Centerline-thrust multi -- effect on Vspeeds?

[Mach Stall]

I'm curious how a centerline thrust twin-engine airplane design would alter the takeoff (V1, Vr, V2) and approach/landing (Vref) speeds -- if at all -- both from an engineering and regulatory standpoint.

It strikes me intuitively that a centerline-thrust twin design might move certain speeds closer to stall, without having to worry about asymmetric thrust Vmc, critical engine, etc, and could thus improve the takeoff and/or landing distance performance vs the exact same aircraft with asymmetric thrust. But I'm not clear how this would pan out in any detail.

Any thoughts?

[Intruder]

Since there will be no (or very low) Vmcg, there will be no V1 limitation with regard to Vmcg. It will be most pronounced on wet or contaminated runways.

[glendalegoon]

dear mach stall

a very popular twin is the piper seminole. it was designed to stall prior to VMCA. (if memory serves, its been a very long time since I taught in it).

You might want to check it out too, but it is NOT centerline thrust.

I see where you are going with this, and it really doesn't work in practical flying I guess. NOt that many centerline thrust planes.

[Intruder]

IIRC, the Seminole had marginal single-engine performance, too. Maybe the lack of power was a "feature"...

[roulishollandais]

C337 Push-pull
other problems!
don't try to avoid to learn basics and masterize stalls and dissymetry, included on "don't stall" A330 : steady airflow doesn't exists
systems and a/p are there to help the crew not to replace them

[Mad (Flt) Scientist]

One possible problem with a "centreline twin" would be the LACK of response due to engine failure. While a conventional twin goes asymmetric on engine-out, giving a pretty obvious cue to the crew, a "centreline twin" would do no such thing. There would be concerns, i am sure, over the ability of a crew to promptly recognize and respond to engine failure. The concern would not be their inability to maintain centreline - that would be easy - but rather the risk of taking longer to recognize and reject a TO. I would not be surprised to find the current "2 second rule" be extended to some longer time period, and/or some very specific additional warning/monitoring be required.

The regulations do provide for some of this:
§25.107 Takeoff speeds.
(a) V1 must be established in relation to VEF as follows:

(1) VEF is the calibrated airspeed at which the critical engine is assumed to fail. VEF must be selected by the applicant, but may not be less than VMCG determined under §25.149(e).

(2) V1, in terms of calibrated airspeed, is selected by the applicant; however, V1 may not be less than VEF plus the speed gained with critical engine inoperative during the time interval between the instant at which the critical engine is failed, and the instant at which the pilot recognizes and reacts to the engine failure, as indicated by the pilot's initiation of the first action (e.g., applying brakes, reducing thrust, deploying speed brakes) to stop the airplane during accelerate-stop tests.
(1) will likely be trivial, but the recognition time in (2) will then come into the fore.

[john_tullamarine]

over the ability of a crew to promptly recognize and respond to engine failure

.. and the same applies for multi jet - a standard training failure is during late descent with idle thrust set across the ship. Amusing how often the crew doesn't pick the change until they have spun up .. at which stage things can unravel quite rapidly. Generally, one or two exposures and folks are far more alert to the possibility and tend not to get caught out.

[Checkboard]

never flown a Seminole myself but I recall an friend who instructed on it commenting that it had greater endurance if you shut down one engine after take-off (presumably at cruising level) than using both.
All light piston twins have greater range and endurance on one engine, than at normal cruise on two engines - because two engine normal cruise speed is well above the most efficient speed, whilst single engine speeds are very close to it.

... of course slowing to best range speed with both engines running is more efficient than doing so with just one running. :rolleyes:

[barit1]

Although some literature describes this as a single, the video makes it obvious it's a tandem twin.

Three contemporary racers may be seen today at Vigna di Valle (http://en.wikipedia.org/wiki/Italian_Air_Force_Museum), an hour's drive north of Rome.

Macchi M.C. 72 - YouTube

[A Squared]

Although some literature describes this as a single, the video makes it obvious it's a tandem twin.

I guess that would depend on how exactly one defines "engine". Is a "U" engine or an "H" engine one engine or two engines geared together? Is the Deltic engine one engine or 3? In some ways the Fiat AS 6 is two engines, in others it's not Physically, both halves shared the same crankcase casting, and they operated off the same supercharger and induction manifold. The Supercharger was driven off the crankshaft of the rear half, so without the rear half producing power, the front half would only run at reduced power or perhaps not at all, from the video it's apparent that the rear half was started first.

[A Squared]

... of course slowing to best range speed with both engines running is more efficient than doing so with just one running. :rolleyes:

0n a conventional twin {engine on each wing) possibly, due to additional drag generated from counteracting asymmetrical thrust, on a centerline twin, very likely not, depending on the brake specific fuel consumption curves for the engines, prop efficiency curves and such. If you need 150 hp to maintain best range airspeed, and you have 2, 250 Hp engines, it may be more efficient to have one engine running at 150 hp, than two at 75 hp. Note that the Rutan Voyager cruised on the rear engine with the front engine shut down and feathered.

[clunckdriver]

The Cessna 337 is widely used in Canada as a "Fire Spotter", if one is low on fire work and the front engine quits it flies quite well on the rear one, however should the rear one go on a typically hot/bumpy/smoky day , one has ones hands full to stay out of the trees, for this reason its normal to have just one spotter along with the pilot as the tanks are allways full at the start of a flight, skiny spotters are much in demand!

[FE Hoppy]

On the old 3* we had a bloody big light for Eng 2 fail to help the jockeys recognise the failure. Of course the Eng normally pointed it out before the light came on ;-)

[A Squared]

On the old 3*...

Trident???

[FE Hoppy]

TriStar.........

[A Squared]

Ahh, ok, I get it now, asterisk=star