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63 knots best glide speed?

Mike Meehan

Mike Meehan

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This article from AOPA states "As an experimental aircraft, each builder determines the best glide speed of their Van's RV–10. Various online sources cite 63 knots as a recommendation." ???

aopa.org

Change of emergency plan preceded fatal accident

The pilot of a Van's RV–10 attempted to fly a traffic pattern after reporting the need to immediately return to Fullerton Municipal Airport in California and crashed short of the runway on January 2.
aopa.org
 
Start at 1.3 Stall for your config.
Best glide is normally zero flap, is it better reflexed up in the 10? Will you set that if not at it, or battery off?

I owm a 6, like my Pitts, too slow for best glide really cuts to range. There is little gained at Vx/Min Sink vs Vy/best glide.

Best glide is well above 1.3 stall.

63 sounds low for either 1.3 stall or best glide- that's best glide in a heavily loaded 40:1 sailplane with 15+M wingspan. That would mean a sub 44 knot stall speed, if it met both.

Edit to add- establishing your v speeds is part of phase 1. Check the logs or reaccomplish if needed.
 
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Maybe a typo, or maybe they were thinking of DMMS rather than best glide. Although, for my -9A DMMS is 1.404 x Vs = 66 knots. Best glide is 80 kt according to my POH.
 
I'm with Kyle. 63 is really low. I suggest go up and test. Mine is 88-90KTS. 63ish is about what I cross the fence at when landing.
 
My 6.9i sure has a different wing than a 10, but flight tests gave 85 as Vy (which is not Vg, but close enough), and 65 as Vx (best angle). Those being in knots.
 
The RV series is relatively low drag. Such designs have a higher best L/D speed than the 'usual' GA airplane.
 
Mike Meehan said:
This article from AOPA states "As an experimental aircraft, each builder determines the best glide speed of their Van's RV–10. Various online sources cite 63 knots as a recommendation." ???
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Two comments.
1. I gave up my aopa membership some time ago. Too much misinformation, like this one.
2. The adsb trace of the pattern flown shows, imho, a common training problem. As a cfi I used to regularly ask Flight Review pilots to demonstrate landing power off in the first 500’ of the runway. But then, when I pull the power near an airport, they reflexively try to touch down in the first 500’. Some need ‘help’ (power) when they come in short. When I ask them why they didn’t aim for mid-field (far better to go off the end at 30 knots than to come in short at 60) they get the ‘deer in the headlight’ look. “you do what you train for”,
 
Best glide? Most distance or most duration. For max distance in my RV-4 it is 90 - 110 mph, flaps up. Between 90 and 110 it really doesn't make much difference. Clean stall 60 mph indicated.
 
swjohnsey said:
Best glide? Most distance or most duration. For max distance in my RV-4 it is 90 - 110 mph, flaps up. Between 90 and 110 it really doesn't make much difference. Clean stall 60 mph indicated.
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You’d better be just about ready to touchdown at 63 kia (full flap stall is 53-55). 82/85 kia is my best glide.
 
Here's what the FAA says about it:


If it’s distance you want, than you’ll need to use the speed and configuration that will get you the most distance forward for each increment of altitude lost. This is often referred to as best glide speed and, on most airplanes, it will be roughly halfway between V x (best angle of climb speed) and V y (best rate of climb speed).
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If you’re more interested in staying in the air as long as possible to either fix the problem or to communicate your intentions and prepare for a forced landing, then minimum sink speed is what you’ll need. This speed is rarely found in Pilot Operating Handbooks, but it will be a little slower than maximum glide range speed.
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85 kts also seems like a good number for my RV-8, even though the calculated value based on my flight tested Vx (70) and Vy (97) is 83.5 kts.
 
I think we should be careful comparing other RVs to the RV10.
The 10 has a totally different airfoil, a significantly higher wing loading and stall speed. It’s an apples and oranges comparison.
63kts will see a 10 at gross descending like a brick.
90 is what I aim for best glide. Maybe a touch lower at lighter weight.
 
