Exactly the same problem Johnny: Too little kinetic & potential energy already stashed onboard to withstand any interruption in power without the ground smiting thee.
In that POH reference I’d meant to contrast single & 2-engine performance. See PDF pages 91 & 93 = handbook pages 5-13 & 5-14. Ignore page 5-13a between them.
Note that both the vertical and horizontal scales are very different, so you can’t just eyeball how far up towards the upper right (good) corner those sloping lines are. IOW, the single engine lines are a lot closer to the lower left (bad) corner than they appear. Notice also there’s a gear-down+half-flaps line for the two-engine case. There isn’t one for the single engine case because it won’t fly that way.
We had similar curves for submarine operations. Basically it is dangerous to be going very fast and very deep, because if the control planes get stuck in the down position, it may not be possible to correct the situation before you have an uncontrolled depth excursion that takes you below crush depth. We actually engaged physical stops on the planes and rudder above certain speeds to mitigate this.
This was really interesting. Thanks for detailed post, @LSLGuy!
I went back and looked at that H/V curve you linked to a little more closely, @Johnny_L.A. I never realized that the flight profile for helicopters was so restrictive.
How does this work for a helicopter in the example (a Bell 204B) taking off or landing from the roof of a moderately tall building (i.e. one that’s one or two hundred feet tall)?
As my instructor told me, ‘It’s a calculated risk.’ There are situations where you have to fly outside of the envelope; but the odds of losing your engine are very, very small. Of course another reason for autorotation might be a tail rotor failure. That’s also unlikely to happen – except one of the times you might be in the left shaded area is when you are getting in or out of a confined area where you might strike your tail rotor. So basically if you can’t avoid an operation in the shaded area, you calculate the risk and decide if you like the odds.
As far as a JetRanger operating from the roof of a tall building, you would probably be operating pretty much within the safe envelope because helicopters generally don’t take off and land straight up and down. They fly patterns much like airplanes do. So landing on a tall building (known as a ‘pinnacle landing’) or a ridge (‘ridge landing’), you still have forward speed. FWIW, pinnacle landings and ridge landings have a steeper approach than landing on a wider area.
And thanks in turn for this. I had never considered that possibility, but it’s obvious as all get out once you explain it.
Launching a mighty hijack about subs …
As I’ve said before I find sub ops fascinating. There’s a weird Alice-in-Wonderland distorted mirror symmetry between airplanes and subs, especially at the high performance end of each.
In addition to the uncontrolled dive to excess depth issue, since you mention rudder stops I wonder …
Given attack subs’ need for high maneuverability even at low speeds, I expect your rudder control surfaces are large, have large throws, and have the actuating power to go stop-to-stop quickly. Much more so than on e.g. a destroyer or a merchantman.
Which if mistakenly fully engaged at high speeds might overstress the structure, either of the rudder & fixed fin, or of the whole sub; getting the hull too sideways to the waterflow (excess yaw) might be real hard on it. Especially since those asymmetric forces are in addition to the symmetrical forces already imposed by your depth.
Is any of the above accurate?
In jets we have a similar issue. Rudders & vertical fins are sized for the worst case demand on yaw control power: engine failure during takeoff. Said another way, the designers grow the rudder & fin until the Vmca I described above is reduced to a speed well below flying speed. That’s just one of many ways jets are required to meet higher safety standards than are light piston twins.
That big fin with the big rudder is nice and safe for takeoff but comes with a downside: Any excess rudder use by the crew, or a hardover malfunction is very dangerous at speed. So every big jet I’ve flown, and AFAIK every jet even bizjets, have some sort of automatic rudder limiter that greatly limits mechanical rudder throw, or rudder actuating power, or both as speed increases. Which also introduces emergency procedures for malfunctions in either direction: being unrestricted while fast or restricted while slow.
Another related thought …
In airplanes, and especially swept-wing airplanes there’s a phenomenon called roll-yaw coupling. Without delving into why, a yaw excursion promotes a roll in the same direction & vice versa. e.g. if the aircraft yaws left, it will also try to roll left after a brief delay unless compensated for by the crew or the FBW computers.
It occurs to me that a sub has all the same and maybe more. A yaw to the left will have the sail plowing sideways and that asymmetrical force applied above the longitudinal centerline will induce a left roll. The sail force being ahead of the fore/aft CG (in most designs) also makes it destabilizing: a yaw started by the rudder will be amplified by the sail, not damped by it. And, as with jets, the timing of the roll will be delayed and the amplitude a second or third-order effect and hence sorta unpredictable / counterintuitive to a human helmsman.
I’ve been noodling on whether there’s any inherent roll-to-pitch or yaw-to-pitch coupling in subs; I can almost pull my ideas into focus but not quite. Which suggests there might be some but I haven’t hit on the right mechanism yet.
I mentioned pinnacle landings and ridge landings, and it reminded me of a couple of anecdotes. The pinnacle landing is the one I don’t remember exactly. By my recollection, a pilot in Los Angeles (Israeli guy, IIRC) suffered a power failure and had to autorotate. He made a safe landing on top of a building. As for the ridge landing, that helicopter – which I had flown – was destroyed when the pilot made a too-shallow approach while attempting to land on a ridge north of L.A. You land into the wind, so you approach the lee side of the ridge. The helicopter was an R22 Beta which, as I posted earlier, don’t have a lot of power – especially when it’s hot. By coming in too shallow, he encountered the downdraft coming off the ridge and didn’t have the power to climb out of it. He hit the ridge and rolled about 100 metres down the slope. He had some bruises and abrasions, but the helicopter was destroyed. (Two other helicopters I’d flown were destroyed in crashes, with no injuries to the occupants.)
