EAS vs TAS vs GS
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EAS vs TAS vs GS
Can somebody educate me as to why different speeds are used in different graphs in the manual? For eg, why is EAS used in a Drag graph but TAS is used in a Power graph and IAS used in a HV graph?
Also, why is HV graph based on IAS and not Groundspeed?
Also, why is HV graph based on IAS and not Groundspeed?
EAS is a function of dynamic pressure (q), drag is also a function of q (=.5 x density x TAS^2); (almost) all structural loads are first of all a function of EAS.
TAS is what you need for navigation so is generally quoted in the performance data as a function of altitude and power (and in bigger aeroplanes, weight).
IAS is the only thing indicated in the cockpit, so used for any piloting task as the reference speed, whether that's climb profile, stall, approach, flap limiting speed...
G
TAS is what you need for navigation so is generally quoted in the performance data as a function of altitude and power (and in bigger aeroplanes, weight).
IAS is the only thing indicated in the cockpit, so used for any piloting task as the reference speed, whether that's climb profile, stall, approach, flap limiting speed...
G
TAS is used in power graphs because power required is proportional to TAS cubed.
Because the relationship between EAS, IAS and TAS varies with altitude, it cannot be said that power required is proportional to EAS or IAS in any straight forward way.
Because the relationship between EAS, IAS and TAS varies with altitude, it cannot be said that power required is proportional to EAS or IAS in any straight forward way.
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The HV charts in many of the helicopters I've flown just said 'Speed'. There is no way to measure the airspeed accurately below 40 Knots in any case. (regardless of what the requirements say)
As I was taught many moons ago at PPSC (RIP) "TAS is for Navigators and P**fters"...pretty much sums it up. (Thanks DW, every one a gem!)
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I don't agree to the HV graph indicating the IAS.
Consider this .... even though the Y axis of the graph states this as IAS, and the conditions for the 'Avoid area' curve are dependent on the Density altitude and NIL wind conditions (which in any case has to be stated for its validity) doesn't it become a GS axis?
You may also like to consider the factors considered for drawing and validating the various segments of this graph.
The IAS/EAS (in case of helicopters where the compressibility plays a limited role in performance related issues) usage in the manuals are for aircraft 'performance' related factors... TAS would be valid for reasons of navigation.... generally speaking.
Nevertheless, if the IAS values read from graph are valid for a given density altitude it would mean that the TAS has been reduced to be read IAS !
Consider this .... even though the Y axis of the graph states this as IAS, and the conditions for the 'Avoid area' curve are dependent on the Density altitude and NIL wind conditions (which in any case has to be stated for its validity) doesn't it become a GS axis?
You may also like to consider the factors considered for drawing and validating the various segments of this graph.
The IAS/EAS (in case of helicopters where the compressibility plays a limited role in performance related issues) usage in the manuals are for aircraft 'performance' related factors... TAS would be valid for reasons of navigation.... generally speaking.
Nevertheless, if the IAS values read from graph are valid for a given density altitude it would mean that the TAS has been reduced to be read IAS !
Last edited by peeush; 20th Jul 2011 at 17:59.
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For helicopters, the H-V avoid area is usually given as a function of IAS. Takeoff and landing procedures must be developed to preclude entering the H-V avoid area, and pilots use IAS during takeoff and landing.
H-V diagrams are validated during full scale flight test by introducing engine failures at discrete points along the perimeter of the H-V avoid area. The test points are set-up by flying at a specified IAS and height. Some H-V diagrams are shown as a series of density altitude lines for a fixed weight, some are shown as a series of weight lines for a fixed density altitude, while others are shown as a fixed line but require that weight be reduced with increasing density altitude in a prescribed way. Regardless of how the H-V diagram is portrayed, it must be validated by flight test over the range of density altitudes for which the helicopter is approved for takeoff and landing.
I’m puzzled by Shawn Coyle’s input that “there is no way to measure the airspeed accurately below 40 Knots in any case.” On the helicopter models I’ve tested the airspeed system position errors were measured at airspeeds in the 15-30 kt range.
H-V diagrams are validated during full scale flight test by introducing engine failures at discrete points along the perimeter of the H-V avoid area. The test points are set-up by flying at a specified IAS and height. Some H-V diagrams are shown as a series of density altitude lines for a fixed weight, some are shown as a series of weight lines for a fixed density altitude, while others are shown as a fixed line but require that weight be reduced with increasing density altitude in a prescribed way. Regardless of how the H-V diagram is portrayed, it must be validated by flight test over the range of density altitudes for which the helicopter is approved for takeoff and landing.
I’m puzzled by Shawn Coyle’s input that “there is no way to measure the airspeed accurately below 40 Knots in any case.” On the helicopter models I’ve tested the airspeed system position errors were measured at airspeeds in the 15-30 kt range.
