Thermals and Discs
Thread Starter
Joined: Apr 2000
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From: West Midlands, UK.
Thermals and Discs
Flying Hang Gliders has left me with an obsession regarding what HG Pilots’ call ‘Thermals’ (but I guess there is a more correct meteorological term for them).
I’ve experienced some powerful ‘Thermals’ in the UK and heard tales from around the World where HG’s have been inverted and subsequently broken up because of them.
A couple of days ago in the UK my ever-patient instructor was demonstrating how setting the power correctly in an R22 would keep the airspeed and height and sure enough we settled down to perfect straight and level (He had control). But not long after we started gently climbing (same power and a/s) – instructor explained we were in a thermal.
Back home I calculated the R22 MR disc as 490 sq ft and a Cessna 152 wing area as 159 sq ft.
Are helicopters more susceptible to rising hot air and if so am I right in thinking that the whole disc is influenced by the ‘Thermal’ with a consequent increase in aoa but no reduction in torque?
Thanks for any further information.
I’ve experienced some powerful ‘Thermals’ in the UK and heard tales from around the World where HG’s have been inverted and subsequently broken up because of them.
A couple of days ago in the UK my ever-patient instructor was demonstrating how setting the power correctly in an R22 would keep the airspeed and height and sure enough we settled down to perfect straight and level (He had control). But not long after we started gently climbing (same power and a/s) – instructor explained we were in a thermal.
Back home I calculated the R22 MR disc as 490 sq ft and a Cessna 152 wing area as 159 sq ft.
Are helicopters more susceptible to rising hot air and if so am I right in thinking that the whole disc is influenced by the ‘Thermal’ with a consequent increase in aoa but no reduction in torque?
Thanks for any further information.
Joined: Sep 2002
Aviation Qualifications: CPL
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From: Great South East, tired and retired
You are flying in the air, and the air is rising ( or falling) - what is so hard to understand about that?
When you talk about thermal turbulence, where there are lots of parcels of air rising at different rates, you need a high wing loading to avoid getting bounced around. An F104 Starfighter, with 3 square feet of wing supporting 7 tons of plane, punches through turbulence easily. A Cessna with a low wing loading will bounce your fillings out, and a chopper, with higher loadings, will experience less bounce than that. Despite this, the teetering head of the chopper will make turbulence into something quite scary. Fully articulated and rigid heads cope better with turbulence.
When you talk about thermal turbulence, where there are lots of parcels of air rising at different rates, you need a high wing loading to avoid getting bounced around. An F104 Starfighter, with 3 square feet of wing supporting 7 tons of plane, punches through turbulence easily. A Cessna with a low wing loading will bounce your fillings out, and a chopper, with higher loadings, will experience less bounce than that. Despite this, the teetering head of the chopper will make turbulence into something quite scary. Fully articulated and rigid heads cope better with turbulence.
Joined: Oct 2003
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From: Over here
Calculating the rotor disk doesn't give you a true picture. The disk isn't solid, it's only very slightly filled. Calculate the area of the blades themselves, and that's what your area is. Helicopters react far less to turbulence than fixed-wing, in general terms, especially spam cans. As AC said, it's a matter of wing loading.
Joined: Jul 1999
Posts: 3,931
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From: Warrington, UK
When we're boring holes in the sky waiting to see if football fans are going to kick off, I'm often on the lookout for thermals(don't know of any other name for them), just to pass the time. Set power for 60kts and wait for the VSI to kick and away we go! OK, so it doesn't leap upwards like a glider, but I can make 2-300 feet in a cou[ple of turns. It's the way to fly!
Joined: May 2000
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From: poor gps coverage
