Flaps 20

[hanspetter]

Flaps are typically "10", "20" and "30 - 40". What exactly do these numbers mean? I guess they are degrees of deflection relative to the center of wing (the imaginary line from leading to trailing edge).

Basic flaps tilt down at the trailing edge. Then we have Fowler flaps that tilt down as well as adding extra wing surface by sliding back, slats along the leading edge (basically front mounted basic flaps) and Krueger flaps that swing out rather than deflect along the leading edge.

Flaps add camber -- the difference in shape between the upper and the lower surface. The more a wing bulges on top the more camber.

In an aircraft cfg we'll typically see lines as,

flaps-position.1= 10.000, 0.000
flaps-position.2= 20.000, 0.000
flaps-position.3= 40.000, 0.000

Then we have flaps response "scalars" in the cfg and probably also in the airfile. Ideally, we should be able to set deflection as well as to which extent the flaps add to the planiform surface of the wing. Could you please elaborate on the issue of flaps starting out with the basics?

[Tom Goodrick]

The most common measurement of flap deflection today is degrees. "Flaps 20" means set 20 degrees of flaps and, as you describe, means the flaps are rotated downward 20 degrres from the zero-deflection position which gives a clean wing airfoil. But sometimes you run into the terminology "first notch," "second notch." etc which are used with aircraft in which the flap deflection can be set only in increments. In some aircraft, the pilot moves a lever between detents to set the flaps. Thus "first notch" means the first detent. in other aircraft, the pilot moves a switch either to drive the flaps out or in until he sees a proper position from the indicator.

FS9 is set up only to use incremental flap deflection. The amount of flap deflection you can get is set only in the aircraft.cfg file. But the effect of the flap deflection is set only in the .air file as an increment to lift, to fdrag and to pitching moment at full deflection. A key here is that the full change in lift, drag and moment occurs at the max flap deflection setting whatever that may be as set in the aircraft.cfg file. Changing the max deflection from 30 degrees to 40 degrees will not change the max flap effect. You get half the effect at half deflection, etc.

The thing that can be helpful to the pilot is the choice of intermediate flap settings. These are used to help on takeoff (improving the rotation), and preparing for the approach in stages. The effect may not be linear so you do not necessarily want even incrememnts. I often adjust this when tuning FD's.

It really does not matter to the pilot how the flaps create the increase in lift. As you mention, there are many ways. Even the engineers boil it all down to an increase in the lift coefficient where that lift coefficient is based on the nominal wing area without regard to whether the flaps actually increase the effective wing area. The only things you have to keep in mind as a pilot is 1) at what speed can flaps be deployed and 2) what is the change in stall speed. The change in stall speed is, of course, the main reason we have flaps on airplanes. In many cases I put a FASP or Flaps Air SPeed light on the panel that turns green when you can lower the flaps. Two bad things happen if you try to lower the flaps when flying too fast: you damage the flaps mechanism and you may rip the wings off the aircraft. One bad thing that can happen that FS9 does not permit is a differential flap deflection between the flaps on the left side and the right side.

Here are some examples of the change in stall speed provided by flaps (Stall speed is always given in KIAS at max gross weight):

AIRCRAFT____NO FLAPS_____FULL FLAPS
Cessna 172____51__________46
Cessna 182____54__________49
Bonanza A36___62__________52
Baron 58______84__________74
Mitsubishi MU-2__104________78
King Air B200___99__________81
King Air 300____100_________81
Learjet 35A____"NA"_________99
Westwind 2____113_________99
Hawker 800____94__________85
Falcon 50______131_________77
Beech 1900____104_________88
Dash 8________96__________72
Lockheed Jetstar__140_______108

The main job of the flaps is to increase the lift coefficient so that the aircraft can fly at lower speeds. The flaps also increase the drag which helps to slow down to the lower speeds. The pitch effect is complex. The nominal pitch effect is a nose-down moment. But because the increase in lift causes an upward change in the flight path with the nose moving up a bit to follow the path, the pilot notices an upward pitch. The complication here is the fact that pitch angle encompasses both inertial orientaion and angle of attack orientation. The moment tends to reduce the angle of attack slightly but the slight climb tendency points the nose up slightly. These effects are experienced in different degrees in different aircraft.

Only in the .air file can you set the lift drag and moment increments directly. In the aircraft.cfg file you can set lift, drag and pitch "scalars" which are simply multipliers of the increments set in the .air file. It is handy to simply change these scalars a little to adjust the flap effects. But without knowning what the actual increments are, you are somewhat "in the dark" as to what change you might experience.

