We know all textbooks say that flaps are supposed to increase drag and cause a steeper approach angle w (w/out increase in airspeed) during a landing, while in a flapless landings, the a/c must fly a shallower approach and higher speed to maintain the same amount of lift. But when my instructor showed me a flapless landing, the "picture" did look shallower, but we still maintained a 2 reds 2 whites glideslope? He said in a flapless landing, just like in a flap extended landing, it is still possible to maintain a VASI glideslope. This totally confused me: if the approach is shallower with a flapless, why didn't I see 3 whites and one red or maybe all whites, as this would have been consistent with a shallower approach? Please unconfuse me

We've got some bits an pieces mixed up here! First thing, lets keep something constant. The descent angle would be good and we'll keep that at 3 degrees. With a normal approach as you add flap you add drag and lift at the same time, but not necessarily in the same proportions. You will also have have decreased your stalling speed. Now to keep the speed constant, you'll have to add power and lower the nose. The additional power to overcome the extra drag, the decrease in pitch to cater for the fact you have changed your wing (the chordline remember goes from the leading edge to the trailing edge, which is now a bit lower because of the flap movement). We are still on the 3 degree descent path. But what nomally happens on an approach is that you keep the power constant(ish) and allow the speed to bleed off (still maintaining your 3 degree approach) and you'll then see that the pitch has now remained similar, the power the same but the speed is reduced (I think the designers had something to do with this!).

Now, lets do a flapless approach. Set up your three degree approach and the power will be less than above. The pitch will also be a bit higher (you'll not see as much runway) but you'll still be on the same approach angle. Remember, we are talking about losing 320' per nautical mile.

Now let's forget about the three degrees (but they were a good group!). We'll now see how steep we can make a descent. We'll do that at idle power ie. a glide approach. Which will allow the steepest approach - the one more drag or the one with less drag (ie. flapless)? And when we have set up the descent, which will have the lowest pitch?

You can answer this yourself, I'm sure.

Best of Luck,

PM

Given that information i would say a flap landing, although the flaps will add lift for a short while, no?

Regards,
Chris

They will, but the lft:drag ratio being so much lower with than without flaps, the effect would be short lived i.e. with flaps, you can fly slower than the stall speed 'clean', but with no power set, the extra drag will slow you to the stall speed very quickly. Hence why, if your engine fails, you do not lower any flaps unless you're too high. When I was young, reckless and playing flight sim at the age of 10, to extend any glide, I'd raise the nose and add flaps. Wonder why that never worked for me, huh?

So, let me get this straight. The whole 'flapped approach is steeper than a flapless approach' thing only applies when it is a glide approach? Otherwise, the power means you maintain your approach angle, although more power is needed with flaps than without.

Yea that seems to make sense!

Regards,
Chris

You both are right (ish). A classic example is at LCY which has a 5.5 degree approach. In a F50 at heavy weights you cannot make that approach unless you have full flap and gear down because even at Flight Idle, the thing would be accelerating away from you. You also add addditional drag by selecting Go-around power, which puts the props up to 100%. On a normal approach (3 degrees), you can easily get away with no flaps and no gear (although the landing would be a be a trifle noisy!). So, you don't have to do a glide approach, but say you want to perform a steep approach to clear close in obstacles or to land closer to the boundary (a short strip senario), would you use A) No Flap B) Some Flap or C) Full Flap? Answers on a postcard please.

And "more power is needed with flaps than without" is, when all else is equal, true.

But lets be clear on lift. The Lift of a wing = C_L*1/2*Rho*Vs²S. Or in English, the lift generated by the wing is equal to (C_L) which covers the shape and efficiency of the wing plus it's angle of attack, the density of the air, your speed squared (small, change = big change) and the surface area of the wing - all multiplied together. Deploying the flaps gives lift as long as the flaps are out. Your stalling speed will also be reduced as long as that is the case as well. The only transient effect is the initial pitch change caused by their extension (or retraction) caused by the change in the position of the centre pressure of the wing.