Seems like there is no single defined term as to what is best glide that is being used in this discussion. In sailplanes, there are two related terms. Best L/D is the speed at which you can cover the most distance for the least rate of sink. Minimum sink is the least sink over time. Unless we specify whether we are comparing best L/D or minimum sink, we aren't comparing the same parameters. So what is "best glide speed" referred to in this discussion?
 
A few points to ponder:

1. All experimental airplanes have differences in pitot/static configuration and plumbing. This means each airplane has a unique static source pressure error, therefore you can't directly compare airspeed unless it's derived from flight test and expressed as calibrated airspeed. No two airplanes will have an identical IAS (unless it's pure coincidence) for the same condition at the same gross weight, even when they are the same type (e.g., RV-10 and RV-10).

2. Maximum range glide occurs at an L/Dmax condition in still air. L/Dmax is not related to stall speed aerodynamically, but the AOA for L/Dmax is engineered into the airplane, and AOA controls airspeed, so the very first step is to flight test to determine what an L/Dmax conditions either in terms of calibrated airspeed. Either the EAA test cards or AC 89-90C are a good place to start. You may also want to google "bootstrap flight test technique". The latter is an interesting approach, but is a bit more complicated with a controllable prop like you have on your RV-10.

3. Maximum endurance glide occurs at a minimum power condition. This condition is aerodynamically related to L/Dmax. For velocity, Vmin power = V L/Dmax * .76 and in terms of AOA its AOAmin power = AOA L/Dmax * 1.73. That math is discussed in this short "how to" article: Some Comments on Angle of Attack System Calibration.

4. As pointed out, glide speed needs to be adjusted for actual wind conditions. To do this correctly, a computer is required. It's practical to apply a WAG adjustment to the Mark I eyeball assessment, but ultimately the only way to work the physics is with some digital help. Some avionics provide this capability. In this video, I'm testing a phone-based application that does the glide physics for me and provides guidance to high key (a known energy state, that is to say altitude and airspeed, over the the TDZ on the intended runway). From this position, a simple 360 degree descending spiral is flown to touchdown. Half way through the turn, the airplane is at a normal power off base position, called "low key." From low key to touchdown, it is 95% Mark I eyeball, but any "negative energy state" (insufficient altitude/airspeed combination) to reach the runway is detected by the computer and conveyed to the pilot graphically and aurally. In the test, you'll hear me use the term "on speed" which is the AOA associated with minimum power required/minimum sink glide. When the engine quits I only fly a single AOA/airspeed and let the computer handle the heavy lifting. It knows my glide ratio and planned bank angle for maneuvering, and since I'm flying an AOA, gross weight is automatically compensated for. I'm also applying drag (in this case Flaps 40) to compensate for residual thrust produced by the fixed pitch propeller at IDLE power, since glide angle at IDLE power is not the same and glide angle engine OFF. That's discussed in this thread if you are interested in more information. Use of a single glide parameter (in this case min sink/max endurance) is purely technique that reduces pilot workload at the expense of some glide performance. You could just as easily fly an L/Dmax condition to high key and then slow to min sink for the final portion of the dead stick. Although not an aerodynamically bullet proof statement of fact, 1.3 Vs for current gross weight is generally going to be pretty close to max endurance/min sink if you don't have AOA. A good glider pilot can likely easily transition between L/Dmax and min sink just looking out the window. I'm not a good glider pilot :), but I've got an iPhone and an AOA tone that works like a variometer:


The bottom line is that the only way to determine best glide speed for your specific airplane is by flight test. Such is the nature of experimental aviation, even though we don't think of ourselves as such, we are all experimental test pilots from time to time. Pretty cool. If your GPS/moving map display has some sort of glide envelope prediction, use it to the fullest extent and practice with it from time to time.

