Forward slipping a swept wing, transport category jet

[seagull]

(1) Only if the tail is glued on and constructed exactly in accordance with (and not one iota beyond) the requirements of Part 25. Boeing qualifies their vertical stabs (or at least did on airplanes up to and including the 777) to withstand rudder doublets a la American 587. Airbus doesn't. I don't know what the Jungle Jet builders do. Yaw maneuver conditions are specifically governed by 14 CFR 25.351. The applicable speed range is from Vmc to VD.:

Not sure where you got this impression, but it is incorrect. Boeing's will structurally fail if you do the doublet also. Boeing ran the scenarios after that accident, but did not advertise the results!

 

[seagull]

(1) Only if the tail is glued on and constructed exactly in accordance with (and not one iota beyond) the requirements of Part 25.

Oh, and with regard to this, remember the B777 wing that Boeing made a big deal about due to it not failing before 154% of the design load? Well, the requirement is that it meet 150%. Barely beyond. Their goal was "not one iota beyond", but they missed it. Most aircraft exceed it by a lot more than that.

 

[Boris Badenov]

Not sure where you got this impression, but it is incorrect. Boeing's will structurally fail if you do the doublet also. Boeing ran the scenarios after that accident, but did not advertise the results!

This is my understanding as well. Putting on our critical thinking caps, consider that the forces involved in a dynamic motion (like a doublet) are at least one order of magnitude more complex than steady state forces. Then add to that that these same dynamic complexities must be tested over the entire range of speeds (not to mention air densities and temperatures) that the aircraft is certified to operate in, and I suspect that you have a functionally intractable problem. I suppose we're all going to have to suffer through and just try not to do anything wildly stupid. No net!

 

[ryan1234]

2. What would the affect be on the engines? How would a forward slip reduce airflow into the engines? On my aircraft, with the engines mounted on the tail, I'm willing to bet you'd either (hopefully) manage a compressor stall on the engine that is being blocked from air coming in by the fuselage, or (possibly) manage to flame the thing out because it doesn't have enough air flow into it. There's no buffet margin chart for forward slips in a transport category jet, so you have no idea how far you could take the maneuver before you can one of your engines.



.



EDIT: I should also add, I'm picturing in my head a situation where you're 2,000' high at the marker, and you put a full control input in, in the form of a slip, in order to lose a ton of altitude and make the runway.

Compressor stalls are actually harder to do than one might think. Surely you wouldn't be at idle or low % anyway (spool time vs. descent rate), so let's say that the power was brought up 80+% or whatever would be needed and boards out...shouldn't be a problem. I've seen a medium bypass turbofan aircraft do a tailslide at power around 87% with no hint of a compressor stall... other than slightly creeping temps.

 

[Hacker15e]

Compressor stalls are actually harder to do than one might think.

Depends on the airplane.

In the T-38 with the GE-5 engines (the PMP mod), they practically compressor stall if you look at them sideways. In the F-15 with the PW-100 engines (and to some extent the -220 also), they would also compressor stall with some regularity in close-to-normal maneuvering regimes.

That being said, I've had a number of compressor stalls in both aircraft, and they're not always that big of a deal.

 

[Autothrust Blue]

Not sure where you got this impression, but it is incorrect. Boeing's will structurally fail if you do the doublet also. Boeing ran the scenarios after that accident, but did not advertise the results!

Yeah, I don't know what I was thinking when I wrote that. Self-administered slap on the back of the head. The document I misread/misremembered/otherwise failed to eidetically recall was Use of Rudder on Transport Category Airplanes (PDF target).

Quote:

Boeing airplanes are designed to withstand the structural loads generated
by a full rudder input out to the airplane’s maximum operating airspeed, Vmo/Mmo.
Some Boeing airplanes meet these requirements out to the design dive speed. This
means the structure has at least a 50% safety margin over the maximum load
generated by this kind of maneuver. As previously mentioned, Boeing airplane
vertical fins can also sustain loads if the rudder is rapidly returned to neutral from the
over yaw sideslip or the rudder is fully reversed from a full steady state sideslip.
Boeing airplanes are not designed to a requirement of full authority rudder reversals
from an “over yaw” condition. Sequential full or nearly full authority rudder
reversals may not be within the structural design limits of the airplane, even if the
airspeed is below the design maneuvering speed. There are no Boeing Procedures that
require this type of pilot input. It should also be pointed out that excessive structural
loads may be generated in other areas of the airplane, such as engine struts, from this
type of control input. In addition, large sideslip angles may cause engine surging at
high power settings.

Thank you. I was wrong, and got to learn something. And I got to re-read that document, which was quite enjoyable too. Seriously, thank you, and :beer: is on me next time.

