Flaps-down G limits?

[India Four Two]

I could have posted this on the Questions forum, but I figured that there are enough knowledgeable people on this forum and that the answer would be of interest to PPLs.

Why is the flaps-down G limit for most (all?) aircraft restricted to +2.0 to 0.0 G? I am particularly interested in the reason for prohibiting negative G.

These limits apply to all the light aircraft that I have flown and I see that it also applies to transports - the 737 for example. I have done some Internet research, but haven’t found any conclusive information.

Another related question concerns the ASI white arc. Is the flap limiting speed (Vf?) applicable to all flap deflections? I have always assumed it is.

[ShyTorque]

If the flaps are up, they can flex across their chord, along with the wings and their associated hinge points because they all lie in the same orientation. If they are down, when the wings flex, the hinge points will move with the wing but the flaps won't bend because they are not in the same orientation. This will put a big load on the hinge points.

Try simulating this using a sheet of paper. It all flexes very easily. If you crease a third of it, lengthways to make a "flap" and then "lower" the flap and try flexing it again, the paper will crumple along the bend.

P.S. EMUAS Rule OK!

[Pilot DAR]

The plus 2, minus zero G limits for flaps extended are based upon the certification requirement. I'm sure that a wise aeronautical engineer back in the day realizes that there is no plausible reason to deliberately enter negative G with the flaps extended, nor be pulling more than 2G. That said, the airframe will withstand more, it is just not needed for certification.

[MarcK]

Originally Posted by India Four Two

Another related question concerns the ASI white arc. Is the flap limiting speed (Vf?) applicable to all flap deflections? I have always assumed it is.

Vfe is the limit with (full) flaps extended. Many aircraft allow partial flaps at higher speeds. Those limits will be on a panel placard.

[finestkind]

Combat flaps??????

[djpil]

With the large nose down pitching moments from the flaps the tail balancing loads are increased. Can be a critical design case for the tail structure.

[Genghis the Engineer]

Originally Posted by Pilot DAR

The plus 2, minus zero G limits for flaps extended are based upon the certification requirement. I'm sure that a wise aeronautical engineer back in the day realizes that there is no plausible reason to deliberately enter negative G with the flaps extended, nor be pulling more than 2G. That said, the airframe will withstand more, it is just not needed for certification.

Exactly my opinion. The standards only require +2/-0, there's no need for more than that, so nobody ever bothers to certify beyond those limits. But this only tells you what was evaluated, not how strong it really is.

G

[Genghis the Engineer]

Originally Posted by finestkind

Combat flaps??????

Is not a setting found on most aircraft flown by PPLs.

G

[Jim59]

Originally Posted by India Four Two View Post
Another related question concerns the ASI white arc. Is the flap limiting speed (Vf?) applicable to all flap deflections? I have always assumed it is.
Vfe is the limit with (full) flaps extended. Many aircraft allow partial flaps at higher speeds. Those limits will be on a panel placard.

EASA Certification Specifications provide for multiple markings on the white arc to show Vfe for different flap settings.

[Pugilistic Animus]

I just love it when windshear kicks me way beyond the white Arc

[Jim59]

A bit off topic but for amusement the following ASI from a current production aircraft has two Vne and two Vra values. Lower values Utility category and higher aerobatic.

[Fitter2]

Sensible answers so far, although nobody has mentioned spanwise lift distribution, which gives a discontinuity (stress concentration) at the flap end. Multiple stress cases are a bore....

[Pugilistic Animus]

I haven't seen the term spanwise load distribution in a while...some old stuff I wrote about spanwise load distribution hopefully nothing is wrong and maybe a little helpful

Effect of Span-Loading on Aircraft Performance

[eckhard]

[QUOTE]With the large nose down pitching moments from the flaps the tail balancing loads are increased. Can be a critical design case for the tail structure.[/QUOTE]

If you’ve ever flown an Aztec or a Cessna 152, you’ll be surprised that they both pitch up quite strongly when the flaps are initially extended.