PM

So are you basically saying that while a flap and flapless landings still maintain the same approach slope, the term steep only refers to the picture you see out the window? THat's the only thing that makes sense to me

No not at all. Let's gid rid of "steep" first of all. Steep refers to the amount of height lost per mile flown. The greater the height loss per mile, the steeper the approach. The more flap you have, the steeper you can approach. We haven't talked about attitude yet - the picture out of the window. Now to have a contrast, you have to keep something constant. So let's have slope X. If you fly down slope X, with flap and speed Y, you'll see a picture. Do the same slope at the same speed without flap, you'll have two differences. One, you'll use less power and the other, you'll see less out of the window. Your attitude will also be higher. Does this explain it?

PM

This must be my night for not being sure about who is confused about what… I’m going to go back to AirbusA380’s original question – and this looks like its likely to be another “rant” – sorry Becky!

Hey AirbusA380: your instructor is correct … you should maintain on or above the glide slope (electronic or visual) for any landing – with or without flaps. Recall that approach speed should be 30% above the stalling speed for the configuration used – plus one-half of the wind component and all of the gust component (and the additions shouldn’t be changed). So to calculate your reference speed with flaps and gear extended, compute your stalling speed for that configuration and add 30% to that figure, and in a no wind condition, that is the speed you will fly; and you will fly on (or above) the glide slope. To calculate your reference speed with the gear extended and the flaps/slats retracted, compute your stalling speed for that configuration and add 30% to that figure, and in a no wind condition, that is the speed you will fly; and, again, you will fly on (or above) the glide slope.

The things that will “change” in these cases are the configuration (flap settings), the pitch attitude, the airspeed, and, very likely, the power required. The point that may be confusing is that the approach gradient should not be substantially different – not if you fly the ILS glide slope as an example. In fact, maintaining the same descent path will require you to descend at a slightly higher rate with no flaps – why? Because you are coming down the final approach at a higher airspeed, and you have to loose the same altitude in the same horizontal distance. If you’re covering that horizontal distance at a higher rate, guess what … you’ll have to lose that altitude at a higher rate or you won’t wind up at the same place.

Let’s use some numbers so you can see that I’m not handing you a line of the proverbial “bullstuff.” The typical FAF is about 4 nautical miles (or 24,304 feet) from the runway end. Add 1,000 feet to the touchdown point and the distance from the FAF to TD should be 25,304 feet. The typical FAF is about 1500 feet above the ground. Let’s say that we have an airplane with a gross weight that would give us a clean wing stalling speed of 120 knots and a full flaps stalling speed of 94 knots. These are realistic numbers for a smaller transport category turbo-jet like the B737 or the DC9. This would mean that the flaps retracted approach speed would be 120 knots + 30% of 120 knots (which is 36 knots) or a total of 156 knots. The full flaps approach would be flown at 94 knots + 30% of 94 (which is 28.2 knots, rounded up to 29 knots) or a total of 123 knots.

How long will it take to fly from the FAF to the TD in each case?

The clean wing case, 156 knots on final approach, will take 1 minute 36 seconds to cover the 25,304 feet to the TD point. To lose 1,500 feet in 1 minute 36 seconds you will have to descend at an average rate of 938 feet per minute.

The full flaps case, 123 knots on final approach, will take 2 minutes 2 seconds to cover the 25,304 feet to the TD point. To lose 1,500 feet in 2 minutes 2 seconds you will have to descend at an average rate of 739 feet per minute. I don’t know if your instructor has ever talked to you about standard rates of descent on an ILS, but for a reasonably wide range of turbo-jet airplanes, choosing an initial rate of descent at the FAF of about 750 fpm will put you in the “ballpark” for more easily maintaining the glide slope.

