GlassCollector

Hi

I noticed a picture of a Marshalls Aerospace Hercules with the test engine in what I assume is the No 2 position. Also assuming that the the props rotate clockwise as seen from behind, why is this position chosen for the test engine and is it used at take off or is a ferry TO in order?

GC

WeekendFlyer

Not sure about the prop rotation direction or the power setting used during takeoff, but the engine position will have been chosen carefully for a number of reasons:

1. The engine can generate significantly more thrust then a standard herc engine, thus to minimise asymmetric effects it is best to have it near to the fuselage (No.2 or No.3 engine positions). The same applies for structural considerations such as wing bending moments. Look a photos of the test aircraft and you will see an additional support strut between the engine and the fuselage. Lastly, there is the issue of moments of inertia (roll and yaw) and lateral asymmetry (which can adversely affect roll and yaw control). Given the additional mass and thrust of the test engine compared to a standard herc engine it is far better to have the test engine nearer to the fuselage in order to minimise the change in inertias and thrust moments.

2. Being a test engine there is a risk it could fail unexpectedly, so you really do not want it on the No.1 or No.4 positions due to the large yawing moment that a sudden failure would cause.

3. In addition to the above there will have been some good engineering reasons for putting the engine on the No.2 position as opposed to the No.3 position, for example to minimise the change in yawing moment caused by the engine compared to the original engine. The position chosen will be the one that minimises the nasty aerodaynamic effects in the event of an engine failure. Incidentally, anyone care to state which engine is the critical engine on a Herc K?

Hope that goes some way to answering your questions!

WF

GlassCollector

Thanks, great reply.

GC

ClippedCub

The test engine is run during takeoff at reduced setting to minimize the risk of failure. The crew know what it has and is sitting on go to react. Additionally, runway length, takeoff speeds, atmospheric conditions, and weight are chosen to substantially increase the margins. Even if you have one, the test engine, set to fail, engine number one, in this case, would have to let go at the same time to really screw things up. So the odds of crew and airplane loss are small and considered acceptable for the payoff. The Herc's engines are co-rotating, so there is one, critical, engine.

FTE Pruner

Interestingly there was a paper presented at SETP Toulouse last week from Airbus and Marshalls about their Test Bed.

I seem to remember that they said there was a requirement to configure the Test Bed to be representative to that of the A400M configuration (See here for what I think they are trying to achieve http://www.a400m-countdown.com/index.php?v=2&spage=7).

So based on GlassCollector's OP don't think it rotates "clockwise as seen from behind" I think it must be the opposite (and hence opposite from the normal engine next to it).

They also said that they will use full power at rotate by the means of a reduced number 1 (but not so far reduced that they can't handle a test engine failure if it were to happen).