flaps lever extends the slats. Boeing said it was just a myth. Yet, pilots believing more in fairs than in Boeing were using it, until one of them did it wrong and they crashed.

        Let's do a first order approach.

        As you know, the drag is the sum of the parasite and induced drags. The parasite goes with speed squared and the induced with 1/speed. ("speed" here is the equivalent airspeed, or we can say IAS). If you go too slow, you have too much induced drag. If you go too fast, you have too much parasite drag. There is a "sweet spot" speed where D is minimum. This sweet spot happens to happen at the speed where the induced drag equals the parasite drag.

        If you go a distance x with a drag D you need to do a work of D.x, which in turn needs to consume an equal amount of energy of D.x. If you do so a constant altitude (i.e. the potential gravitational energy doesn't change) and at constant speed (i.e. the kintic energy doesn't change), that energy will need to come from the fuel burned.

        The "usable" energy stored in a mass of fuel m is of course proportional to the mass burned, so it is k.m. So you have D.x = k.m and the MPG, m/x = D/k. Minimize D and you minimize the fuel burn.

        Minimum D happens at best glide speed. Indeed, the glide ratio is equal to L/D (lift divided by drag), and since L is constant equal to the airplane's weight, best glide ratio happens at minimum D.

        So, ask yourself a question... How does the plane glides better? With or without flaps? Without, so even if the best glide speed with flaps is slower than without flaps, you you use more energy to go a distance x. Think of this: you are at best glide speed without flaps. Parasite drag = induced drag, or, in other words, total drag = parasite + induced = 2 parasite = 2 induced (since they are equal). You extend some flaps. The induced drag doesn't change. The parasite drag increases, so you are not a best glide anymore. You have to slow down to go to best glide. By doing so, you reduce the parasite drag and increase the induced drag, so the new sweet spot happens at an induced drag that is higher than without flaps, and since in both cases the total drag = 2 induced, with flaps the total drag will be greater, and so will be the fuel consumption.

        "Wait a minute", you may say. "That analysis is valid if you cruise at best glide. One typically cruises at more than best glide".
        Well, more or less. But let's take that as true. So? You want to use flaps to cruise slower? Why you don't slow down to best glide clean first? You don't need flaps for that. And, once you got there, slowing down any slower won't help, flaps or no flaps.

        Now (and here it comes the previous "more or less"), best glide speed happens at a given IAS. The same IAS can represent different true airspeeds, depending on your altitude. So the best you can do is fly the highest that you can while keeping best glide.

        For example, in the Tomahawk, best glide is 70 kts. I never got this high, but according to the manuals, at about 12000 ft you will get 70 kts with the throttle wide open, but the engine power (and the fuel consumption) would be just 40%, which is just about what you need to keep best glide i.e. 70 kts at sea level. The difference is that at sea level you will be moving 70 nautical miles every hour, while at 12000 ft you will go 84 nautical miles every hour with the same fuel burn per hour. That doesn't seem to be a lot, but it is a 20% increase in the MPG for a 20% reduction in flight time. Sure, if you go at sea level you can cruise much faster, up to 110 kts IAS=TAS at max continuous power. But the GPH will be more than 100% higher for 30% more airspeed, so your MPG will go way down.

        --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
        --- Defend what you say with arguments, not by imposing your credentials ---

        Comment


        • #5
          I think you missed my point and brought up another.

          You said 'best glide is the best you can do'.

          Yes.

          Therefore is best glide + 'takeoff flaps' maybe still better than going fast?

          You brought up altitude.

          As I think about altitude, suddenly I'm more Evan like: Hmmm, the Dash-8 down low might have had markedly more fuel burn...don't see that as a major concern, but curious.
          Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

          Comment


          • #6
            Originally posted by 3WE View Post
            I think you missed my point and brought up another.

            You said 'best glide is the best you can do'.

            Yes.

            Therefore is best glide + 'takeoff flaps' maybe still better than going fast?
            At one given altitude, best glide no flaps is better than any other thing in MPG terms.
            Best glide with take-off flaps may or may not be better than going faster than best glide with no flaps. It will depend on how much faster than best glide.

            And, again, all the previous explanations was a first order approximation. Engine efficiency, Mach effects, and other effects were not taken into account.

            --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
            --- Defend what you say with arguments, not by imposing your credentials ---

            Comment


            • #7
              Originally posted by Gabriel View Post
              ...It will depend...
              Flame-war expletives to you!

              That was a lame answer! Yeah, I know it is correct, BUT...

              Look at the image below entitled, "Typical Airplane Drag Curves". Please note that the right side of the graph goes a lot higher than the left side of the graph.

              I can look at that, and parlour-ass-hat-assume that going fast (cruising) generates a BUNCH of drag. Going near the best glide speed produces much less.

              So, take "much less" and add "a little more from flaps" and the sum likely falls way short of the drag at those high cruse speeds.

              To hell with your aeroengineering degree and black and white ISO-9002 mentality, the aggie-bull-crap-shoveler says, "for a whole lot of flight modes, and airplanes, you will probably burn less fuel going slow with a few flaps than at higher-power cruise scenarios."

              Indeed, you will be able to cite exceptions, and my comment had several fuzzy wiggle-words, but I put a beer behind it and also say that [getting back to our Dash-8-follow-me affair], the impact of the flaps on fuel economy was very likely minimal...

              Going back to the question I asked you- I say "probable" is the correct answer (even though there may be exceptions and that "it depends".)

              ...now, the effect of flying down low...mild curiosity here and glad that the Dash-8 QRH for following slow twins says, check your fuel quantity and range before committing to the 'rescue'.
              Attached Files
              Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

              Comment

              Working...
              X