[PDR1]

Not really a surprise, just the transient vs the steady-state case. Flaps mostly (nearly always) produce a steady-state nose-down trim change, but in some aircraft there is a transient nose-up trim change due to a combination of a variety of factors ranging from the changing downwash over the tailplane to the different pitching moment of the flapped airfoil. The transient effects are (as always) less noticeable on larger aircraft because the greater moments of intertia damp the transient before it can do much.

I was always taught to hold the pitch attitude with stick pressure and then retrim once the transient had subsided - I always assumed this was universal.

PDR

[biscuit74]

Originally Posted by Pugilistic Animus

I just love it when windshear kicks me way beyond the white Arc

Likewise - Austers, especially the Terrier with quite low flap limit speeds were a pain in strong
wave turbulence. Often needed to get rid of flaps fast - those cantilevered flap hinges produced large twisting loads.

[Pugilistic Animus]

Originally Posted by biscuit74

Likewise - Austers, especially the Terrier with quite low flap limit speeds were a pain in strong
wave turbulence. Often needed to get rid of flaps fast - those cantilevered flap hinges produced large twisting loads.

It seems like the very second that you take your eyes off of the ASI that's when the windshear hits you...
I also get those flaps retracted at the speed of light as soon as I recognize it.

[India Four Two]

Lots of interesting input. This was obviously a question that was lurking and just waiting to be asked!

I'm sure that a wise aeronautical engineer back in the day realizes that there is no plausible reason to deliberately enter negative G with the flaps extended, nor be pulling more than 2G. That said, the airframe will withstand more, it is just not needed for certification.

Thanks Pilot DAR (and GTE for the follow up). There is an analogous situation with demonstrated crosswind velocity in gliders. The regulations require demonstration of satisfactory landing characteristics up to 0.2 Vso. Given the generally low stall-speeds of gliders and the unwillingness of manufactures to spend more money than is necessary during certification, we end up with ludicrously low demonstrated crosswind values in the POHs. For example, the Flight Manual of the DG1000S that I fly has this in the Limitations section:

2.12 Crosswinds
The demonstrated crosswind velocity is 15 km/h (8 kts.) according to the airworthiness requirements.

However, later on in the Normal Procedures section, we have this informative but at the same time useless wording:

4.5.1 Launch
... Take-off with strong crosswind is possible.

PDR1 wrote:

Not really a surprise, just the transient vs the steady-state case. Flaps mostly (nearly always) produce a steady-state nose-down trim change, ...

The C182 that I used to fly required a strong push and lots of nose-down trimming when going from zero to 40º flap. Are you saying that if I didn't push, it would have eventually stabilized in a nose-down attitude? I can't do a test, because it was written off last year.

[Pilot DAR]

Are you saying that if I didn't push, it would have eventually stabilized in a nose-down attitude?

Conditionally, yes. I will occasionally entertain myself on a very long, straight in final, trimming the plane flaps up/low cruise power, for a slight descent. Thereafter, I will aim to fly the remainder of the approach without pitch nor pitch trim inputs, while reducing power and extending full flaps incrementally. Remove a little power, nose dips. Add a little flap, nose rises, and airspeed reduces. This can be repeated without any pitch inputs, until you're flying with full flaps, and low power/low speed, on more or less the same approach path.

Though some Cessna POH's state that it is possible to actually flare and land without an elevator input, I've only once done this, while flying with a skilled safety pilot, who was about to take over and land, as I did not have my hands on the control wheel at all. Getting to the top of the flare with no elevator control is relatively easy, flaring not so easy....

[PDR1]

Originally Posted by India Four Two

The C182 that I used to fly required a strong push and lots of nose-down trimming when going from zero to 40º flap. Are you saying that if I didn't push, it would have eventually stabilized in a nose-down attitude? I can't do a test, because it was written off last year.

In principle, yes. I have tried (at a safe altitude) it in a Beagle Pup and a Chipmunk, and I found that if I set power to the expected setting, then dropped flaps and held it on the stick rather than re-trimming then once the speed stabilised I was actually pulling (slightly) rather than pushing. But aircraft vary and I expect these effects are more or less pronounced in different types. I once flew (under instruction) a PA38 and my memory suggests this had much less transient nose-up with flaps - the engineering in me wonders if the high tailplane made it less sensitive to the downwash change as flaps were applied. But that was 35 years ago and the memory could be false.

PDR