The other differences will be the pitch attitude and power settings you will have to hold to maintain those airspeeds and rates of descent. For the clean wing case, the pitch attitude will be noticeably higher – putting your aim point much lower in the forward windscreen. This tends to give you the impression that you are flying a “flatter” approach – but, we’ve just shown you that it is not flatter – it is, actually, quite considerably steeper in terms of rate of descent – 200 feet per minute steeper. With the full flaps approach, the pitch attitude will be much lower, putting your aim point much higher in the forward windscreen, tending you give you the impression that you are flying a much steeper approach, but you now know differently. However, in each of these cases, the airplane should be on, or very close to, the normal, descending flight path – that is, on or slightly above the desired "glide path" or electronic "glide slope" – giving you the appropriate visual indication, just like you saw – 2 reds and 2 whites.

In these cases the additional problems that might be encountered during the clean wing approach, is that you will use substantially less power and will likely be on the verge of being in, or very nearly in, idle thrust. It is not good to make an idle thrust approach in a turbo-jet airplane – just in case you need the engines to spool up for some reason – they take longer from idle than they do from a slightly higher power setting. The other side of the coin is that if you allow the speed to increase while on final, you’ll have to increase your rate of descent to get the airplane down where you want it down. Also, on most airplanes, there is a maximum tire speed – meaning that landing with a higher speed, may result in tire failure.

Anyway, while you may very well NOT be talking about your efforts in a turbo-jet airplane – it is possible that you were using a smaller aircraft. I’m sorry that I can’t give you a similar dissertation for a smaller airplane – its just that its been a very long time since I instructed in anything but a turbo-jet – and habits are hard to break; but the premise is the same in either case.

Let me know if I can attempt to answer any additional questions you might have.

Air Rabbit, I appreciate the in depth explanation of what is going on. I am now fairly convinced that it is logical to stay on a VASI glideslope because after all, that dictates where you will wind up if flying the indication correctly, regardless of configuration. As you have explained to me, on a flapless approach, the plane is traveling faster, and therefore needs to loose more altitude in the same amount of distance, while in a flap landing, it's slower forward speed means it must loose less altitude per nautical mile. OK.
What i am still confused about is THIS!!!: if we're still on glideslope, which we have to be, WHAT IS ACTUALLY STEEPTER about an approach that uses flaps vs one flapless? What does the word "STEEPER" in the terminology refer to? What exactly do textbooks and all aerodynamic explanations assume is the image that pilots refer to in their mind when trying to understand the concept of "flaps permit for a steeper approach" You've JUST shown me, I myself have also deducted, and plenty others have confirmed that in a flapless situation, there is MORE STEEPNESS in terms of descent rate per unit distance, compared to a shallower descent rate using flaps. So, Do textbooks then just assume that because the pitch attitude LOOKS steeper, that the plane is also flying an actual steeper angle than in a flapless situation? Because, what they and plenty other articles online try to depict w/ flapless landings is a side view where they make it look like if you draw a horizontal line representing a runway, a plane doing a normal landing will show a normal descent path, while a plane doing a flapless will show a line shallower; here is the link to one such article:

http://blogs.warwick.ac.uk/csmith/entry/lesson_17_flapless/

Understand that in my mind, this if how I've managed to visualize the explanation, in terms of the glide slope deviation I assumed would happen in a flapless configuration. This worked fine for me, until I've actaully done a flapless approach, and what I EXPECTED to see was different than what I saw. With this being said, I guess I am confused, since two different trains of thought are tugging away at me. I don't know who if the textbooks are wrong, If I interpreted the explanation wrong, or what. Either way, I would appreciate any further elaboration until I understand this. Please feel free to express any further clarification, especially using any analogies in real life you might come up with.
THANKS SO MUCH, and continue with your replies

"...it's slower forward speed means it must loose less altitude per nautical mile." No, it loses less feet per minute but exactly the same number of per nautical mile.

"What i am still confused about is THIS!!!: if we're still on glideslope, which we have to be, WHAT IS ACTUALLY STEEPTER about an approach that uses flaps vs one flapless?" Nothing

"What does the word "STEEPER" in the terminology refer to? " The number of feet per nautical mile lost.

