RV9 best glide and ratio

[ALagonia]

I?m using 90 mph for Vg, best glide speed and 12:1 for glide ratio in setting up the function on ForeFlight. I have an O-360 with three blade Catto prop on my 9. Does this sound right?

[Bevan]

What does your POH say? Don?t have one? Go test. These are not ?type? certified airplanes.

Bevan

[Steve Iacoviello]

Originally Posted by Bevan

What does your POH say? Don?t have one? Go test. These are not ?type? certified airplanes.

Bevan

I would not have expected much difference in RV-9 airframes. Maybe indicated airspeed errors between airframes?

For my 9A I settled on 80kts. I am interested in what others have determined.

[rjcthree]

I set my engine for a lower than stable ground running idle, like maybe a really rough 450rpm, and during those tests it stayed above 600rpm per my notes. 80 knots indicated was my number once the data was crunched, but the bucket was pretty flat with not much difference 75-82 knots indicated. At 85kt it starts to brick. At 10 deg of flaps 75kt is my number. I use 10-1 as my number for glide ratio, it makes the math easier once the fan is distributing.

The data was taken between 1470 lbs and 1540 lbs, me, a little baggage and mostly full fuel.

[BobTurner]

Don’t forget best glide speed varies with weight...

[RV8JD]

Originally Posted by BobTurner

Don’t forget best glide speed varies with weight...

The plot below illustrates what Bob said. It is for a sailplane (hence the low airspeeds and sink rates), but the idea is the same. The heavier the aircraft (i.e., higher wing loading), the lower and farther to the right the polar is for that aircraft. The Best Glide Speed is the at the line's tangent point to the polar from the origin (0,0) of the plot. The Best Glide Speed is higher at higher weights, and vice versa. The plot below is for still air.

The plot indicates that the Maximum Glide Ratio of the sailplane is approximately 21-to-1 (very low by today's sailplane standards).

Not shown on the plots, but a horizontal line drawn tangent to the polars would show the Minimum Sink Rate and the Airspeed at which it occurs (the Minimum Sink Speed). The lighter aircraft has a lower sink rate than the same aircraft at a heavier weight.

Note that the Minimum Sink Speed is less than the Best Glide Speed at a given weight.

Flying at Best Glide Speed in still air maximizes the distance flown, while flying at Minimum Sink Speed maximizes the time in the air.

The plot illustrates why sailplanes carry water ballast when the lift conditions are strong; the sailplane achieves the same glide ratio but at a higher speed and that increases their average cross country speeds. When the lift gets weak, the water ballast can be dumped and the Minimum Sink Rate is reduced so weak lift can yield a climb rate.

[Ranger]

https://cafe.foundation/v2/pdf_cafe_apr/rv-9a.pdf

[scottmillhouse]

Test it for your POH. My 9 had a Hartzelle constant speed and with power pulled off it went flat creating a lot of drag. Best speed was 80 mph at about 9:1. With an earlier over pitched cruise fixed pitch prop it was closer to 13:1.

[sailvi767]

I tried to find the best glide in the Rocket. It doesn?t exist! Calling it a brick is generous!
G

[sbalmos]

I did some decent experimenting a few weekends ago with my 9a during my flight review with a local 7a driver. I have a 150hp IO-320-E1A and a 3 blade Catto. We ended up agreeing on 70-75kts at just shy of 800fpm loss. Way better than his 7a brick.