OGE Hover, Service ceiling
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Rich is quite precise. Let me try to restate: For the purposes of determining hover performance when you have only two charts, an IGE chart and an OGE chart, and an examiner asks you to determine what performance you have, you answer, "Ige at x feet skid height or OGE at any higher hover."
Many military helos have a chart that computes power required for any height, as do some FAA manuals. For these, there are several correct answers, "I don't know" is not one of them!
One nice thing about pprune is that there are lots who want each slant, the "legal" one and the technical one.
Many military helos have a chart that computes power required for any height, as do some FAA manuals. For these, there are several correct answers, "I don't know" is not one of them!
One nice thing about pprune is that there are lots who want each slant, the "legal" one and the technical one.
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OGE, IGE
So tell me, surface type. Does a different surface type really make a noticeable difference on power requirements for hover IGE??? Sadly as a student I am lacking in practical experiences concerning this matter 
...What requires most/least power during hover IGE egs...snow, water, grass, ice, sand...does hovering low over a tree/forest canopy constitute hover IGE???
...What requires most/least power during hover IGE egs...snow, water, grass, ice, sand...does hovering low over a tree/forest canopy constitute hover IGE???
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IGE hover height given in the FM for civil helicopters is based on a takeoff technique (constant collective), and done over a smooth flat surface.
Asking the effect of other surfaces opens a can of worms similar to light fixed wing aircraft using wet grass runways, or soft muddy ones and trying to determine takeoff distance from the FM chart. No one has the answer, and the list of variables is quite long.
Anything that waves, like long grass or trees can soak up downwash and kill performance - but by how much? Wish I knew.
Asking the effect of other surfaces opens a can of worms similar to light fixed wing aircraft using wet grass runways, or soft muddy ones and trying to determine takeoff distance from the FM chart. No one has the answer, and the list of variables is quite long.
Anything that waves, like long grass or trees can soak up downwash and kill performance - but by how much? Wish I knew.
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Being both an engineer and a pilot I find it quit amusing how heated the debate sometimes gets between the worlds of theory and practise.
Two years ago during a flight I got really worried because of an increase of oil temperature. For some time the engineer took over and I stopped flying the helo. Since then I made up my mind that on the ground I would be an engineer and in the air a pilot : in the green is in the green, point. That does not mean that on the ground an equally simplistic approach to the problem should be taken.
As far as ground effect is concerned there is a fair understanding of the effect, even quantitative. An attempt to over-cook this is useless. Some people in this thread stated 1 diameter is OGE and as a pilot I would say this is a good average decision point, as far there is no tail wind, and no unusual ground surface. As a pilot I also rather would monitor descent rates, tail wind and power settings instead of inches of skid hight (as I do not have a telemeter I probably would anyway make a very rough estimate). From there making the leap to stating : all that is not IGE is OGE is fair. But then consider the engineers view point : power needs are easier to calculate OGE than IGE (cfr my previous remarks). When doing tests constructors want a reference. The lower that is, probably the easier the tests are and the better the machine is going to look. So from that view point it is in the pilots interest the OGE does not get defined as too low.
So all together 1.5 skids above the ground is not all that bad. This weekend I played around with it: first low hover, then increase power a bit and let the helicopter bounce up and down until it settles. Keep on increasing the power untill you really take off (carefull for HV...). When doing the tests this rigourously I had the feeling that ground could be felt slightly higher than 1 diameter..
Two years ago during a flight I got really worried because of an increase of oil temperature. For some time the engineer took over and I stopped flying the helo. Since then I made up my mind that on the ground I would be an engineer and in the air a pilot : in the green is in the green, point. That does not mean that on the ground an equally simplistic approach to the problem should be taken.
As far as ground effect is concerned there is a fair understanding of the effect, even quantitative. An attempt to over-cook this is useless. Some people in this thread stated 1 diameter is OGE and as a pilot I would say this is a good average decision point, as far there is no tail wind, and no unusual ground surface. As a pilot I also rather would monitor descent rates, tail wind and power settings instead of inches of skid hight (as I do not have a telemeter I probably would anyway make a very rough estimate). From there making the leap to stating : all that is not IGE is OGE is fair. But then consider the engineers view point : power needs are easier to calculate OGE than IGE (cfr my previous remarks). When doing tests constructors want a reference. The lower that is, probably the easier the tests are and the better the machine is going to look. So from that view point it is in the pilots interest the OGE does not get defined as too low.
So all together 1.5 skids above the ground is not all that bad. This weekend I played around with it: first low hover, then increase power a bit and let the helicopter bounce up and down until it settles. Keep on increasing the power untill you really take off (carefull for HV...). When doing the tests this rigourously I had the feeling that ground could be felt slightly higher than 1 diameter..
