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Max Endurance

scrollF4

Moderator, Asst. Line Boy
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Has anybody successfully tested and determined a max endurance -- slash -- L-over-D Max chart or speed for their RV-7?

How did you ascertain that curve and speed?

What did your test card(s) look like?
 
I’ve created a chart for my -4, I probably should graph it, but here’s what I did..
Every flight, at different altitudes in cruise, I would write down the altitude, manifold pressure (in .5 increments from full throttle down to very little 17.5 inches or so) and the corresponding fuel flow (leaned both best power and max range) and TAS. I also divided TAS by Fuel Flow to come up with a MPG column. Over time I got most of the altitudes from 4,500 up to 17,500 feet. While I didn’t get slow enough to L/D, I now know how to get as far as I could want out of my plane for a given flight. My longest flight to date in my -4 with standard 32 gallons of fuel onboard was 4:29 and 650 nautical miles. I landed with 5 gallons (30+minutes)
 
Has anybody successfully tested and determined a max endurance -- slash -- L-over-D Max chart or speed for their RV-7?

How did you ascertain that curve and speed?

What did your test card(s) look like?

Here is the FAA's view:

https://www.faa.gov/news/safety_briefing/2018/media/SE_Topic_18-05.pdf

I think the EAA flight test cards can help with this, if you have not seen them, they are worth the money. I don't have them handy as I'm traveling, but perhaps someone who has them nearby can explain further.
 
There's some solid math here you can use:

https://www.nar-associates.com/technical-flying/angle_of_attack/FundamentalAoA_wide_screen.pdf

It's about AOA, but the formulas are around the middle of Page 3.

If you know your L/Dmax speed, you can easily calculate minimum power required speed (max endurance) or Carson speed.

Lenny

This is in line with other passages I have read, but it takes more than just my own fingers and toes to work the math. I have 'close enough' best glide speeds. "If you know your L/Dmax speed..." ...that's exactly what I'd truly like to nail.
 
Here’s what I did:
Pick a smooth air day (minimal up/down drafts). Pick a safe altitude. Pull the power to idle, and stabilize the airplane in a 65 kias glide. Record the vertical speed (VS). Repeat at 70, 75, 80,……, 110 kias.
Now go home and plot the VS as a positive number, against KIAS. Draw a smooth curve thru the data. The lowest point of the curve is minimum sink speed.
Now hold a straight edge on the x-axis, anchor the end at the origin so it can pivot but stays on (zero, zero). Rotate the straight edge upwards (keep it pinned on the origin) until it just touches the curve. That airspeed is best glide speed, and is also (lift/drag)max speed. If you want to find best glide over the ground with a wind, just move your straight edge anchor point 20 knots (kias) left if there’s a 20 kt (ias) tailwind, rotate up until you touch the curve. Move the anchor point to the right for a headwind. The ratio of airspeed to vertical speed (use the same units!) will give you the glide ratio. Finally, if you were below gross weight, multiply your best glide speed as tested by the square root of the ratio of gross weight to as-tested weight, to get best glide at gross.
 
Here’s what I did:
Pick a smooth air day (minimal up/down drafts). Pick a safe altitude. Pull the power to idle, and stabilize the airplane in a 65 kias glide. Record the vertical speed (VS). Repeat at 70, 75, 80,……, 110 kias.
Now go home and plot the VS as a positive number, against KIAS. Draw a smooth curve thru the data. The lowest point of the curve is minimum sink speed..

That calculates best L/d provides the best distance covered with minimum power. I think, min sink rate is really what is needed, but it must be taken with zero prop drag. That is not simple.

If you just fly level flight at various speeds in say 10 Kt steps from 70KIAS to 120KIAS with power adjusted to maintain altitude, then record fuel flow for each speed, the one with the least FF GPH will give you best endurance (time in the air).
 
That calculates best L/d provides the best distance covered with minimum power. I think, min sink rate is really what is needed, but it must be taken with zero prop drag. That is not simple.

If you just fly level flight at various speeds in say 10 Kt steps from 70KIAS to 120KIAS with power adjusted to maintain altitude, then record fuel flow for each speed, the one with the least FF GPH will give you best endurance (time in the air).

I agree completely. Usually max endurance (under power) will be a higher airspeed than max endurance (prop windmilling)(same as minimum sink) not only because of the drag of the windmilling prop, but also because the prop efficiency on the cruise props we all use is increasing with airspeed in this relatively low speed range.
 
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