A question for you wheels up guys
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A question for you wheels up guys
What kind of airspeed gain does having retracts offer for the average machine and are the max gear extended speeds similar to FW or can they cope with a lot more ?
Thanks.
Thanks.
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I believe on the 365N series the airspeed is reduced by 5% with the undercarriage extended. After modification (i.e. removal) of the undercarriage doors, there is no airspeed limitation incurred for extension or retraction.
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For an S-76, it is about 5 knots at 125 knots, while holding constant power - and therefore constant fuel flow. Calculating the fuel savings is easy, just multiply the speed change by the fuel tank capacity, and realize how much fuel you save with each full tank. Multiply by the number of refuels per year, and realize why skids and fixed wheels are not very popular any more.
For an S-76, 5 knots /125 knots = 4% x 1850 lbs fuel = 72 lbs per tank, or 3000 gallons per year (1,000 hr x 500 lb/hr = 500,000 lbs per year x 4% = 20,000 lbs fuel saved = 2,985 US gallons)
For an S-76, 5 knots /125 knots = 4% x 1850 lbs fuel = 72 lbs per tank, or 3000 gallons per year (1,000 hr x 500 lb/hr = 500,000 lbs per year x 4% = 20,000 lbs fuel saved = 2,985 US gallons)
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For the S76 (the only retractable I've flown) the maximum gear extended speed is 130 kts, while Vne is 155 kts. Vne is rather easily reached in level flight (in fact it's the first limit this time of year), so it's certain that the maximum gear extended speed can be exceeded.
The gear on helicopters doesn't have to cope with speeds much above that of fixed-wing, because they can't go that fast in any case.
The gear on helicopters doesn't have to cope with speeds much above that of fixed-wing, because they can't go that fast in any case.
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Any thoughts about why retractable skids never caught on? These were fitted to the Lockheed 186 and looked a very practical feature. The skids did not fully retract, but the arms rotated backwards to hold skids flush to fuselage.
Mart
Mart
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As part of several design teams, Graviman, I can assure you once a helo design spends the effort to retract its gear, the few advantages of skids vanishes, and the advantages of wheels predominates.
Advantages
Skids - Cheap, simple, a bit lighter
Wheels - Operationally much more suitable - taxi without wind, turning, towing, running takeoff and landing, slope operations, rough field operations, smoother touchdowns, less gear penetration on soft terrain (less landing footprint pressure)
Disadvantages
Skids - more dangerous due to entrapment/dynamic rollover and running landing rollover; no braking control, poorer running takeoff capability, poor ground handling, hover taxi required (and its bothersome downwash). High drag
Wheels - higher maintenance, somewhat higher weight, higher cost
Advantages
Skids - Cheap, simple, a bit lighter
Wheels - Operationally much more suitable - taxi without wind, turning, towing, running takeoff and landing, slope operations, rough field operations, smoother touchdowns, less gear penetration on soft terrain (less landing footprint pressure)
Disadvantages
Skids - more dangerous due to entrapment/dynamic rollover and running landing rollover; no braking control, poorer running takeoff capability, poor ground handling, hover taxi required (and its bothersome downwash). High drag
Wheels - higher maintenance, somewhat higher weight, higher cost
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Bigmike,
Typically not, the area of the skids is usually smaller than that of the tires, believe it or not. The skids can produce a bigger area as they sink into the terrain, but that is dangerous if the sod plops up above the midpoint of the skid so that the skid gets entrapped.
If you measure the area of the skid shoes, and divide by the weight of the helicopter, and compare it to the tire pressure of a helo that is designed for soft terrain, they are quite equal - not surprisingly, the engineer's calculators work the same at every manufacturer!
Typically not, the area of the skids is usually smaller than that of the tires, believe it or not. The skids can produce a bigger area as they sink into the terrain, but that is dangerous if the sod plops up above the midpoint of the skid so that the skid gets entrapped.
If you measure the area of the skid shoes, and divide by the weight of the helicopter, and compare it to the tire pressure of a helo that is designed for soft terrain, they are quite equal - not surprisingly, the engineer's calculators work the same at every manufacturer!
Last edited by NickLappos; 18th Dec 2005 at 18:30.
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Sven's quite correct.
The Sea King will fly at any speed with the gear down but there is a bit more drag. At light weights one does notice a lower nose down attitude for a given IAS.
It looks far better with the gear retracted.
Some guys in Devon found that the landing was little noisier with the wheels tucked away!
It will certainly go faster when on the water with the gear retracted!
Er...., don't ask!
The Sea King will fly at any speed with the gear down but there is a bit more drag. At light weights one does notice a lower nose down attitude for a given IAS.
It looks far better with the gear retracted.
Some guys in Devon found that the landing was little noisier with the wheels tucked away!
It will certainly go faster when on the water with the gear retracted!
Er...., don't ask!
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Dear Nick, can you put some figures together to prove your statement ?
I've been trying to think this out, and I just can't get my head around it - but you're the expert.......
Typically not, the area of the skids is usually smaller than that of the tires, believe it or not.
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Skids vs Tyres
peachpilot:
Skids are usually round, therefore the bit that actually touches the ground is very narrow, whereas tyres are able to squish down a bit meaning the bit touching the ground is a lot wider than a skid.
If your skids had flat bottoms, then they would absolutely have more surface area touching the ground.
Lets say a skid is 1.5m long and only 3mm of the skid touches the ground, then the surface area for one skid is 1.5 x 0.003 = 0.0045 SqM.
Guessing at the size of a tyre (I have only had the pleasure of flying machines with skids - Little old Master-vegas, NZ doesn't usually see anything bigger than a five-hundie!) for the same sized machine that would have 1.5m skids, I would say they would be roughly 80mm wide, and once the weight of the machine was on them, it would be fair to say that you would get that same area front to back as well, giving us 0.08 x 0.08 = 0.0064 - and you would usually expect to have more wheels than skids.
So...after all that bollicks, I guess there isn't really that much difference.
God, I'm soooo boring sometimes
Dear Nick, can you put some figures together to prove your statement ?
If your skids had flat bottoms, then they would absolutely have more surface area touching the ground.
Lets say a skid is 1.5m long and only 3mm of the skid touches the ground, then the surface area for one skid is 1.5 x 0.003 = 0.0045 SqM.
Guessing at the size of a tyre (I have only had the pleasure of flying machines with skids - Little old Master-vegas, NZ doesn't usually see anything bigger than a five-hundie!) for the same sized machine that would have 1.5m skids, I would say they would be roughly 80mm wide, and once the weight of the machine was on them, it would be fair to say that you would get that same area front to back as well, giving us 0.08 x 0.08 = 0.0064 - and you would usually expect to have more wheels than skids.
So...after all that bollicks, I guess there isn't really that much difference.
God, I'm soooo boring sometimes
