Isn't it ....

"TT - any scientific basis for the assumed difference in coefficient of friction between asphalt and grass? Any factor applied for the lentgh of the grass? Or the temperatures of the surfaces? Runways and roads seem to be designed to have a high coefficient of friction rather than a low one."


Well Crab, the short answer to that is no.


Mind you, I have been attempting to get a bit of project off the ground that may shed a bit of light on the matter and have approached Wimbledon and Lords about using their well-mown surfaces to measrue some solid run-ons. Still waiting for a reply there. But I have had an intruiging offer from Aintree saying I could take part in an event involving a variety of terrain, namely grass; asphalt; hedge row(!) and even water. To be honest, I got the impression that someone was sending me on a bit of a wild steeple chase.


In the meantine, I will have to concede that I haven't got a jot of evidence. Somewhere in there is the thinking (righlty or wrongly) that above grass the laminar flow will be more disturbed, and that even if tarmac is the higher friction surface, this "stickiness" will occur very close to the surface allowing a more laminar flow to develop almost immedeatly above.


Point taken about temperature differences. Late morining with a baking hot sun on the asphalt there might be some pretty thin air above the runway. Maybe that adjacent grass strip in the shade deserves a second look after all.


TT

TT, if you would like the ultimate in large, smooth and firm grass areas to test run-ons, I suggest you contact the greenkeeper at the Old Course, Royal and Ancient Golf Club of St Andrews! :)

Ta John!


Letter in the post.


I see it all unfolding clearly: the high speed run-on; the "favourable" gradient; that ever-so-low drag coeffient: "Is it a birdy? is it a plane?


TT

TT - having done some interesting run-ons teaching student instructors on EOLs- - I can tell you that when going from grass to tarmac and back to grass again (that was the runway crossing) the tarmac was definitely the grippiest on the skids:ok: .......I think we are going lonnnnnnggggggg...

Maybe a bunker with some sand might have been useful.............engine off golf - now there's an idea;)

Well, you know what they say about horses and water. :)

Crab
Sorry it's late as on holiday in South Africa but and it is a big but you certainly know if the donkey has given up in a piston engine as the machine has yawed by about 45 to 90 degrees in a split second ! Seeing as many are taught on Pistons there reaction is qued by this rapid yawning with punching the lever down. So your assertion that they don't know what is going on is false. Now in a turbine that would be true. I do throttle chops in both types, all pilots react quicker in a piston because of its reaction probably just as well as most don't have a high inertia head. In a 300 at 60 it's , auto speed 48 kts snap the throttle and the rrpm is off the green with a yaw of about 45 degrees. By the time the lever is down.Thankfully this type really drops its nose with the lever down so easy to restore rrpm. Part of the lesson though is to always try and fly above best auto speed so you have energy to play with. Back to the pool now. Although my only engine failure was at 80 ft and about 35 knots,just bent the rear cross beam

Hughes - hope you enjoy your hols:ok:

Do you ever do throttle chops when the student is in the middle of changing a frequency or squawk or has a map in his hand?

That is a real-world scenario rather than a pre-meditated training one and the reaction will be a lot slower.

Hughes500,

Don't get lulled into turbines being slow to decelerate in a real engine failure.

Closing the throttle on a serviceable turbine when it is running is only showing you the FCU decal rate that it can achieve without flaming out!!

When they stop for real its a bit more interesting!

PS How many of you RR A250 operators do regular decal checks just to be sure?

RVDT

Bit difficult to actually do anything else in training as I am sure if engine seized the result would be the same as a piston ! Never had the turbine donkey go in flight had one shut down as we pulled pitch to go to hover but again it was a run down as there was contamination in fcu filter.
Crab you are quite right do try and chop throttle at inconvenient times. Only once stopped the engine for real. In a 300 have to hold the throttle against the "Spring" otherwise as you pull full lever at bottom the correlation gives you about 2200 rpm.Student did a throttle chop as pre take off check all was ok until I throttle chopped it over the airfield downwind at 500 ft to find to my horror the engine stopped. The student very Cooley said you have control !big lesson to instructor try it from your side as well !
As for the holiday now in Addo, just love walking in to see elephants, and be Jesus was the big bull big, puts a new meaning on things when on foot