912 Sq Miles!?

[N941WR]

The other day I ran across this post here on the VAF forum:

The CAFE Foundation measured the following glide ratio for the RV-9A [1]:

* Best glide ratio, idle power, coarse pitch, 1738 lb, 95 mph CAS: 12 to 1
* Min. sink rate, idle power, coarse pitch, 1725 lb, 81.7 mph TAS: 664.2 fpm

Using the 12 to 1 Best Glide Ratio I came up with the following:
Cruising along at 8,500 MSL above the 1,000 MSL dirt and you have an engine failure. Assuming the prop stops, providing the lowest drag configuration, you should have 912 square miles in which to set the plane down in.

Here's how my math breaks down:
8500 - 1000 = 7500 feet AGL
7500 * 12 = 90,000 foot radius
90,000 / 5280 (feet in a mile) = 17 miles
3.14 * 17 ^ 2 = 912 Square miles (Pi R Sq - You do remember that formula?)

Using the same math, if you have the same engine failure at 1000 AGL, you still have a 16 theoretical sq mi circle in which to land.

Now, using that 95 MPH Best Glide speed and figuring your decent rate goes up to 700 FPM, from 7500 AGL you will be on the ground in 10.7 minutes. From 1000 AGL it will only take you 1.4 minutes.

Just some more food for thought on this Thanksgiving day.

 

[asav8tor]

No.

It says idle power. As in some power. As opposed to no power. Which would include oil pressure. As opposed to no oil pressure. To change the pitch of the prop. Unlike dead stick. With no ability to change the pitch of the prop.

If the engine quits with a C/S speed prop you are coming down like a manhole cover. Get that mindset now so if it happens you will not be if such a state of shock/denial you have enough brain wave energy to deal with the forced landing.

 

[pierre smith]

Interesting...

......math, Bill. Every time I have a transitioner do an emergency approach to a field, the good glide ratio of these airplanes gets them to overshoot the field. I've done it myself and I remembered Kahuna's advice to go out and practise dead sticks. I now do 180 degree approaches almost every time I land and it gets easier as well. Pull the power all the way off on downwind abeam the numbers and adjust your turn radius in order to make the runway, using flaps as necessary. You'll learn a lot about forced landings and how far these airplanes glide. It'd be a shame to glide so well, land in the field only to plow into the trees at the far end.

Cya,

 

[asav8tor]

The difference between a F/P in idle and a C/S dead stick is night and day.

 

[Mel]

No.

It says idle power. As in some power. As opposed to no power. Which would include oil pressure. As opposed to no oil pressure. To change the pitch of the prop. Unlike dead stick. With no ability to change the pitch of the prop.

As long as the prop is windmilling, you will have oil pressure and control of the prop. Assuming of course that loss of oil pressure was not the cause of your engine failure.

 

[Bryan Wood]

I didn't make it home on my test

One day after going for a ride I decided to try to glide from the reporting point back to the airport to see if I would make it. I cross this point at 3200' and it is 8 miles from the airport. I didn't do any math and still haven't, but if there would have been a real engine failure there would have been a landing in a neighborhood. The throttle was pulled back all the way and the prop to full course which allows the plane to lunge forward noticably. Trimmed to around 90 mph there was a lot of time to plan for which street would make the best field, but not enough to make the actual field. From the way it seemed it would have taken another 1,000' to make it home from where power was pulled.

Also, practicing landing with no throttle is definetely good to do once in a while. With my 9A if power is chopped on downwind at the numbers I have to slip to get it down in time if the prop is full course and no flaps.

 

[TGRV7]

CS vs FP Dead Stick

The difference between a F/P in idle and a C/S dead stick is night and day.


Curious: Which configuration has less drag with a dead engine (CS or FP)?

Thanks

 

[Kahuna]

As long as the prop is windmilling, you will have oil pressure and control of the prop. Assuming of course that loss of oil pressure was not the cause of your engine failure.

Not in my plane Mel. My blue lever does nothing on my windmilling prop unless I an diving over 140kts and the prop is really turning fast. ALso my prop will not cycle on the ground unless my RPM is over 2100. I have a hard time keeping the tail down at that RPM so prop cycle is NOT on my checklist.
I believe it is dependent on the governor type and its related pressure settings.

Best,