This is more or less accurate. The rudder on a submarine is indeed larger than that of a typical surface ship such as a merchant*, as it extends above and below the stern planes. Along with the control planes, it is hydraulically controlled with very robust actuators, and can be very quickly moved.
With that said, surface warships such as destroyers are also very maneuverable, with similar hydraulically-controlled rudders. And while they don’t have a rudder that extends above and below the screw like a submarine, many of them do have double rudders.
So far as engaging a large rudder at high speed, as I recall the main issue was controllability, because the effectiveness of control surfaces increases in direct proportion to the square of the speed of the vessel. However, overstressing the control surfaces might also have been a concern and another reason for the limits.
Thinking of American Airlines Flight 587, I don’t recall any prohibition on moving the rudder back and forth repeatedly.
Interestingly, a sub on the surface acts like a surface vessel. In other words, if you throw the rudder over the left, the vessel heels (or rolls) to starboard.
However, a submerged submarine acts more like an airplane. If you throw the rudder over to the left, the sub rolls to port. (I’m not sure if this is due to the sail or some other effect, though.) As the sub rolls, this means that the rudder is now acting somewhat like a stern plane. The upshot is that a large rudder on a submerged sub will tend to make the sub dive, which needs to be counteracted by the planesman. The effect is greatly exacerbated at high speed. This is why at high speeds both the planes and the rudder are limited.
To be clear, I did not crash them. (My instructor crashed the first one, while I was waiting for him to get back for my lesson. I went up in a Skyhawk that day instead.)
I don’t know of any airplane I’ve flown that later crashed. I have flown lightplanes that others previously or subsequently damaged pretty heavily, but not destroyed. Lots of planes I’ve flown have been scrapped. I flew one that had previously been blown up & one that had been hijacked.
Thank you very much for the detailed explanation, with cites, of the issues surrounding engine-out emergencies on takeoff, and immediately afterward. I learned a great deal and, like inadvertently stepping off a sandbar while wading, realize just how deep these mysteries can be. Neat!
On your quote, I don’t remember where I read it, but supposedly an airship pilot has similar lag and sensitivity in their control of the craft’s orientation, to that of a submarine helmsman + diving planes operators.
Given that pumpjet propulsors are the ‘new’ hotness for submarine propulsion, I wonder if diverted thrust is also used to control orientation? Like how a pumpjet boat can be controlled? Probably way too noisy.
I now realize that we/I have managed to thoroughly hijack two different @Whack_a_Mole threads about airplanes. And that my long-winded post above really only makes sense to laymen within the context set in WaM’s other thread:
So I’m guilty of using acronyms and ideas without introduction, just as I gently chastised @Gray_Ghost for. Whose name I also misspelled last time.
I really appreciate the insights provided by our resident airline pilots here. I know that the limitations of light piston-engine twins on a single engine were discussed before by one or both of them. It’s one of those surprising things that are non-intuitive. At one time I flew in chartered light planes quite a lot when working on a project in a town that had very awkward air connections from our home base, so my employer opted for charters instead. The most frequently used plane was a Piper Navajo light twin, and I would have felt a lot less comfortable had I known how vulnerable such designs are to engine failure, relatively speaking. At least, I would have made more liberal use of the open bar at the back! Occasionally instead of the Navajo we’d get a Cessna Citation jet, which was fun and elegant but I personally found it rather cramped and preferred the space of the slow old Navajo.
Just as a total aside, I’m just reading another great Erik Larson book, The Splendid and the Vile, about Churchill during the war years. It mentions that his favorite plane, which was equipped with plush armchairs and which he frequently and rather fearlessly used for personal travel during the war, was the de Havilland DH.95 Flamingo, a late-30s era piston twin somewhat on the borderline between a typical private light twin and a real airliner. Churchill flew it several times to see Prime Minister Reynaud in 1940 just before the fall of France. It was never very popular otherwise and not many were made, although besides Churchill’s plane and a few used as RAF transports. for a while one was reserved for the “King’s flight” should the royal family have to escape the country in the event of an invasion.
I naturally wondered how such a plane would fare if it suffered an engine failure, and Wikipedia provides the answer. I don’t know about stability issues but it had rather enormous engines for a relatively small plane and looks like it could do quite well on one engine. The large engines and high wing gave it an amazingly short takeoff run, somewhat like today’s STOLs like the Dash-8, coincidentally also made by de Havilland:
Powered by 890 hp (660 kW) Bristol Perseus engines, [the DH.95 Flamingo] had a maximum weight takeoff in 750 ft (230 m) and the ability to maintain height or climb at 120 mph (190 km/h) on a single engine.
R22s look small no matter what, but lying on its side it looks like a toy. I can see that the fuselage is only 44 inches in width, and in the video it doesn’t even look like that much (maybe it got crushed a little).
I assume that was a total loss? The main fuselage doesn’t look like it’s in that bad a shape, though I suppose you wouldn’t really trust any of the equipment at that point.
You might need to remove the doors anyway if you’ve had a large meal recently (Wikipedia says they weigh 10.4 lbs, out of a total capacity of 389 lbs).
The Navajo is a so-called “cabin-class” or medium twin. Still not jet airliner takeoff safety performance, but much more capable than true light twins.
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Occasionally instead of the Navajo we’d get a Cessna Citation jet, which was fun and elegant but I personally found it rather cramped and preferred the space of the slow old Navajo.