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Vx, Vy

erich weaver

erich weaver

Well Known Member
Patron
What are typical values for best angle of climb and best rate of climb on the -7A?

Looked for this info in the POH that is available from the home page, but it has conflicting information - it indicates Vy is 110 mph on one page, and 90 mph on another page

thanks

erich
 
Based on the test results I have on my RV-8, plus a hypothetical performance model I created for the RV-8 based on a combination of CAFE Foundation and Van's data, I predict your Vy will be somewhere around 95 to 100 kt CAS at 1800 lb weight. Note that there will be a fairly wide speed range around Vy where the rate of climb won't fall off too much. For an example, see my initial climb test results. Full story here.

Vx is harder to predict. My test results found a Vx that was about 8 kt faster than my model predicted. I'm not sure whether my model is off, or whether my test results were shaky. There also may be quite a difference in prop efficiency at low speed between your WW and my Hartzell, which could affect Vx. In any case, I predict Vx at 1800 lb at sea level to be somewhere in the range of 60 to 70 kt CAS.

This info is worth what you paid for it, so there is no guarantee it is correct.
 
Kevin's predictions seem to fit in pretty well with what is listed in Roberta Hegy's "Airspeed Limitations" section of her POH:

Vy = 110 mph (96 knots)
Vx= 80 mph (70 knots)

Her -7A has an O-360 and a Hartzell constant speed

I was hoping to hear from a few other -7A flyers with real numbers. Y'all completed the appropriate flight testing and documented your numbers, right? :)

erich
 
Keep in mind that each aircraft has its own airspeed system errors, which are a combination of ASI instrument error and pitot-static system position error. So, even with two seemingly identical aircraft, there may be a several kt variation in Vx and Vy if you consider them in terms of IAS.

Also, it is very hard to nail down exact Vy and Vx via flt test, so the data from any given flight may lead you to quote values that vary a few knots one way or the other. It is easy to get into "measure with a micrometer and cut with an axe" territory here. I'm more guilty than most here.
 
Kevin is right on, especially about the fact that the Vy varies hardly at all over a pretty wide speed range in the RV's.

I never really bothered to pin down Vx because the airplane climbs so quickly on takeoff, there is very little point to knowing the best angle - if the obstacle/slope ahead of you is so steep that it matters, you'll probably think twice about it anyway! Not scientific, of course, but just how good the RV's are at climbing out...

Paul
 
Safety

Interesting discussion. Mike Seager recommends to climb at 120kts for safety considerations, especially until you get to a reasonable altitude. Climbing at 70 to 90 is a very high angle of attack with bad visibility and likely temperature rise. If the engine quits there, you are on the edge of a stall.

I usually set the auto pilot to get a climb of 120 kias. Occasionally, I'll set to get 110 kias. Either one of these is going to get you a great rate of climb.
 
Hmmm... point taken regarding Vx - now that you mention it, I dont think I would be comfortable with the view climbing out at 80 mph

Vx and Vy seemed so important in that C-150 I trained in...

erich
 
Excellent advice.....

......to climb around 120 knots/138 MPH, exactly what I use and recommend. I try to always see the horizon for several reasons, traffic, CHT's and quicker enroute time. Matter of fact, I just showed a guy how well my FP -6A climbs at 160 MPH...right at 1000 FPM!

You guys won't believe 170 MPH indicated and 800-900 FPM, so I suggest you go out and try it. With a FP prop, you make more RPM at the higher speeds, therefore more horsepower resulting in better climbs.

Regards,
 
A little math and technique to help out

With a CS prop (a major issue for this) the best rate of climb ought to be very darn near the speed for best glide and the best angle of climb ought to be 76% of best glide. On my 7A the IAS for best glide is usually about 96 kts but the instrument is reading a little high. Yours will vary. CAFE says their 6A tested at 110 mph best glide and that's about 96 kts.

This ignores good advice such as keeping the nose down for visibility, etc. Best is what it is, but smart flying is more complicated.

There is one easy way to nail down best glide for a given combination of weight and atmosphere - mostly weight since we are talking about CAS as suggested by IAS.

Set your throttle to give you partial power such that you are sinking. It doesn't matter exactly how much, but if you use 500-750 f/m this will work well. Test your speeds until your glide ratio is best at that power setting; that's the best glide speed regardless of power setting.

Oh - did I forget to mention that the whole Garmin handheld family has a data field that you can choose to display which is a real-time glide ratio? Just find the speed at which the number is highest. It matters not what the ratio is. You will discover if you play with this that varying the power while holding the speed constant will change the rate of sink but the same speed will still be best.
 
H. Evans: Thats a good tip about Garmin units - dont remember that function but I will defintiely try and find it on my 296.

One nit pick - I seem to remember that best glide speed is independent of weight - no?

erich
 
independent of...

n5lp said:
No, best glide speed varies with weight. Best glide angle is independent of weight.
Click to expand...

The best glide CAS happens when induced drag and parasite drag are equal. If you are heavier, then at a given speed your induced drag is greater. But for a given speed, parasite drag is the same. I think this plays out that your best glide will be faster if you are heavier. You can visualize it from there, probably. I think you'll find it's at the same Angle of Attack, not the same angle with respect to the ground/horizon.
 
I didn't respond to this thread earlier, as I've had a very, very busy week.

hevansrv7a said:
With a CS prop (a major issue for this) the best rate of climb ought to be very darn near the speed for best glide and the best angle of climb ought to be 76% of best glide. On my 7A the IAS for best glide is usually about 96 kts but the instrument is reading a little high. Yours will vary. CAFE says their 6A tested at 110 mph best glide and that's about 96 kts.
Click to expand...
Unlike the story with speed for best glide and the speed for minimum rate of sink, there is no hard ratio between Vx and Vy. The difference between those two speeds gets smaller as the altitude increases, and Vx = Vy when you are at the absolute ceiling of the aircraft.


hevansrv7a said:
The best glide CAS happens when induced drag and parasite drag are equal. If you are heavier, then at a given speed your induced drag is greater. But for a given speed, parasite drag is the same. I think this plays out that your best glide will be faster if you are heavier. You can visualize it from there, probably. I think you'll find it's at the same Angle of Attack, not the same angle with respect to the ground/horizon.
Click to expand...
True, the speed for best glide will always be at the same angle of attack, no matter what the weight (ignoring the effect of Mach number, which is pretty much legligible for our airfoil at RV speeds). Ignoring any thrust or drag from the prop, the descent angle will equal the ratio of lift to drag, and this will be same at the speed for best glide, no matter what the weight is.
 
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