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Peeush,
HV diagrams are based on completing the landing with 0 GS, but initiating the entry with speed relative to the surrounding air.
Although it may work for very low wind speeds, you can't generalize to consider the HV diagram being height vs GS.
HV diagrams are based on completing the landing with 0 GS, but initiating the entry with speed relative to the surrounding air.
Although it may work for very low wind speeds, you can't generalize to consider the HV diagram being height vs GS.
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Matthew P,
H-V diagrams are not based on completing the landing with zero ground speed, especially in the case of twin engine wheeled helicopters where one engine inoperative H-V diagrams are presented.
HT
H-V diagrams are not based on completing the landing with zero ground speed, especially in the case of twin engine wheeled helicopters where one engine inoperative H-V diagrams are presented.
HT
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HeliTEster:
There may be data for airspeeds in that range, but unless you have a specifically shaped pitot tube (a la MD900) you're not getting anything worthwhile in terms of real data - a small off-axis angle (more than 12°) will rapidly degrade the results.
There may be data for airspeeds in that range, but unless you have a specifically shaped pitot tube (a la MD900) you're not getting anything worthwhile in terms of real data - a small off-axis angle (more than 12°) will rapidly degrade the results.
How extremely kind of you old chap. 
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Do you really know what you are talking about................???
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There may be data for airspeeds in that range, but unless you have a specifically shaped pitot tube (a la MD900) you're not getting anything worthwhile in terms of real data - a small off-axis angle (more than 12°) will rapidly degrade the results.
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Shawn C,
The following requirements are excerpted from CFR Parts 27 and 29 for Normal and Transport Category Helicopters....
§ 27.1323 Airspeed indicating system.
(a) Each airspeed indicating instrument must be calibrated to indicate true airspeed (at sea level with a standard atmosphere) with a minimum practicable instrument calibration error when the corresponding pitot and static pressures are applied.
(b) The airspeed indicating system must be calibrated in flight at forward speeds of 20 knots and over.
§ 29.1323 Airspeed indicating system.
For each airspeed indicating system, the following apply:
(a) Each airspeed indicating instrument must be calibrated to indicate true airspeed (at sea level with a standard atmosphere) with a minimum practicable instrument calibration error when the corresponding pitot and static pressures are applied.
(b) Each system must be calibrated to determine system error excluding airspeed instrument error. This calibration must be determined—
(1) In level flight at speeds of 20 knots and greater, and over an appropriate range of speeds for flight conditions of climb and autorotation; and
(2) During takeoff, with repeatable and readable indications that ensure—
(i) Consistent realization of the field lengths specified in the Rotorcraft Flight Manual; and
(ii) Avoidance of the critical areas of the height-velocity envelope as established under §29.87.
These requirements must be met in order to obtain FAA certification. If this necessitates the use of specifically shaped pitot tubes (a la MD900, S76, S92, et al) then so be it. Some models I've tested required custom designed pitot tubes to comply with the requirement, while another was able to demonstrate compliance with an off-the-shelf AN5816 pitot tube.
HT
The following requirements are excerpted from CFR Parts 27 and 29 for Normal and Transport Category Helicopters....
§ 27.1323 Airspeed indicating system.
(a) Each airspeed indicating instrument must be calibrated to indicate true airspeed (at sea level with a standard atmosphere) with a minimum practicable instrument calibration error when the corresponding pitot and static pressures are applied.
(b) The airspeed indicating system must be calibrated in flight at forward speeds of 20 knots and over.
§ 29.1323 Airspeed indicating system.
For each airspeed indicating system, the following apply:
(a) Each airspeed indicating instrument must be calibrated to indicate true airspeed (at sea level with a standard atmosphere) with a minimum practicable instrument calibration error when the corresponding pitot and static pressures are applied.
(b) Each system must be calibrated to determine system error excluding airspeed instrument error. This calibration must be determined—
(1) In level flight at speeds of 20 knots and greater, and over an appropriate range of speeds for flight conditions of climb and autorotation; and
(2) During takeoff, with repeatable and readable indications that ensure—
(i) Consistent realization of the field lengths specified in the Rotorcraft Flight Manual; and
(ii) Avoidance of the critical areas of the height-velocity envelope as established under §29.87.
These requirements must be met in order to obtain FAA certification. If this necessitates the use of specifically shaped pitot tubes (a la MD900, S76, S92, et al) then so be it. Some models I've tested required custom designed pitot tubes to comply with the requirement, while another was able to demonstrate compliance with an off-the-shelf AN5816 pitot tube.
HT
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HeliTester,
I agree with you that OEI HV charts for a twin may not be based in completing maneouvre with zero airspeed.
The point I was addressing is that the manoeuvre starts with an airspeed and ends with a groundspeed (or groundspeed range) vice an airspeed. That point really doesn't change if you include run-on landings.
