Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Tilt on February 09, 2005, 03:23:35 AM
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I always assumed that the best climb speed would be sort of proportional to the MP curve.
Hence typically if power backs off as I climb then (for optimum climb rate) I would lift the nose to reduce air speed proportionally...
In trying to understand Vy I read elsewhere curves indicating that whilst power drops of (due to altitude), the defined point of Vy can infact increase.
Ignoring the effect of 2nd stage super chargers putting steps in the MP curve...............
1) Does it always increase?
2) For best overall ROC should Vy be followed?
Sorry to be dense.....................
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Vy is a definition. Vy is the airspeed for maximum climb at a given set of conditions. The maximum climb equates to maximum excess thrust.
With a given set of conditions, a specific airspeed exists to give best rate of climb. That is Vy. AH uses a Vy based on sea level conditions. In theory this is not optimum. However, it is practical.
Here is an EAA link for a climb test procedure to determine Vy at different altitudes:
http://members.eaa.org/home/homebuilders/testing/articles/Climb%20Test%20Procedures%20-%20In-flight%20data%20collection.html
Vy is at its highest value (in IAS) at sea level. At maximum ceiling altitude, Vy is the same as Vx. Vx is the maximum excess horsepower speed. These speeds are important for a pilot to know.
Regards,
Malta
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I never came across Vy except as a notation for the vertical component of f a velocity vector e.g. (Vx,Vy,Vz). I'm not a pilot, though, but an engineer.
Also I think Stantond made a small typo.
"The maximum climb equates to maximum excess thrust"
Max climb rate max occurs at max excess power. In fact it can be shown that climb rate is equivalent to specific excess power. dh/dt=(T-D)*V/W
Climb angle max occurs at max specific excess thrust (T-D)/W.
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Well, I suppose it is all a matter of defintion, but if you read the EAA article they define Vy based on excess thrust. Vx, which I did not fully define, is the maximum climb angle speed or maximum climb rate relative to ground distance. Again, both are important for a pilot to know.
If you fly a certain plane quite a bit, knowing these numbers will help you in gaining the most altitude per minute, or the most altitude per distance on the map. Depending on the situation, one may be preferable over the other. As a bit of trivia, I found the F4U-1 Vx in AH2 to be 114 mph (IAS) at a ceiling of 40,000 ft.
Regards,
Malta
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Vy as you define it would occur at the speed which gives maximum excess power, Vx would occur at the speed which gives maximum excess thrust. I had to derive all this for homework once. :)
And yeah this stuff helps a ton in the game. Also speeds and power settings for best endurance and range are huge in scenarios. I don't play anymore though, I just post here because I'm neurotic. :)
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Yes,
I see what you are saying, after looking at the units. I used the EAA article definition of Vy which is the "maximum excess thrust airspeed".
These guys at:
http://www.mopilots.org/sgf/feb2001sgf2.htm
describe Vx as based on excess thrust and Vy based on excess horsepower, which as you point out, are dimensionally correct.
However, as Von Misses points out in his book, "Theory of Flight" (pg 402), the thrust (T) is power available per speed (Pav/V) and drag is power required per speed (Pre/V) so that (T-D) can be expressed as excess power per speed (Pav-Pre)/V. Which I would call excess thrust. Of course, what is also true is dh/dt=(Pav-Pre)/W, which can be expressed as excess power but it occurs at a specific speed, so it could be called excess thrust at that speed since dh/dt=(T-D)*V/W.
I don't think the EAA article is incorrect, but it seems the definitions of excess power and excess thrust can be interchanged with regard to Vy and Vx. However, Vy and Vx definitions are not affected by the definition of excess thrust or excess power. The only explanation for the definition discrepency is that they must have British origins.:)
Regards,
Malta
p.s. this is like a hobby of mine, I do not have a pilots license, but plan to get one this year. The main reason I have for a getting a pilots license is to make sure I will want to fly (in real life) the plane I am planning to build. I am not a pilot (yet).
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Damn limeys. :)
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So when HT states that the default auto speed is Vy at SL then this should give the best ROC at SL?
