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  #1  
Old 12-10-2020, 12:39 PM
Harvey rv12 Harvey rv12 is offline
 
Join Date: Apr 2015
Location: Olympia WA
Posts: 188
Default Best angle climb speed Vx

To all you expert aerodynamicist's out there, my question:
Does best angle climb speed vary with weight in the same way as maneuvering speed or stall speed?.
I understand best angle climb speed is a function of excess thrust. Best rate climb speed is a function of excess power. What I don't have is an equation for the curves of induced drag, parasite drag, and thrust available.
Stall spd: Vs new = Vs gross x √(Vs new/Vs gross)
maneuvering spd: Va new = Va gross x √(Va new/Va gross)
So does Vx new = Vx gross x √(Vx new/Vx gross)

I understand best angle climb speed goes up with altitude, as best rate climb speed goes down with altitude, until they meet at absolute ceiling.

Anyone?.
I expect to get schooled here :-)
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the bitterness of poor quality
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Last edited by Harvey rv12 : 12-10-2020 at 01:28 PM. Reason: typo
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  #2  
Old 12-10-2020, 12:56 PM
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revenson revenson is offline
 
Join Date: Mar 2005
Location: Tucson, AZ
Posts: 173
Default

Vx goes up with density altitude...but very little.
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  #3  
Old 12-10-2020, 02:13 PM
A64Pilot A64Pilot is offline
 
Join Date: Dec 2020
Location: Weirsdale Fl
Posts: 28
Default

VX increases with weight by if memory serves by half the percentage of weight increase.
Meaning if you add 10% aircraft weight, VX will increase by 5%.
Not so much difference with an RV, but say for example and Ag airplane that can carry its own weight, it’s a pretty big difference.

Other similar example is that glide distance doesn’t go down with weight increase, but glide speed increases to maintain the glide ratio and rate of sink of course increases.
Don’t rely too much on the percentages I said above, that off of memory and is likely one of those close enough rules, an approximation.

On edit, I believe actual Vx speed may increase significantly, but if we are using indicated airspeed that sort of compensates, I think.

Last edited by A64Pilot : 12-10-2020 at 02:15 PM.
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  #4  
Old 12-10-2020, 02:44 PM
Ed_Wischmeyer's Avatar
Ed_Wischmeyer Ed_Wischmeyer is offline
 
Join Date: May 2005
Location: Savannah, GA
Posts: 1,433
Default

I've wondered a lot about Vx, but haven't ever done the analysis or flight test. Here are some interesting questions -- at least, I find them interesting:
* Vx is maximum altitude gain per distance covered in still air. So if you're interested in gaining altitude vs distance covered over the ground, how does Vx change with headwind? or tailwind? Enough to make a difference?
* The big idea behind climbing at Vx right after takeoff is that if the engine quits soon after takeoff, you've got altitude to enable a further glide to a "best" landing site, on or off-airport. How much does that glide capability differ from a glide from a climb at Vy when the airplane has less potential energy but more kinetic energy?
* What are the pitch angles for Vx and Vy? What little testing I've done in the RV-9A suggests that 13į and 15į might be about right -- 160 HP, constant speed prop. That seems like surprisingly little difference.
* If you climb at Vx and the engine has problems, is that a more challenging situation than an engine problem at Vy with a lower pitch angle?
* Why do POH for certificated airplanes never seem to give rate of climb at Vx, or changes in Vx with weight or density altitude?
* How much difference is there in rate of climb and climb gradient between Vx and Vy?
* How do all these Vx/Vy comparisons vary with thrust (power) to weight ratio?

When I got the RV-9A, it had cooling problems on summer days, so "normal" climb was 110 knots. Those problems have been fixed, but deck angles are pretty steep at Vx and even Vy. Plus, the plane climbs like mad anyway and I normally fly from long runways, so I tend to climb at Vy+25 or so. And if there are buzzards around the airport, best to keep the nose down so you can see and avoid.

