denningte
Member
Over on the other site Tom D and Jim W were answering up on a XW and VG issue and noted that XWs at different field elevations (lower) are harder to handle than the same XW at high altitudes where they both fly and so do I.
Here is some thoughts on why that is real and not perceived:
Gentlemen,
The point you bring up is a classic merge point of IAS/CAS/TAS/GS understanding for the landing zone.
If you consider the difference in measurement between a pressure differential system (pitot-static IAS) and how wind speed is measured on the ground (free-wheeling cup or rotor vane system), it becomes clear than at sea level you are dealing with a different wind triangle for landing than you would at higher density altitudes like CO or a hot NV day. The wind speed measurement given at a field from AWOS or a local controller is closer to TAS than CAS or IAS but not truly TRUE because of minor drag inefficiencies of the rotors or cups in the air stream. ( I estimate negligible.) The reason TAS is preferred for the landing area is because it is next to GS in most calculations and the runway and tires only care about GS during and after contact. I would suggest that a way to figure out what is “like home-drome” limiting conditions elsewhere is to convert your regular approach/landing IAS to TAS and use the same RATIO of forward to cross as a guideline for operating in other elevations.
I also note that the placard and POH in my aircraft AND the extra POH I have from Aviat for an A-1B all say 15 MPH which is 13 KTS. 15 KTS is 17.2 MPH. When you’re on the edge of performance a difference of 15% can be eye-watering (or other dampening farther south).
Before the next section is read, it is important to understand that CAS (IAS corrected for installation error) is what your wings and control surfaces care about and describes the amount of controllability in compressible flow. (We will need a few more mods from Tom to get the Husky into the transonic region)
So, if the factory says 15 MPH at AFO on “normal” as opposed to “standard” day where the temperature is say 20C, the recommended XW approach speed is 68 MPH IAS (77 MPH TAS) but the felt max demonstrated crosswind of 15 MPH true would be equivalent to 13.25 MPH CAS. Now let’s say that in this speed range a good installation makes the difference between CAS and IAS negligible. I am using the recommended approach speed only as a reference point for comparison. The hardest controllability problem will be around 40-20 MPH in the roll-out but the CAS is not available there. If you tried to read it there I would suggest that your tail-dragging priorities need a reshuffling. This is where in my experience the VGs can come out to bite you after you think you’ve lived through a high XW touchdown.
77/15 = 68/13.25 = 5.1/1 forward to cross ratio in the same speed type (IAS/CAS/TAS).
At sea level std day – 68 IAS/15 IAS is the same as 68 TAS/15 TAS = 4.5 forward to cross. The smaller number means you are dealing with more “felt XW” throughout the rollout than in a high density altitude environment.
TD
Here is some thoughts on why that is real and not perceived:
Gentlemen,
The point you bring up is a classic merge point of IAS/CAS/TAS/GS understanding for the landing zone.
If you consider the difference in measurement between a pressure differential system (pitot-static IAS) and how wind speed is measured on the ground (free-wheeling cup or rotor vane system), it becomes clear than at sea level you are dealing with a different wind triangle for landing than you would at higher density altitudes like CO or a hot NV day. The wind speed measurement given at a field from AWOS or a local controller is closer to TAS than CAS or IAS but not truly TRUE because of minor drag inefficiencies of the rotors or cups in the air stream. ( I estimate negligible.) The reason TAS is preferred for the landing area is because it is next to GS in most calculations and the runway and tires only care about GS during and after contact. I would suggest that a way to figure out what is “like home-drome” limiting conditions elsewhere is to convert your regular approach/landing IAS to TAS and use the same RATIO of forward to cross as a guideline for operating in other elevations.
I also note that the placard and POH in my aircraft AND the extra POH I have from Aviat for an A-1B all say 15 MPH which is 13 KTS. 15 KTS is 17.2 MPH. When you’re on the edge of performance a difference of 15% can be eye-watering (or other dampening farther south).
Before the next section is read, it is important to understand that CAS (IAS corrected for installation error) is what your wings and control surfaces care about and describes the amount of controllability in compressible flow. (We will need a few more mods from Tom to get the Husky into the transonic region)
So, if the factory says 15 MPH at AFO on “normal” as opposed to “standard” day where the temperature is say 20C, the recommended XW approach speed is 68 MPH IAS (77 MPH TAS) but the felt max demonstrated crosswind of 15 MPH true would be equivalent to 13.25 MPH CAS. Now let’s say that in this speed range a good installation makes the difference between CAS and IAS negligible. I am using the recommended approach speed only as a reference point for comparison. The hardest controllability problem will be around 40-20 MPH in the roll-out but the CAS is not available there. If you tried to read it there I would suggest that your tail-dragging priorities need a reshuffling. This is where in my experience the VGs can come out to bite you after you think you’ve lived through a high XW touchdown.
77/15 = 68/13.25 = 5.1/1 forward to cross ratio in the same speed type (IAS/CAS/TAS).
At sea level std day – 68 IAS/15 IAS is the same as 68 TAS/15 TAS = 4.5 forward to cross. The smaller number means you are dealing with more “felt XW” throughout the rollout than in a high density altitude environment.
TD