[hanspetter]

As always, good and informative answer
Regarding FS we should ensure that we get the right aerodynamic response to flaps deployment. I remember aircraft requiring trim adjustments in EITHER direction and that's hardly correct. The higher the flaps setting the further down (forward) the correct trim providing the bonus of better forward view during finals.

As anside, if you set flaps at excessive speeds they will blow in Flight Unlimited III. Then you'll have to try to land the aircraft without being able to use flaps. Would it be possible to set up flaps damage in FS?

After I wrote the post that started this thread I got think of "flaps 50". Yes, airliners may have "flaps 50". Provided the number defines the deflection "50" sounds absurd -- 45 degrees would be at a right angle to the wing! Maybe there's more to the (high) numbers than just the angle of deflection? My best guess is that the high numbers are used for a combination of flaps and slats.

[flaminghotsauce]

90 degrees would be a right angle to the wing.

[hanspetter]

Absolutely correct, 90 degrees would be a right angle rather than 45 degrees My bad. Still, 50 degrees sounds extreme. Are the flaps really at 50 degrees for "flaps 50"? If so, that explains why normal operation tends to do with less than full flaps.

[flaminghotsauce]

Yeah. I was taught to fly a 172 to the runway with less than full flaps. I rarely used the third "notch" of flaps. Usually only in no wind situations, or practicing short field ops.
We regularly practiced varieties of flap settings for landing, but normally not full flaps.

[Tom Goodrick]

Yes, in any aircraft, full flaps are only used on calm days and then only when you need to land on a short runway or at minimum speed (as when you have problems with wheel-shimmy).

On jet airliners, the wing flaps are segmented with gaps between segments. The angle applies to the last segment extended relative to the mean chord of the wing. The last segment can be 80 degrees to the wing chord but looking at it from the side you would see a cascading curved flap with several gaps. Yes, in FS9, leading edge slats are included in the first one or two flap positions.

Here is a description from Jane's for the 747-400 flaps:

"[There are] three sections, fore flap, mid flap and aft flap moving rearwards as single panel for only 5 deg; thereafter three sections separate progressively to form three slots and camber angles relative to each other increase progressively."

The max flap deflection may be mainly drag for braking on the ground. They don't give stall speeds.

[hanspetter]

Here's a related issue that we might discuss, namely how to deploy flaps without too much pitch upsets. Autopilots do this job brilliantly.

A less brilliant sequence goes like this: The aircraft is trimmed for a speed somewhat above the stall limit. You set a notch of flaps / another notch and the plane pitches up and slows down to stall speed. You push the stick / yoke forward and recover and then start trimming it down to attain healthy flight again. After a period of phugoid you get it right.

I've been quite successful in avoiding the upset by trimming the aircraft down prior to setting flaps while holding some stick back pressure. That is, the aircraft is actually trimmed for flying faster but I hold it at a higher angle of attack manually. Then, when flaps are deployed and I feel the nose is rising I gently release the stick / yoke back pressure to counter / balance out the pitch up. With this approach there's no frantic re-trimming since the trim is quite right for the (next) flaps notch as you release the stick / yoke back pressure.

[flaminghotsauce]

While your method works, it seems backwards to me as I read it. Pre-trimming?

Typically, in the 172, I will be in the traffic pattern at the bottom of the green cruise speed. When I get to the end of the runway, I pull the power down to 1800 RPM, and hold the nose up until the airspeed reaches the white arc, flap safe speed. Hit the first notch, and three "throws" of trim wheel and it's settled in the descent. Turn base, second notch, trim. Turn final, and adjust power to keep the runway in the center of the windscreen. This is how I was taught, and still do it.

When turning base to final, I should be 400' AGL, thereabout. Flap deployment shouldn't be so close to stall that there would be a recovery phase. I like 90 knots in the pattern, flaps as soon as I hit the white arc, 80-kts. I'm not sure the speed on base, just as soon as I get wings level I go to flaps. Final I was taught 70 knots with those two flaps, which is well above stall speed.

I never get so close to stall, so much so that I don't even recall the stall *1.3 speed.

[Tom Goodrick]

Hans, your method is, in general, terrible. Don't get into that bad habit.

Flaming, yours is, of course right for the 172 flying a normal rectangular pattern (you meant to say you are on downwind across from the runway when you pull back to put the KIAS in the white), but it does not work well for Learjets or other high-performance aircraft.

Hans, you must always stay well above stall. In many planes, flirting with stall when low and slow on near or during the approach is a very needless risk.

In the past few days I have been flying the C-182 (touch down 55 KIAS), the Learjet 35A and the Astra SP (touchdown about 130 KIAS). I just made some careful adjustments to the Astra SP to handle the problem of slow flight because I read a pilot report that said it should stall at a slower speed. (That report was somewhat wrong because they were at a very light weight. I fixed up the V2 and Vref gauges for that aircraft after I changed several parameters affecting the slow flight.)