<-- Dons Dunce cap and continues.

The next paragraph in that document is of interest:

It is important to note that use of full rudder for control of engine failures and
crosswind takeoffs and landings is well within the structural capability of the
airplane.

(this is also a certification requirement.)

Oh, and with regard to this, remember the B777 wing that Boeing made a big deal about due to it not failing before 154% of the design load? Well, the requirement is that it meet 150%. Barely beyond. Their goal was "not one iota beyond", but they missed it. Most aircraft exceed it by a lot more than that.

Hey, if Airbus is doing it, it must be safe, right? :insane:

 

[Hacker15e]

3. The CFM doesn't allow for it.

4. There's no training on it.

Well, hell, if the manual doesn't say you can do it....and if they don't TRAIN you to do it, it obviously can't be done!!

In case anyone doesn't detect the sarcasm there, I could fly the Queen Mary sideways through the gap between what's written in the flight manual and what is taught in training and the actual performance capabilities of the aircraft. Both the manual and training courses live squarely in the "warm, chewy center" of the flight envelope, and not out at the edges (which is what we're talking about in this case).

 

[ryan1234]

Depends on the airplane.

In the T-38 with the GE-5 engines (the PMP mod), they practically compressor stall if you look at them sideways. In the F-15 with the PW-100 engines (and to some extent the -220 also), they would also compressor stall with some regularity in close-to-normal maneuvering regimes.

That being said, I've had a number of compressor stalls in both aircraft, and they're not always that big of a deal.

Interesting... the resident Tomcat guy here said that he never had a problem with tailslides in the F-5E (-21 GEs?) or the F-14 with the GE motors... the TF30s were a different story.

 

[Matt13C]

What does a compressor stall feel like and how is it detected?

I am assuming thrust is lost as there is limited air going through the engine, but can it damage the engine or cause it ti flame out? Also, just curious, but what does it feel like? Does the plane feel like the engines quit and go nose down suddenly?

 

[mikecweb]

What does a compressor stall feel like and how is it detected?

I am assuming thrust is lost as there is limited air going through the engine, but can it damage the engine or cause it ti flame out? Also, just curious, but what does it feel like? Does the plane feel like the engines quit and go nose down suddenly?

I had a few compressor stalls before. It makes a rapid banging sound. No real pitch change and a little roll.
So you know when you take power out on tail mounted engines the nose comes up at first.

 

[Matt13C]

I had a few compressor stalls before. It makes a rapid banging sound. No real pitch change and a little roll.
So you know when you take power out on tail mounted engines the nose comes up at first.

Thanks for the info.

What causes the nose to come up? Are the engines mounted slightly inclined to the horizontal axis providing some nose down pressure?

 

[Hacker15e]

What does a compressor stall feel like and how is it detected?

I am assuming thrust is lost as there is limited air going through the engine, but can it damage the engine or cause it ti flame out? Also, just curious, but what does it feel like? Does the plane feel like the engines quit and go nose down suddenly?

It's usually audibly a pop, bang, or buzzing sound. It will sometimes (usually?) be accompanied by some sort of change in RPM (usually decreasing) or ITT/EGT/FTIT (usually increasing). Visually, it may be accompanied by some smoke or a flash out either end.

I've had three compressor stalls; one in the F-15E and two in the T-38. In the F-15E (PW-220 engine) it was a big bang (like a gunshot) with a 15-foot flame shooting out the intake for a second. There was a quick jump in FTIT, but everything else stayed the same in terms of engine indications. There was a slight lurch simultaneous to the 'bang', but no other noted loss of thrust. The engine recovered and had normal performance throughout the envelope.

Two very different stalls in the T-38. One was a small pop, followed by a puff of smoke out the exhaust, and then a constant buzzing sound (and vibration through the airplane). RPM slowly started to decrease and EGT inreased rapidly. The engine quit, and I did not attempt to restart it.

A second time was a violent buzz as I pushed the throttles into afterburner, with both decreasing RPM and EGT. I ripped the throttle back to idle (IAW the checklist non-memory memory item) and the engine recovered at idle.

EDIT: Forgot to add: of note here is that the Pratt 220 engine is controlled by a DEEC, and the GE-5 engine in the T-38 has a manual fuel control. There is a direct correlation between the incidences of compressor stalls and the use of a digital engine/fuel control (ergo, there are less compressor stalls with DEECs/FADECs than without).

 

[ryan1234]

It's usually audibly a pop, bang, or buzzing sound. It will sometimes (usually?) be accompanied by some sort of change in RPM (usually decreasing) or ITT/EGT/FTIT (usually increasing). Visually, it may be accompanied by some smoke or a flash out either end.