For my flying, almost entirely long, paved runways, Vx climbs seem to be extra risk with little benefit. I probably should practice Vx more (although that would probably look like I was showing off), and at some point I should use the G3X digital flight data recording capability and generate some more up to date performance measurements. And I should also practice Vx climbs in gusty air (at altitude) so that I have a good feel (and aural warning) for stall margin.

Your mileage and risk profile are sure to vary...
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  #5  
Old 12-10-2020, 04:04 PM
BobTurner BobTurner is offline
 
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Location: Livermore, CA
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Getting a workable but simple formula for Vx is complicated by the fact that many of the simple approximations often used donít work at low airspeeds, e.g., induced drag cannot be neglected, and is probably greater than parasitic at these airspeeds. Prop efficiency tends to be falling as airspeed decreases, even with a CS prop. For most GA aircraft, Vx is close to stall speed, so there is no option to fly slower (as you would want to do, to answer Edís question about a headwind). etc.
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  #6  
Old 12-10-2020, 05:34 PM
andrewtac andrewtac is offline
 
Join Date: May 2014
Location: Friendswood TX
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" The big idea behind climbing at Vx right after takeoff is that if the engine quits soon after takeoff, you've got altitude to enable a further glide to a "best" landing site, on or off-airport. How much does that glide capability differ from a glide from a climb at Vy when the airplane has less potential energy but more kinetic energy?"


Wouldn't you always be higher and faster at Vy? Best rate of climb.

Edit to add, I guess for a brief period of time transitioning to By, and slightly after.

I thought Vx was for obstacle clearance.
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Last edited by andrewtac : 12-10-2020 at 05:47 PM.
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  #7  
Old 12-10-2020, 06:57 PM
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Ed_Wischmeyer Ed_Wischmeyer is offline
 
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Location: Savannah, GA
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Quote:
Originally Posted by andrewtac View Post
" The big idea behind climbing at Vx right after takeoff is that if the engine quits soon after takeoff, you've got altitude to enable a further glide to a "best" landing site, on or off-airport. How much does that glide capability differ from a glide from a climb at Vy when the airplane has less potential energy but more kinetic energy?"

I thought Vx was for obstacle clearance.
You're right. But I have heard it both ways over the years.
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RV-9A at KSAV (Savannah, GA; dual G3X Touch with autopilot, GTN650, GTX330ES, GDL52 ADSB-In)
Previously RV-4, RV-8, RV-8A, AirCam, Cessna 175
ATP CFII PhD, so I have no excuses when I screw up
Too many safety posts purged without notice...
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  #8  
Old 12-11-2020, 03:19 AM
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rv8ch rv8ch is offline
 
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Location: LSGY
Posts: 3,756
Default Kitplanes article

Interesting article from Nigel related to this.

https://www.kitplanes.com/using-leve...b-performance/

Climbing in my RV-8 even at MTOW the sight picture at Vx and Vy is nothing but sky. I've been doing a lot of this as required for my "phase 1" testing, and really don't like it - no forward visibility. Outside of testing, as soon as I've cleared obstacles (a few seconds after liftoff) I get the nose down and climb at a rate where I can see forward.
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  #9  
Old 12-11-2020, 03:32 AM
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pierre smith pierre smith is online now
 
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Location: Louisville, Ga
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Quote:
Originally Posted by rv8ch View Post
Interesting article from Nigel related to this.

https://www.kitplanes.com/using-leve...b-performance/

....... Outside of testing, as soon as I've cleared obstacles (a few seconds after liftoff) I get the nose down and climb at a rate where I can see forward.
A wise move in the interest of safety but much improved CHT cooling in the summer. My -10 climbs like a homesick angel with the nose on the horizon anyway.

Best,
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  #10  
Old 12-11-2020, 03:53 AM
Capt Capt is offline
 
Join Date: Feb 2017
Location: Australia
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I've got a headache reading all that I just point my 8 skyward and it's an express elevator at any angle/speed
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