The pilot report said "Deployment of the electrically actuated flaps for the stall sequence did not give any ballooning motion to the aircraft." (Aviation Week & Space Technology Business & general Aviation Aircraft Pilot Reports, McGraw Hill).

So, you see that it does not always happen. Now consider that the Astra always flies at close to 250 KIAS until preparing to land and the first 10 degrees of flap come down at 250 KIAS. You see that they ahve done some fancy engineering to keep pilots happy.

It's a bad idea to fly any aircraft, in real life or in FS9, without boning up on the Pilots Operation Handbook (POH). Before my first flight in the 172 I bought the POH and studied it for two days. I did the same before my first flight in the American Aviation Trainer (aka "Yankee"). I read all the time of people spending a week in ground school before they transition to a new turboprop or fanjet and these are people who are already rated in similar aircraft. Even magazine test pilots get the POH to study before their first flight though they have a top-notch test pilot sitting next to them.

What can we do? If there is a checklist and, perhaps even a tech note with the aircraft, read it and stuidy it. Know the weights and speeds for all phases of flight. Don't just grab the stick and push the throttle forward. Set trim and flaps properly for takeoffs. Often this will depend on weight. On takeoff, raise the flaps as you pass 400 ft and get clean shortly after (by 10,000 ft in a 747).

If you don't have a checklist that you trust or a tech note, look at the flap settings in the aircraft.cfg. Choose flap speeds for each segment that are about 20% below the lowest damage speed.

Find out the Max landing Weight (MLW). If you can't, assume you must land with 50% fuel or less - never at max gross weight.

If you can, look in the .air file at the change in pitching moment for flaps and for gear. They should both be small positive numbers (giving a downward pitch) on the order of 0.005 to 0.02. If they are negative, change them! In some cases they can be zero because the designers may have fixed the controls to compensate for the extension of gear and flaps.

If you have the right weight (near MLW) when you first deploy the flaps and you have slowed to the proper speed for first flaps, there will be very little disturbance. Whatever there is you can handle with a little sensitive touch on the stick.

Practice this effect with progressively complex aircraft - Baron, King Air, and then various jets from my site. The Laerjet 31A is a good starter jet. It can land on a 3000 ft runway at standard conditions. For a challenge to see what I have been talking about, try the Falcon 50. That can fly from a 3000 ft runway on the East Coast of US to a 3000 ft runway in Norway with no problem. But you'll have a tiger by the tail if you try dropping the flaps above 160 KIAS. (I'll have to double-check that one. It's been a couple years since I flew it from Bangor to Freiburg, landing on a glider port.)

[hanspetter]

To make sure we understand each other -- I DO NOT hit the flaps when I'm trimmed up close to the stall speed anymore. That has been done in the past to keep a wide berth to flaps overspeed. What I have been doing is to anticipate the flaps deployment pitch reaction and run the trim wheel forward while holding the trim manually with stick back pressure. By doing so I can easily "lean the weight forward" by releasing the back pressure and smoothly counter the flaps induced pitching.

[Tom Goodrick]

I just spent some time flying the Falcon 50 and found that all I said about it is correct. Spoilers are used to slow from 250 KIAS to 200 KIAS, gear is extended at 200 KIAS and used to slow to 160 KIAS where first flaps can be deployed. Vref on final is about 90 KIAS. It works. My landing was at 80.55 KIAS and -152 fpm. Contrast this with a landing at 113.1 KIAS and -50 fpm in the Astra SP and at 130.6 KIAS an -219 fpm in the LJ35A.

Here's an excerpt from a very pertinent article in July '09 AOPA PILOT magazine by Barry Schiff, a highly experienced pilot. (See www.barryschiff.com.) Note his reference to the nose attitude caused by flap deployment while flying steady on final.

_________________________________________________

"Why all the flap?"

Anyone who has spent time watching landings has wondered why so many pilots land on all three wheels simultaneously in tricycle-gear airplanes. In many cases, it is because some aircraft are more difficult to land properly (nose high) when flown with a forward center of gravity (no rear-seat passengers or baggage). The use of full flaps contributes to this problem because this often diminishes the elevator authority needed for the pilot to hold off the nosewheel after main-gear touchdown.

This makes me wonder why landings are always made with full-flaps in such aircraft. It is easier to make a two-point landing when only partial flaps are deployed. Depending on the airplane, this would mean landing with half, two-thirds or three-quarters flaps. This makes it easier to hold the off the nosewheel until well after the mains roll onto the runway. Such landings are typically smoother too.

