ERJ 145 pitching moment? Please help

[RobbyErj145]

Hi Guys,

I'm battling to understand why the application of speed brakes causes a dramatic nose pitch down in the ERJ. I can't seem to understand the couples. Is it the lift weight couple or the thrust drag couple that create this pitching moment.

[TopBunk]

Robby

Does it really matter?

Seriously, I have no knowledge of the ERJ145, never having flown them, but in my previous types, I just accepted the characteristics and lived with them. Maybe I was simplistic, but as I saw it, understanding wasn't going to change anything.

Just saying - keep life simple and take the money!

[Basil]

You may get a sensible answer in the Flight Testing forum.

[TopBunk]

Basil

By sensible, I presume you mean 'more technical'. I would suggest my response is nonetheless 'sensible'!

Just like the last inch of movement of the B747-400 spoiler deployment causing a pitch over, or the selection of flap 2 in an A320 causing a 'bunt over' - they happen, I can't stop them but can be aware and plan appropriately.

[jack11111]

"Seriously, I have no knowledge of the ERJ145, never having flown them, but in my previous types, I just accepted the characteristics and lived with them. Maybe I was simplistic, but as I saw it, understanding wasn't going to change anything."

I'm glad Sir Issac Newton felt differently about life.

[galaxy flyer]

Probably lift-weight coupling. The spoiler is located forward of the CofL, so extension causes lift forward of the CofL to be spoiled and the aft loaded component causes the nose down pitch.

[Basil]

COULD also be degraded tailplane downforce in turbulent airflow but the manufacturer should be able to provide a definitive answer.

[Le Flaneur]

I have never flown an ERJ-145, but the general first order principle is this - any time you change the lift distribution (via flaps, spoilers, flow separation due to stall/Mach Buffet) on a swept wing, you will get a pitching moment. So the speed brakes on the ERJ-145 must be positioned on the wing so that when deployed, the resulting net lift distribution is moved slightly inboard. Generally, the manufacturers attempt to provide no change in pitching moment with speed brake extension.

For example, the B-52H uses roll spoilers for most of its roll control power. The roll spoilers are positioned so that when they are deployed in a roll, the net lift distribution moves outboard, resulting in a pitch up. So unlike every other aircraft I've flown, you have to apply nose down pitch when rolling.

[westhawk]

I used to wonder about this when I flew the Westwind and the Lear 60 concurrently. They reacted in opposite ways to each other! The Westwind has a straight, double tapered wing design and the Lear has a very slight sweep of the leading edge. (13 deg, IIRC) IAI call the drag devices on the Westwind speed brakes and Lear calls them spoilers. Both are positioned similarly at about 1/3 span from the wing root and I'd guess somewhere around 65 - 75% MAC or so. (just a guess mind you) The Westwind speed brakes are selectable to either deployed or stowed by a two position switch. At 300 KIAS, selecting speed brakes deployed requires a firm push on the yoke to counter the pitch-up reaction. 1 1/2 seconds of nose down pitch trim (moveable horizontal stabilizer) is just about right to counter the effect at 300 KIAS. In the Lear it's exactly opposite! The nose is pulled down a little less, but it can be countered in the same way. Of course it's much less abrupt in the Lear because unlike the Westwind with it's binary switch control, one can control how quickly the spoilers deploy on the Lear by how quickly the the control lever is moved.

As to WHY the pitch reactions are opposite in these two types when the panels appear to be similarly positioned on the wing, I have a couple of thoughts as to why that may be so, but possess no supporting data to rely upon as proof. So I too decided that I must simply accept that it is so and anticipate the pitch reaction accordingly. Perhaps someone with the aerodynamics theory chops to explain the difference in reaction between these two airplane types might choose to chime in, but I don't think I'll hold my breath while waiting! The most interesting answer I've gotten in response to the question was from a Bombardier Training Centre instructor: "I haven't a clue, eh"

[PDR1]

You'd need to see the lift distribution data for the specific config to really be sure. There is a general guestimate which suggests that if the effect is mild it's the result of changes in the chordwise (straight wing) or even spanwise (swept wing) lift distribution over the wing, but if it is "not mild" then it's probably due to mainplane downwash angle changes which change the tailplane AoA. This guestimate is simply based on the moment arms those forces will be acting on. If there is a large trim change AND a reduction of elevator response then it's probably a general disruption of the flow into the tailplane which is "blanking" or "masking" it. Hopefully that one is obvious.

On the other hand if it seems to mainly correspond to the arrival of flight deck coffee it's because the FO is careless about which switches he/she puts her coffee cup down on...

:E

[Centaurus]

Pitching moment reminds me of Mach Tuck in the single seat DH Vampire Mk 30 twin boom RAAF fighter in the late Forties and early Fifties. In a dive, compressibility would occur around Mach 0.76. The RAAF version had two extra air intakes called Elephant Ears, installed aft of the cockpit which led to the RR Nene engine.

In a dive, shock waves would form over the curved intakes and destroy elevator effectiveness. The Vampire would pitch down into an almost vertical dive. There were several fatal accidents attributed to this phenomena with the pilots unable to level out before hitting the ground. A modification was introduced which changed the intakes position to under the fuselage rather than on top. In turn that caused the aircraft to pitch up during a high speed dive and which was controllable.

[bafanguy]

Probably lift-weight coupling. The spoiler is located forward of the CofL, so extension causes lift forward of the CofL to be spoiled and the aft loaded component causes the nose down pitch.

gf,

[Disclaimer: I was just a garden-variety line pilot...not an aero engineer !!]

I had somewhat of the same thought when I looked at the diagram of the E145:


If those IB panels are the only speedbrakes available in flight when the lever is pulled, it may be that a nose-down pitching moment is created due to sweep. In fact, the CV880 (35 degrees of sweep, IIRC) used just such a principle to over come a jammed stab. It had a "split spoiler" switch which isolated either the IB or OB spoilers so when the lever was pulled, the desired pitch moment was created.

Just a thought...not a definitive pronouncement. ;-)

[galaxy flyer]

In the C-5, we did something similar in the "loss of pitch control" procedures, only we had to depower some the spoiler panels.