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OGE hover ceiling vs. service ceiling
Can somebody of you briefly explain, why there usually is a big difference between the OGE hover ceiling and service ceiling?
I read about the Bell Jet Ranger (250-C20J) that is has a service ceiling of 13500ft whereas the hover ceiling OGE is at roughly 5000ft. Does this mean when flying at 13000ft you are not able to enter a hover but that you have to descent to or below the hover ceiling OGE first and if so, why?
Or do i miss something completely? I apologize in advance for such a silly question but i did not find an explanation yet.
Regards,
Marcus
I read about the Bell Jet Ranger (250-C20J) that is has a service ceiling of 13500ft whereas the hover ceiling OGE is at roughly 5000ft. Does this mean when flying at 13000ft you are not able to enter a hover but that you have to descent to or below the hover ceiling OGE first and if so, why?
Or do i miss something completely? I apologize in advance for such a silly question but i did not find an explanation yet.
Regards,
Marcus
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Forward speed results in additional lift to the rotor thrust. In OGE hover you don't have forward speed (in no wind condition), so you can only use the rotor thrust to maintain altitude.
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Service ceiling is the height at which the aircraft can maintain 100 ft/min climb rate, using continuous power, and is always specified at best climb speed. As FixedRotaryWing says, this means the rotor is at its most effecient point, so the service ceiling can be very high.
The HOGE ceiling is the max hover height at takeoff power, always lower than the service ceiling because the hover power required is quite high, so the engine must be at a much lower altitude in order to produce it.
The HOGE ceiling is the max hover height at takeoff power, always lower than the service ceiling because the hover power required is quite high, so the engine must be at a much lower altitude in order to produce it.
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Thanks for your comments. Quite easy to understand, actually. I just wonder that the difference is that high (obviously depending on the type flown). I hope i did not qualify for this years dumb questions award 
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Service ceiling is going to be flown at about 40-50 KIAS (minimum power speed). The power required is much lower than in the hover, so that's why the ceiling is much higher than OGE or IGE hover ceilings.
Another item in the flight manual that is not well explained....
Another item in the flight manual that is not well explained....
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It may not be dependent on the engine, but on the ability of the rotor to produce thrust.
At high altitude, the blade pitch angle and hence angle of attack will be quite high, and the increase in drag for an increase in angle will greatly outweigh the increase in lift.
Since drag has to be overcome with power, with a rapidly increasing drag, the engine may not be able to do anything to increase the lift.
For example, the OH-58, at maximum weight, can't make it much over 16,000' pressure altitude at +5C (it gets hot here in Mojave). At lighter weight, it can go a lot higher, but if you work out the coefficient of thrust, it is the same number (weight over (air density times disk area times Rotor tip speed squared - the formulae is in most helicopter engineering books).
Even going above continuous power, at 16,000' +5C, it won't climb at 50 KIAS.
That's why the flight manual will have some different numbers for service ceiling (or should, if it has it at all)
At high altitude, the blade pitch angle and hence angle of attack will be quite high, and the increase in drag for an increase in angle will greatly outweigh the increase in lift.
Since drag has to be overcome with power, with a rapidly increasing drag, the engine may not be able to do anything to increase the lift.
For example, the OH-58, at maximum weight, can't make it much over 16,000' pressure altitude at +5C (it gets hot here in Mojave). At lighter weight, it can go a lot higher, but if you work out the coefficient of thrust, it is the same number (weight over (air density times disk area times Rotor tip speed squared - the formulae is in most helicopter engineering books).
Even going above continuous power, at 16,000' +5C, it won't climb at 50 KIAS.
That's why the flight manual will have some different numbers for service ceiling (or should, if it has it at all)
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????????
So the service ceiling would be reached when you first hit either 1) max out the ability for the engine to produce power, or
2) reached maximum torque design limits for the components, or 3) run out of aerodynamic ability for the rotor system to produce lift at a rate of a 100 foot per minute climb.
??????
So the service ceiling would be reached when you first hit either 1) max out the ability for the engine to produce power, or
2) reached maximum torque design limits for the components, or 3) run out of aerodynamic ability for the rotor system to produce lift at a rate of a 100 foot per minute climb.
??????
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Service ceiling is defined as '100 feet per minute rate of climb'. Done at VY (minimum power airspeed) The determining factor could be engine limits, rotor capability or something else.
The main point is that it has nothing to do with hover ceilings or capability.
The main point is that it has nothing to do with hover ceilings or capability.