Cheers,
Matthew.
I agree with you that OEI HV charts for a twin may not be based in completing maneouvre with zero airspeed.
The point I was addressing is that the manoeuvre starts with an airspeed and ends with a groundspeed (or groundspeed range) vice an airspeed. That point really doesn't change if you include run-on landings.
Cheers,
Matthew.
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HeliTester:
I realize what Part 27 and Part 29 say.
If what you say is true, why is minimum airspeed IFR for so many helicopters well above 20 Knots?
Or why are so many Vtoss figures also so high?
My experience is that on takeoff, the airspeed indicators read consistently lower than actual speed (tests done at zero wind and using GPS groundspeed for equivalent to airspeed).
The regulations may say one thing, but the practical reality is that very few, if any airspeed system works well below 40 KIAS -there just isn't enough difference between dynamic and static pressure to be accurate.
I realize what Part 27 and Part 29 say.
If what you say is true, why is minimum airspeed IFR for so many helicopters well above 20 Knots?
Or why are so many Vtoss figures also so high?
My experience is that on takeoff, the airspeed indicators read consistently lower than actual speed (tests done at zero wind and using GPS groundspeed for equivalent to airspeed).
The regulations may say one thing, but the practical reality is that very few, if any airspeed system works well below 40 KIAS -there just isn't enough difference between dynamic and static pressure to be accurate.
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Shawn,
What I said was “on the helicopter models I’ve tested the airspeed system position errors were measured at airspeeds in the 15-30 kt range.” I then cited CFR paragraphs to confirm that a requirement exists to determine airspeed system position error at airspeed down to 20 kts. Those statements were not intended to be contentious, merely statements of fact.
I agree that the accelerated takeoff position error is different from the level flight position error. The takeoff IAS typically lags (as you said), but the CFR requirement is met as long as the indication is repeatable and readable….important for TDP, field length, and H-V envelope avoidance. I would argue that at speeds below 40 kts airspeed system performance is compromised more by rotor downwash than by dynamic to static pressure differential. The best performing helicopter airspeed system I’ve ever seen is that of the S-61N where the pitot-static probes are mounted several feet above the rotor mast.
In any of my work, Vtoss was never predicated on airspeed system performance, but on helicopter performance (OEI ROC WAT Limited performance). Some Category A certified helicopters have selectable Vtoss and others have a fixed Vtoss. For those helicopters having a fixed Vtoss, that value is typically in the 50-60 knot range. Anything lower would cause the WAT curve to be limited by the first takeoff segment Vtoss OEI climb performance requirement rather than the enroute Vy OEI climb performance requirement which produces the best possible Category A WAT limited performance. For those helicopters featuring selectable Vtoss, the minimum value is typically set on the order of 40 kts, because anything lower would cause the operating weight to be so low that there could be very little payload.
I believe it is typically the IFR handling qualities requirement, most likely the lateral-directional stability requirement that sets the minimum IFR airspeed. I think that 50-60 kts is the minimum airspeed at which most helicopters can demonstrate compliance with the IFR handling qualities requirement.
HT
What I said was “on the helicopter models I’ve tested the airspeed system position errors were measured at airspeeds in the 15-30 kt range.” I then cited CFR paragraphs to confirm that a requirement exists to determine airspeed system position error at airspeed down to 20 kts. Those statements were not intended to be contentious, merely statements of fact.
I agree that the accelerated takeoff position error is different from the level flight position error. The takeoff IAS typically lags (as you said), but the CFR requirement is met as long as the indication is repeatable and readable….important for TDP, field length, and H-V envelope avoidance. I would argue that at speeds below 40 kts airspeed system performance is compromised more by rotor downwash than by dynamic to static pressure differential. The best performing helicopter airspeed system I’ve ever seen is that of the S-61N where the pitot-static probes are mounted several feet above the rotor mast.
In any of my work, Vtoss was never predicated on airspeed system performance, but on helicopter performance (OEI ROC WAT Limited performance). Some Category A certified helicopters have selectable Vtoss and others have a fixed Vtoss. For those helicopters having a fixed Vtoss, that value is typically in the 50-60 knot range. Anything lower would cause the WAT curve to be limited by the first takeoff segment Vtoss OEI climb performance requirement rather than the enroute Vy OEI climb performance requirement which produces the best possible Category A WAT limited performance. For those helicopters featuring selectable Vtoss, the minimum value is typically set on the order of 40 kts, because anything lower would cause the operating weight to be so low that there could be very little payload.
I believe it is typically the IFR handling qualities requirement, most likely the lateral-directional stability requirement that sets the minimum IFR airspeed. I think that 50-60 kts is the minimum airspeed at which most helicopters can demonstrate compliance with the IFR handling qualities requirement.
HT
