Handling with flaps

[UCKAN]

Hi all.

This is my first post. I've been lurking this site for years. I think it is time to register and ask questions.

My first question is about handling with flaps extension.

I know flaps extension makes nose down pitch movement on low wing aircrafts because center of lift is behind center of weight so that any increase in center of lift pitches nose down but is there also some other effect so called balloon effect? I dont have much info about it.

These are the values i got them from official QRH Unreliable speed section.

50 tons 737ng in level flight 5000 ft

flaps up vref40+70 / 5.5 pitch att 54 N1
flaps 1 vref40+50 / 5.0 pitch att 56
flaps 5 vref40+30 / 5.5 pitch att 57
flaps 15 gear down vref40+20 / 5.5 pitch att 63

Say we are manually flying 737ng in level flight with flaps up. our speed vref40+70 with 5.5 pitch att 54 N1 and we want to slow down to vref40+50.

After selecting flaps 1, 737 wants to pitch down. according to the values above, we shouldnt let nose drop too much because vref40+50 needs 5.0 pitch att. Is that proper technique? What about balloon effect? How do you handle with that?

Also according to the values above, we dont need reduce thrust to slow down. In contrast we should add %2 more power. Is that because flaps 1 creates enough drag to slow down the 737? Is the same concept applicable for the next flaps extensions?

During flaps extensions, hold the attitude or minor changes and let the aircraft do the job? Is that simple?

Thanks in advance.

[172_driver]

During flaps extensions, hold the attitude or minor changes and let the aircraft do the job? Is that simple?

I think you are spot on. There will be some trim changes you need to anticipate and cancel out. Just let the plane do its job and find new equilibrium - job done. Good seat of the pant exercise for the simulator.

[UCKAN]

Thanks for reply.

There are still two questions in my mind.

1) Some people say flaps shouldn't be used for slowing the aircraft. but when i look the values for 737ng, after each flap selection, you don't need to reduce thrust to reduce speed, in contrast you have to increase thrust a little bit.

My understanding is extending flap creates drag and drag slows the aircraft. my question is why do some people say don't use flaps to slow the aircraft? Am i missing something?

2) My second question is what is balloon effect? In one thread there are lots of different explanations on balloon effect. http://www.pprune.org/tech-log/253852-flap-2-a320.html

Could you tell me what balloon effect is and how do you handle with it?

Thanks.

[eckhard]

Hi UCKAN,

Good questions!

1/ The flaps on many transport aircraft are big and complicated. They may have several panels, e.g. The wing, a slot, fore-flaps, a second slot, the main flap and even another slot followed by a third section.

The drive mechanism and support structure is designed to move the flaps in and out and to hold them in postiton for a short period of time at a reasonable manoeuvring speed. As with all things in aircraft design, there is a compromise between strength, weight and durability.

Sure, you could extend the flaps at the limit speed and use them to help decellerate the aircraft. This may be fine the first hundred times but on the 101st time you may start developing cracks, deformation or other damage in the flaps or in the drive and support mechanism. So, to avoid unnecessary maintenance costs, operators generally have an SOP that discourages use of flaps as speedbrakes and also encourages waiting until 15-20 knots below the limit speed before extending the next stage.

The FCOM for the 747-400 for example has a statement that says something like:

"Flight with flaps extended for prolonged periods, other than holding in the vicinity of an airfield, is prohibited. Flaps 10 or greater should not be used for prolonged holding or other extended periods of flight.
When holding with flap extended, limit speed to 250 K for flaps 1 and 230 K for flaps 5."

So, the best way to decellerate is to:
Think ahead (descend early, extra track miles, etc.);
Use speedbrakes;
Use the Gear;
All the time, extending flaps according to the recommended speed schedule.

2/ The balloon effect occurs when a particular flap extension produces a marked change in trim and/or lift coefficient. The effect may be to cause the flightpath to diverge upwards and therefore to require a lower nose attitude to maintain the desired trajectory.

How to deal with it? Again, being aware that it will happen is half the answer. Be ready to adopt the appropriate attitude as the flaps move. Keep in trim. If intercepting an ILS glideslope from just below, watch out for GS capture as the balloon takes effect. If intercepting from above, be aware that you will need to increase drag before extending the flaps to the 'balloon setting'.

If possible, explore all of this in the sim with some free-play time and get the hang of your aircraft!

[Piltdown Man]

Think of the end goal. That is to arrive on the end of your approach, gear down, flaps set at the required speed with an appropriate power setting. Your starting point is in a descent at say 250 knots with idle power. In an ideal world you will slow the aircraft up, lower the flaps stage by stage, lower the gear, lower the rest of the flaps and then set approach power. The tricky bit is to determine your initial descent point which may have been 140 nm away.

The problem is the real world. Your have to share the sky with other people and comply with sometimes ludicrous ATC constraints. This generally means that idle descents are impossible. When in such a system, I fly my plane as clean as possible (less drag) and only lower flap to enable a lower speed. I try not to use flaps to slow down but there are times when you have no other choice.

PM

[UCKAN]

Eckhard and Piltdown Man, thank you both.