I've had three compressor stalls; one in the F-15E and two in the T-38. In the F-15E (PW-220 engine) it was a big bang (like a gunshot) with a 15-foot flame shooting out the intake for a second. There was a quick jump in FTIT, but everything else stayed the same in terms of engine indications. There was a slight lurch simultaneous to the 'bang', but no other noted loss of thrust. The engine recovered and had normal performance throughout the envelope.

Two very different stalls in the T-38. One was a small pop, followed by a puff of smoke out the exhaust, and then a constant buzzing sound (and vibration through the airplane). RPM slowly started to decrease and EGT inreased rapidly. The engine quit, and I did not attempt to restart it.

A second time was a violent buzz as I pushed the throttles into afterburner, with both decreasing RPM and EGT. I ripped the throttle back to idle (IAW the checklist non-memory memory item) and the engine recovered at idle.

EDIT: Forgot to add: of note here is that the Pratt 220 engine is controlled by a DEEC, and the GE-5 engine in the T-38 has a manual fuel control. There is a direct correlation between the incidences of compressor stalls and the use of a digital engine/fuel control (ergo, there are less compressor stalls with DEECs/FADECs than without).

Did you ever fly with the -229s at LN?

 

[Hacker15e]

Did you ever fly with the -229s at LN?

Yep, and it's a much better engine in every way except for fuel consumption. Off the top of my head, never had a notable engine problem with a 229.

 

[dasleben]

Never had a compressor stall in real life, but I do know that the sim makes it quite violent. Lots of dramatic banging and yawing.

EDIT: I'm not one to present myself as having experienced a compressor stall based on what I've seen in the sim; the sim may not replicate it correctly. In fact, I guess I have nothing to add to this.

 

[mikecweb]

It's usually audibly a pop, bang, or buzzing sound. It will sometimes (usually?) be accompanied by some sort of change in RPM (usually decreasing) or ITT/EGT/FTIT (usually increasing). Visually, it may be accompanied by some smoke or a flash out either end.

I've had three compressor stalls; one in the F-15E and two in the T-38. In the F-15E (PW-220 engine) it was a big bang (like a gunshot) with a 15-foot flame shooting out the intake for a second. There was a quick jump in FTIT, but everything else stayed the same in terms of engine indications. There was a slight lurch simultaneous to the 'bang', but no other noted loss of thrust. The engine recovered and had normal performance throughout the envelope.

Two very different stalls in the T-38. One was a small pop, followed by a puff of smoke out the exhaust, and then a constant buzzing sound (and vibration through the airplane). RPM slowly started to decrease and EGT inreased rapidly. The engine quit, and I did not attempt to restart it.

A second time was a violent buzz as I pushed the throttles into afterburner, with both decreasing RPM and EGT. I ripped the throttle back to idle (IAW the checklist non-memory memory item) and the engine recovered at idle.

EDIT: Forgot to add: of note here is that the Pratt 220 engine is controlled by a DEEC, and the GE-5 engine in the T-38 has a manual fuel control. There is a direct correlation between the incidences of compressor stalls and the use of a digital engine/fuel control (ergo, there are less compressor stalls with DEECs/FADECs than without).

One of mine was in an airplane with an inop DEEC.

 

[MikeD]

Interesting... the resident Tomcat guy here said that he never had a problem with tailslides in the F-5E (-21 GEs?) or the F-14 with the GE motors... the TF30s were a different story.

TomCatter?

 

[MikeD]

Depends on the airplane.

In the T-38 with the GE-5 engines (the PMP mod), they practically compressor stall if you look at them sideways. In the F-15 with the PW-100 engines (and to some extent the -220 also), they would also compressor stall with some regularity in close-to-normal maneuvering regimes.

That being said, I've had a number of compressor stalls in both aircraft, and they're not always that big of a deal.

A-10 and the TF-34s had same vis-a-vis compressor stalls and being in the engine disturbance zone in nearly the slightest. More than once have guys been doing BFM/ACM and compressor stalled, sometimes unrecoverable and requiring shutdown.

 

[MikeD]

Insofar as forward slips in a transport category jet, I see the argument as possible vs practical.

It's easily possible and safe, if monitored closely. Been onboard when it was done and have seen it done. But for normal operations, it's not very practical and is more of a "salvage" maneuver IMHO for sloppiness or hamfistedness. Not always, but oftentimes. And that's not mentioning the pax comfort aspect for pax ops.

 

[Av8n4life]

Thanks for the info.

What causes the nose to come up? Are the engines mounted slightly inclined to the horizontal axis providing some nose down pressure?

The big reason is that tail-mounted engines are above the center of gravity, so the line of action of the thrust force passes over (instead of through) the CG.