Long Strop Speed Reduction
Thread Starter
Long Strop Speed Reduction
This is something I have been thinking about for some time. As I have retired the situation will not arise again but I am throwing it into the ring to see what views there are about it. Somebody may know the answer, I do not, but I would like to know.
Scenario 1. A North Sea platform has managed to have both of its cranes U/S so there is a panic to get an engine rebuilt. A 332L arrives with the centre tank removed and a 4.5K load pole fitted because the engine weighs 2 tonnes or so. They use a 40 ft strop to lift the engine out of its housing, place it on the helideck and refuel. Whilst this is going on the 40 ft strop is replaced by an 8 ft strop for the 110 nm return flight. With the fuel required the aircraft is nearly at MAUW on departure. It settles into the cruise and it shows about 115 knots, about 5 knots down from normal.
Scenario 2 Another 332L; two production slots from the other but this time in Honiara, Solomon Islands. The RAMSI has a police station in Rennel Island that requires a weekly resupply with diesel; five drums in a net. It is 125 nm to Rennel; no fuel, so return fuel plus reserves must be carried. Add on other stores to be shifted and the aircraft is at 20,000lbs. The load is on the end of a 60 ft strop for reasons that I will not go into. The routing involves climbing to 7,000ft, dodging cu-nimbs with radar and mountain tops with the GPS until coasting out south of Guadalcanal. Then it is down to 2,000ft, autopilot in, feet up and fags out because the aircraft can only make 90 knots. This 332 has the big sponsons on which give you an extra 5 knots so the result is that it is 30 knots down from normal.
The logical reason is the long strop. There is a centre of drag in any flying combination and five drums 60+ ft below are going to drag it down a quite a lot; possibly below the ventral surface of the aircraft. We have seen pictures of helicopters towing ships that show the extremes. However in both these scenarios the pitch was set at 15.5 and the attitude of the aircraft was very much the same as normal which is the point of this thread.
I would have thought that the attitude of the aircraft in the different situations would have been markedly different. The component that is preventing it is the stabiliser, and anybody who knows the 332 stabiliser knows that it has a pretty extreme profile with a slat to assist. For it to maintain an attitude with the drag point so far down it must be producing an enormous amount of force which is evidenced by the drop in airspeed. Has anybody actually worked out how much stress it is taking and whether it has been designed for it?
One could query whether any conventional single rotor design has allowed for it. I am thinking mainly about those operators who do continuous long line underslung work, ie logging. I know they only do short transits but how short ones make a long one. I do know that one company I worked for blacklisted logging gearboxs.
Has anybody any answers. There may be another explanation why the speed should vary with the strop but I have not noticed such a variation before in 43 years of sling loading and that has included a lot of drums on short strops.
Scenario 1. A North Sea platform has managed to have both of its cranes U/S so there is a panic to get an engine rebuilt. A 332L arrives with the centre tank removed and a 4.5K load pole fitted because the engine weighs 2 tonnes or so. They use a 40 ft strop to lift the engine out of its housing, place it on the helideck and refuel. Whilst this is going on the 40 ft strop is replaced by an 8 ft strop for the 110 nm return flight. With the fuel required the aircraft is nearly at MAUW on departure. It settles into the cruise and it shows about 115 knots, about 5 knots down from normal.
Scenario 2 Another 332L; two production slots from the other but this time in Honiara, Solomon Islands. The RAMSI has a police station in Rennel Island that requires a weekly resupply with diesel; five drums in a net. It is 125 nm to Rennel; no fuel, so return fuel plus reserves must be carried. Add on other stores to be shifted and the aircraft is at 20,000lbs. The load is on the end of a 60 ft strop for reasons that I will not go into. The routing involves climbing to 7,000ft, dodging cu-nimbs with radar and mountain tops with the GPS until coasting out south of Guadalcanal. Then it is down to 2,000ft, autopilot in, feet up and fags out because the aircraft can only make 90 knots. This 332 has the big sponsons on which give you an extra 5 knots so the result is that it is 30 knots down from normal.
The logical reason is the long strop. There is a centre of drag in any flying combination and five drums 60+ ft below are going to drag it down a quite a lot; possibly below the ventral surface of the aircraft. We have seen pictures of helicopters towing ships that show the extremes. However in both these scenarios the pitch was set at 15.5 and the attitude of the aircraft was very much the same as normal which is the point of this thread.
I would have thought that the attitude of the aircraft in the different situations would have been markedly different. The component that is preventing it is the stabiliser, and anybody who knows the 332 stabiliser knows that it has a pretty extreme profile with a slat to assist. For it to maintain an attitude with the drag point so far down it must be producing an enormous amount of force which is evidenced by the drop in airspeed. Has anybody actually worked out how much stress it is taking and whether it has been designed for it?
One could query whether any conventional single rotor design has allowed for it. I am thinking mainly about those operators who do continuous long line underslung work, ie logging. I know they only do short transits but how short ones make a long one. I do know that one company I worked for blacklisted logging gearboxs.
Has anybody any answers. There may be another explanation why the speed should vary with the strop but I have not noticed such a variation before in 43 years of sling loading and that has included a lot of drums on short strops.
