How can I calculate critical loading conditions for max flap extension?

[Rameez_Ul_Haq]

TL;DR Summary

I am trying to calculate the critical loading conditions for Flaps (in order use them inside FEA to determine if the Flaps will be able to bear these critical loads) during the design phase of a civil aircraft.

Below shows the generic V-N diagram for most of the civil aircrafts.

Now, I have the CFD results available for multiple combinations of angles of attack of the aircraft, with the deflection angle of the flap. I have to choose a couple, to input into my structural analysis of the flaps. How to determine the most critical loading conditions (Aircraft's angle of attack and Flaps' deflection at that angle of attack) for the flaps?

Now what is understood is that the greater the flap extension, the more overall Cl of the wing/aircraft (for the same angle of attack), and this will cause flaps to take more load during the flight (assuming that the extra load due to increase in Cl in just coming from Flaps only, and not from the actual wing).

Therefore, when the flaps are deployed to full extension, that is when the flaps should take the maximum load. How should I determine and calculate the load factor at this maximum flaps deflection, and also maximum flaps extended speed for this case? If these both are known, then the angle of attack of the aircaft can be found, and hence that specific CFD result can be implemented in my structural analysis of the flap.

Is there a limit on the load factor that beyond which the flaps cannot be deployed (or fully deployed) during flight? Is it possible for the flaps to be deployed fully when the aircraft is undergoing limiting load i.e. 4.4g and -1.8g (as shown in the V-N diagram above)? Or a completely new and different V-N diagram should be built for the aircraft, where flaps are completely deployed?

 

[Lnewqban]

The only limitation for flaps deployment is airspeed, and strong atmospheric turbulence, perhaps.
The mechanism is the weakest point, which may get stuck in one position due to airspeed that is out of the safe envelope.
They are calculated with a huge safety factor; therefore, mechanical failure under load is not likely.
If the wing is overloaded, it simply can’t sustain level flight, no failure is induced.

 

[jrmichler]

Flap limiting speed is determined by the peak force on the flaps. The airplane is rated for a maximum vertical acceleration. That acceleration is limited by stalling when below the maneuvering speed in the V-N diagram. Above that speed, peak acceleration is limited by the pilot. If the pilot exceeds those limits, structural damage results. The diagram above is for flaps retracted.

The exact failure mode depends on exactly what is the weak link. A Cessna 150 flown by a spoiled rich kid who attempted a loop was damaged when the flap tracks bent. He flew it back, the airplane was repaired, and the kid was thrown out of flight school. During WW2, some bored pilots in the South Pacific read the operator's manual for a PBY-5A, which stated that loops were impossible in that plane. They decided to find out. The entire trailing edge of the wing bent up, that airplane was landed, after which it was only good for spare parts. My father claims he was not involved. And a recent accident involving major structural failure: https://www.planeandpilotmag.com/ne...ash-mooney-in-minnesota-bizarre-and-baffling/.

Summary:: I am trying to calculate the critical loading conditions for Flaps (in order use them inside FEA to determine if the Flaps will be able to bear these critical loads) during the design phase of a civil aircraft.

Is there a limit on the load factor that beyond which the flaps cannot be deployed (or fully deployed) during flight? Is it possible for the flaps to be deployed fully when the aircraft is undergoing limiting load i.e. 4.4g and -1.8g (as shown in the V-N diagram above)? Or a completely new and different V-N diagram should be built for the aircraft, where flaps are completely deployed?

So the answer to your questions is in FAR Part 23: https://www.ecfr.gov/current/title-14/chapter-I/subchapter-C/part-23. Look for structural design envelope first, then outward from there.

 

[FactChecker]

Now what is understood is that the greater the flap extension, the more overall Cl of the wing/aircraft (for the same angle of attack), and this will cause flaps to take more load during the flight (assuming that the extra load due to increase in Cl in just coming from Flaps only, and not from the actual wing).

This should not be assumed to directly indicate the structural load on the flaps. The entire wing, including the flaps, causes increased lift.

Therefore, when the flaps are deployed to full extension, that is when the flaps should take the maximum load. How should I determine and calculate the load factor at this maximum flaps deflection, and also maximum flaps extended speed for this case?

From vague memory: The forces on the flaps are not the most critical structural force. The flap moving mechanism does not need to be very fast and is resistant to stalling or becoming too slow. It is the entire wing aerodynamics and structure that you need to be concerned about.