AH La7 auto speed = 160 IAS ROC SL 3500 (at guage) (default)(WEP off)
AH La7 auto speed = 125 IAS ROC SL 3600 (at guage) (/.speed 125)(WEP off)
RL La7 optimum climb was recorded at circa 125 IAS.
In fact in AH 125 IAS seems to return a better ROC (than 160) at all altitudes. (although this only makes circa 10 seconds difference on the 0 to 15600ft climb time)
Whilst AH power obviously changes with altitude and so effects ROC.......... I struggle to find that changing the auto speed from 125 brings any increase in ROC at differing altitudes.
So my questions are
1)Does Vy always give the best ROC?
2)Does Vy always change with altitude?
3)and if it does change with altitude does it always increase/decrease.
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Upon further review, I think the guy in the EAA article simply made a typo. He transposed X and Y. Everything else I find talks about Vx being best climb angle and Vy being best dh/dt.
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Originally posted by Tilt
So my questions are
1)Does Vy always give the best ROC?
2)Does Vy always change with altitude?
3)and if it does change with altitude does it always increase/decrease.
1. Yes
2. Yes, although the change could be very small.
3. For a prop, the true speed usually increases and the indicated speed usually decreases.
Here's a really nice page for this problem and just about every other question in this forum:
http://142.26.194.131/aerodynamics1/Performance/Page10.html
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Well, I suppose it is all a matter of defintion, but if you read the EAA article they define Vy based on excess thrust. Vx, which I did not fully define, is the maximum climb angle speed or maximum climb rate relative to ground distance. Again, both are important for a pilot to know.
If you fly a certain plane quite a bit, knowing these numbers will help you in gaining the most altitude per minute, or the most altitude per distance on the map. Depending on the situation, one may be preferable over the other. As a bit of trivia, I found the F4U-1 Vx in AH2 to be 114 mph (IAS) at a ceiling of 40,000 ft.
Regards,
Malta
While its true that Vx does give the best gain in altitude per unit distance, it does not provide near the altitude gain per unit time as Vy does. I only point this out because Malta's post might sound (and I don't know if this is what he intended or not) as though you should use Vx to climb when you are going some place that is close on the map and you figure climbing at Vx will get you to your destination the highest altitude in the shortest distance. Use Vy and circle once or twice if you have to. You'll spend less time climbing and getting to your destination.
Use Vx only for short periods to clear an obstacle (high terrain right after takeoff, for instance.) Use Vy for all other climbs if reaching the max altitude in the shortest possible time is a concern.
Finding Vx is fairly simple. On a graph, plot Rate of climb on the Y axis and airspeed on the X axis. The peak of the curve, the highest rate of climb, is Vy (which you already know in AH since Hitech told us it's the default value for Alt-X.) Then draw a straight line from the origin at an angle such that the line is tangent to the curve. At the point where the line touches the curve, drop straight down to the X axis; this airspeed is Vx.
This is a simplified way of finding the average Vx through the ranges of altitudes you measured the rate of climb.
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Originally posted by Tilt
1)Does Vy always give the best ROC?
2)Does Vy always change with altitude?
3)and if it does change with altitude does it always increase/decrease.
Answered well previously. Vy always gives the best rate of climb and decreases with altitude. Vx gives the best angle of climb and increases with altitude. The point at which these speeds converge is the absolute ceiling of the aircraft.
Casca
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Originally posted by Tilt
In trying to understand Vy I read elsewhere curves indicating that whilst power drops of (due to altitude), the defined point of Vy can infact increase.
Hi Tilt, your question has already been answered correctly by others, but I thought I would point you to this thread:
http://www.hitechcreations.com/forums/showthread.php?threadid=137751&referrerid=2314
That provides a combat perspective to the topic.
Badboy
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Don't forget that Vy is computed for the maximum gross weight, if you take only 50% fuel, Vy will change.
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Originally posted by SFRT - Frenchy
Don't forget that Vy is computed for the maximum gross weight, if you take only 50% fuel, Vy will change.
If you have Vy for a specific configuration, then increasing the weight increases Vy, and decreasing weight decreases Vy. For typical WWII fighters and fuel capacities that variation in Vy is small, generally single figures, but increases again if you take into account ordnance loads.
Hope that helps...
Badboy