"Do textbooks then just assume that because the pitch attitude LOOKS steeper, that the plane is also flying an actual steeper angle than in a flapless situation?" The lesson you referred to is describing a glide approach ie. power at idle. If you have more drag you will cover less ground distance for every foot of height you have.

"Understand that in my mind, this if how I've managed to visualize the explanation, in terms of the glide slope deviation I assumed would happen in a flapless configuration." With or without flaps, you can still maintain a 3 degree approach - but this is NOT a glide approach, it is a powered approach. "Steepness" is the same with or without flap. So on a 3 degree slope, assuming the same speed, with flaps (comparing with the flapless case) the difference will be your attitude (which will be lower) and power setting (which will be higher). At a lower speed, the power setting will be similar to a flapless approach but almost certainly but your attitude will be lower.

Keep asking and we'll keep having a go!

PM



AirRabbit: I really enjoyed your explanation and whilst for me too it has been a long time since instructing in small planes, I think (and I'm not trying to be patronising) your numbers and explanation are spot on and your additional information is applicable to all fixed wing aircraft.

Thanks PD-Man. Your answers are right on.

AirbusA380:

The use of flaps allows you to fly a steeper approach – if you choose to do so. The PRIMARY reason for using flaps during an approach is to be able to slow the speed and still maintain an adequate margin above a stall. Of course you can extend your final approach and start down earlier than at the FAF and wind up having a shallower approach during final; but that means you'd have to be familiar with the obstacles beneath the airplane outside of the FAF and if this is what you wound up doing, you would be below the glide slope, both visual and electronic all the way. That may be acceptable in some airplanes - but it certainly isn't acceptable in turbo-jet transports. I know that some people tend to teach that way - but I don't. I don't think its necessary to start a final approach way out in "never-never" land when it can be accomplished the way it's supposed to be accomplished. Maybe that's the answer you were looking for ... but ...

Try this the next time you are out with your instructor – DON’T do this by yourself! I’m not kidding – ONLY with your instructor!

Pick an altitude (a reasonable one – like 2500 feet) and airspeed with a clean airplane; but choose an airspeed that is within the flap limiting speed for a specific flap setting. Now, while maintaining that altitude and that airspeed, lower the flaps to that setting. Remember, keep the same altitude and airspeed. What do you need to do? You should have to increase the power. Now clean up the flaps (retract them) and regain your altitude and airspeed.

Do the same thing again – set it up just as you did the first time; however, this time, leave the power at the setting you had for straight and level flight. Lower the flaps to one-half of the setting you selected the first time and maintain the airspeed – remember, leave the power set where it was. What do you have to do? You should have to descend in order to maintain the constant airspeed. Now, leaving the power set, lower the remainder of the flaps. What happens? Again, you’ll probably have to increase your rate of descent to maintain that airspeed.

After you’ve done this, when you return to the airport, ask your instructor to show you, or talk you through, landing on the runway as close to the approach end as you can, but pretending there is a 50-foot obstacle (a tree or something like that) right on the runway number! To be safe, you should clear that obstacle by at least 50 feet (that means you’ll fly over the runway number at 100 feet AGL. Now, land the airplane as close to the approach end of the runway as you can. What your instructor will probably do (or talk you through) is to lower full flaps, bring the power to idle, and pitch the nose down to maintain the maximum airspeed allowable for that flap setting. You have to be careful, because your flare will have to be started a bit higher than you’re used to and it will have to be a pretty aggressive flare maneuver. The result is getting the airplane down in a relatively short distance down the runway. Why? Because the use of flaps allowed you to make a much steeper approach. Again, DO NOT do this by yourself! I don’t want to read about your crashing on the approach end of a very nice runway!

After you’ve done this, feel free to ask anything more about approach “steepness.”

Power + Attitude = Performance

They are not suppose to increase drag, they do. Flaps means a higher speed for vmc purposes. You need to fly a Flatter and Wider appr, not neccs shallower. You may or not be on glide slope and you do not flare for the landing. You may have the vasi in order but I bet you are fast for a no flap clean config. FE

well, Finally got it; thanks for all the head drilling and perseverence.