I used this technique when teaching students who had particular difficulty in making two-point landings. By using only partial flaps, aircraft attitude on final is not quite so nose-low, and this alone makes it easier to transition from glide attitude to flare and touchdown. With greater elevator authority, students find it easier to prevent touchdown until the aircraft has been flared to the desired node-high attitude. Less muscle is required to achieve that attitude, too. Once these students got the hang of landing with partial flaps, I transitioned them to full-flap landings.

Using maximum flap extension is the optimum way to land, but this is usually not required by airframe manufacturers or by the FAA. Pilot operating handbooks for Cessna singles, for example, simply advise pilots to extend flaps for landing "as desired." The pilot has the option to use as much or as little flap as he desires. One advantage of using full flaps is that this results in the maximum stall-speed reduction. The last portion or "notch" of flap deployment, however, typically does not reduce stall speed by more than a knot or two. In some cases the last 10 degrees of flap extension does not lower stall speed at all. The additional flap is provided mostly to add parasitic drag and allow steeper descents at given approach speeds. It is axiomatic that most stall protection is provided by the first half of flap deployment and most of the drag results from the second half.

To compensate for slightly increased stall speed speed resulting from a partial-flap approach, a pilot should add the same number of knots to one's approach and over-the-fence speeds. The use of less than full flaps does result in more floating in the flare; the airplane decelerates less quickly. This provides pilots with more time to properly position the aircraft for touchdown. Partial-flap deployment obviously increase the landing distance somewhat, but not significantly unless runway length is critical or marginal. At such times, full flap extension should be utilized.

If you have not tried a partial-flap landing in airplanes that tend to land in a three-point attitude, you should. In these and other airplanes, landings are typically improved , and you are afforded more opportunity to "fly" the nosewheel down to an equally smooth touchdown. Another advantage of partial-flap approaches is that this configuration allows an easier transition to a go-around (VFR) or a missed approach (IFR). It is my practice to execute partial-flap approaches when the possibility exists that I may reach minimums and not be able to see the runway. If the runway is found waiting for me, I then have the option of completing the the approach and landing with partial flaps, or I an extend the remaining flaps once I have the runway in sight. (Pilots are cautioned not to change flap settings when less than a few hundred feet above the ground unless they are comfortable with the configuration change and proficient in coping with the pitch change this produces.)

Ordinarily, no-flap landings are made only when there is a failure of the flap system, and the pilot has no option other than to land with the flaps retracted. Although every pilot should practice this procedure once in a while, few do, and they will be completely surprised by the relatively nose-high attitude required on final approach when the real thing occurs. Their tendency is to lower the nose to a more familiar and comfortable attitude, which results in excess speed. Airspeed on final should be increased only a nknot or two more than the difference in the "clean" versus the "dirty" stall speeds as shown between the bottoms of the white and green airspeed arcs. The additional airspeed used to compensate for the higher stall speed should be 1.3 times this difference in stall speeds.

Pilots doing this for the first time are amazed at how much a "clean" aircraft floats during the flare. The runway zips by at an alarming rate, which is why excess airspeed on final approach can be dangerous. This results in a tendency to plant the airplane on the ground prematurely, which in itself can be dangerous. Be patient during the flare, and do not touch down until in a substantially nose-high attitude.

Practice no-flap landings only on long runways. If uncomfortable doing this for the first time when alone, obtain an experienced instructor. Your first no-flap landing should not come during a genuine emergency.
_________________________________________________

This is one of the best descriptions of the experiences using various flap settings I have seen. It deserves some study. Obviously, the risks go up with the approach speed of the aircraft. It's no big deal in a 172, just a notable difference in attitude with a 5-knot difference in speed.

[hanspetter]

I just tried your Falcon and it worked as a charm. I saw no ballooning with this one. I slowed down to about 90 with full flaps and landed with a speed of about 85.

Apart from bad FS models, which aircraft are prone to ballooning? I'll check the airfiles of those FS models that seem to have extreme pitch reactions.

[Tom Goodrick]

You will find the bad ones have a negative pitching moment change with flaps in the pitching section of 1100 (primary aerodynamics). A peculiarity of the sign convention used in FS9 dictates that negative moment causes the nose to rotate upward.

Some of my early FD files shoiw that too. I have kept my Skylane and Skylane RG files up to date but not some of the derivatives. I tested the Turbo Skylane and found it was ballooning because of a negative flap moment.

[flaminghotsauce]

I had to think for a second, Tom, when you said you touched down the 182 at 55 knots. I must admit, I don't have a clue what the actual touch speed is, as I'm not looking at the airspeed when I'm two feet above the runway. I now am wondering what my average touchdown speed is. I am not using any of your fancy gauges in FSX, so I don't have a readout.

I'll try to check that next time.

Good article by Mr. Schiff. I always enjoyed reading his articles.