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-   -   Max L/D speed? (https://vansairforce.net/community/showthread.php?t=180222)

suds 03-07-2020 06:32 AM

Max L/D speed?
 
I?m curious about the Max L/D speed for the RV-6 and can?t seem to find one. Anyone have it? Thanks!

Kyle Boatright 03-07-2020 06:45 AM

The CAFE foundation tested an RV-6A about 20 years ago. You can find a link to it on this page:

https://cafe.foundation/v2/research_aprs.php

They calculated 92 Knots/ 106 MPH as best glide.

suds 03-07-2020 07:52 AM

That?s terrific. Thank you Kyle!

Marc Bourget 03-07-2020 08:21 AM

If you decide to get picky, I think that speed changes with aircraft weight. Not a lot, but, someone who was more knowlegible and smarter than me convinced me it was worth "tucking in the back of your memory"

Seems he had an engine out one time - some distance from an airport. Realized after he got down that he would've ended up in the river, short of the runway, without the added glide distance gained by adjusting his glide speed for weight. [Granted, this is "Sullenberbeger" level pilotage !!!]

FWIW - YMMV

BobTurner 03-07-2020 11:29 AM

Yes, best glide varies as the square root of weight (if 10% below gross, reduce the gross weight speed by 5%). Also, in the real world, where one wants to cover distance over the ground, the wind matters. Into the wind, increase best glide speed; tailwind, decrease best glide speed.

DeeCee 57 03-07-2020 11:57 AM

mine's been determined as 85kts during flight testing... guess having that 3 bladed speed brake at front also affects the result.

sailvi767 03-07-2020 03:58 PM

Quote:

Originally Posted by BobTurner (Post 1412893)
Yes, best glide varies as the square root of weight (if 10% below gross, reduce the gross weight speed by 5%). Also, in the real world, where one wants to cover distance over the ground, the wind matters. Into the wind, increase best glide speed; tailwind, decrease best glide speed.

The wind will generally be the biggest factor. Plus or minus 5 knots is not going to change glide range much. 20 knots of wind will have a huge effect on what airports you can reach.

Vac 03-10-2020 07:25 AM

If you fly L/Dmax AOA, it’s not effected by weight, G-load or density altitude. Flaps up, L/Dmax AOA and max endurance glide (ONSPEED) AOA are different. When you deploy flaps, best overall glide performance occurs ONSPEED. Half flaps in an RV-3/4/6/7/8 provide best L/D compromise and can be thought if as “lift flaps,” so when the engine quits, deploy lift flaps and establish ONSPEED AOA while you are assessing options. Maneuver as much or as little as you’d like, maintaining an ONSPEED condition is a “no brainer” technique to essentially optimize your energy burn. L/Dmax, ONSPEED and best range are important AOA parameters, unfortunately not all AOA systems are capable of capturing them, even when properly calibrated. See “Using AOA When The Engine Fails:” https://www.flyonspeed.org/using-aoa-for-em-engine-out

If you don’t have an AOA system in the airplane, consider running flight test card 8 in the EAA Flight Test Manual to get accurate numbers for your airplane. Those IAS references will be accurate for your airplane, regardless of any pitot/static error Or propeller type (big effect) you have. If your prop is controllable, you’ll need to experiment at different RPM to see how that effects performed. There is a prop-stopped glide discussion in the transition training manual worth reviewing.

Wind is always a factor affecting glide distance; and some GPS and EFIS systems have a glide display based on actual computed L/D.

Cheers,

Vac

sailvi767 03-10-2020 02:51 PM

I would do some testing on deploying any amount of flaps after a engine failure. In most aircraft a clean wing is your best option for maximum glide range. In the RV I plan on flaps being used more in a speedbrake mode in the event of a actual engine out. Any deployment of flaps in my 6 and now Rocket increased rate of decent.

Vac 03-11-2020 06:30 AM

Hi George,

You are absolutely correct. Flight test is the only way to determine glide capability for any airplane, and every RV is unique. Which means every airplane requires flight test to determine actual capability and specific speed references. You are also correct insofar as a clean wing will provide absolute maximum range glide.

The CAFE report is a good place to start for an RV-6.

We've found in instrumented flight test that the "lift flap" rule of thumb (and it's just that, a rule of thumb) provides an excellent compromise in terms of energy, time and maneuverability for the RV's with 23013.5 airfoil sections and plain flaps for a low-altitude engine out situation. At altitude, a GPS derived solution at L/Dmax with flaps 0 that results in 1K-1.5K' overhead intend landing location works well. Without GPS, a rough visual approximation can be made by picturing an ellipse from wing tip to wing tip that extends just forward of the nose of the airplane. Another rule of thumb is that a 45 deg banked turn is a good all-around "bang for the buck" turn rate (deg per second) vs altitude lost when the objective is to point the nose. These energy management drills are easier with a calibrated AOA system that provides accurate performance cues and accommodates multiple flap positions than it is using IAS. And I'll defer to the glider folks who have forgotten more about this than I know and may have even better techniques.

For most folks, EAA test card 8 is a great place to start. Regular "SFO" (simulated flame out) practice is also helpful.

Hopefully, the references in my previous post make this clear. If not, we're always open for help critiquing/editing/correcting/improving any of them.

v/r,

Vac


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