Fly safe,

Vac
 
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Vac said:
3. Maximum endurance glide occurs at a minimum power condition. This condition is aerodynamically related to L/Dmax. For velocity, Vmin power = V L/Dmax * .76 and in terms of AOA its AOAmin power = AOA L/Dmax * 1.73. That math is discussed in this short "how to" article: Some Comments on Angle of Attack System Calibration.
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A minor correction on this, the calculation that is based on assumes a constant drag coefficient for all angles of attack. Ok for a first pass but not so accurate at higher angles where the fuselage, gear fairings, etc. are not as well aligned with the wind. Drag will almost always be higher in that lower speed/higher AOA so the actual maximum endurance glide/minimum sink rate will happen at a higher speed than L/Dmax * 0.76.

Finding the exact speed would take a lot of very careful testing. Critical for a glider, but probably not worth it for an RV. Just aim for "A bit faster than 76% of L/Dmax"
 
Kyle Boatright said:
We are a long way down the road from a thread that started with an inaccurate glide speed reported by AOPA. ;-)
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Yep. All the theory and other models’s numbers are nice philosophically but at end of the day the water cooler discussion needs to stop and the OP needs to grab the EAA or similar flight test card and go fly to determine what’s the best indicated glide speed is for his RV-10 (as suggested back on post #5). Everyone else’s numbers including my own 10’s, much less another model , don’t mean squat except as verification that his stated number is way off hence the need to fly the test.
 
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Auburntsts said:
Yep. All the theory and other models’s numbers are nice philosophically but at end of the day the water cooler discussion needs to stop and the OP needs to grab the EAA or similar flight test card and go fly to determine what’s the best indicated glide speed is for his RV-10. Everyone else’s numbers including my own 10’s, much less another model , don’t mean squat except as verification that his stated number is way off hence the need to fly the test.
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I’d be willing to bet that his number is going to be 85 knots +/- 5…
 
Vans publishes Vbg (Best power-off glide speed for max range) for the RV-14 at 84 knots. Same airfoil. Me thinks 85 knots +/- 5 a safe bet.......
 
dmattmul said:
Vans publishes Vbg (Best power-off glide speed for max range) for the RV-14 at 84 knots. Same airfoil. Me thinks 85 knots +/- 5 a safe bet.......
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Next issue is betting how many pilots can stay +/-5 during an actual emergency…

If my experience during instructional mock checkrides, BFRs, etc. is any indication, +/-10 would be a better bet.
 
rocketman1988 said:
Next issue is betting how many pilots can stay +/-5 during an actual emergency…

If my experience during instructional mock checkrides, BFRs, etc. is any indication, +/-10 would be a better bet.
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I know I would probably struggle to maintain an accurate Vbg. I practice 100kts and have set my EFB "glide ring" for what seems to be what I would get at 100. I try to avoid flying routes where I need that last bit of performance to find a safe landing spot. When I lose my engine, as long as I don't stall/spin, I'll take what I get.
 
Which is why flying an AOA tone is about as caveman simple as it gets :cool:, especially when sucking up the seat cushion. The tone provides simple "push/pull" direction to the pilot, and doesn't require you to look at anything. With the right cuing, +/- 5 knots is surprisingly doable. Unlike airspeed, AOA isn't affect by gross weight. As discussed, the actual performance curves shift in terms of airspeed, but by flying AOA that's completely transparent to the pilot. You simply establish the desired alpha (either range or endurance) and fly the cue (aviate). Adding some computer enhancement (whether it's a displayed "glide ring" on a moving map or a phone app designed to do the physics, etc.) is the next best course of action (navigate). And be ready for a different site picture, because even if you regularly practice engine-out approaches, when the engine is not producing power, glide performance won't be the same due to residual thrust effects (it will be worse, i.e., actual glide engine off is a steeper angle). Things are compounded if the airplane has a controllable propeller as the drag produced by the windmilling prop varies with pitch. The bottom line is that if the engine actually quits, the airplane simply won't glide as well and the pilot is going to be under extreme pressure. Thus, we are measuring with a micrometer for something that will ultimately be cut with an axe--the answer is "good enough" and a simple, accurate, flyable AOA cue is about as easy as it gets to help with the "aviate" part of the "aviate, navigate, communicate" process.

Technique aside, good flight test discussion! And, yeah, interesting pile-ons to "various online sources cite 63 knots."

Fly safe,

Vac
 
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