Short field T/O question

[Murdoughnut]

Which is better (i.e. which uses less runway) in light GA aircraft:

1) Stop at the end of the runway and apply full throttle with the brakes set before rolling out.

2) Turning onto the runway without stopping and going full throttle while keeping forward momentum.

Thanks!

 

[imacdog]

Unless I had to back-taxi to the end of the runway, a rolling takeoff worked fine for me in the 206/7 on shorter runways. It was also more comfortable for the passengers, and there was less chance of the prop picking up rocks. However, most of the short runways I flew off of required a back-taxi, in which case I had to do the power/brakes method.

 

[SPilot]

How fast can you taxi?

Vr = 28m/s (55knots)
Average time it takes for a Cessna or Cherokee to get airborne from Vi = 17 seconds.
a = (Vr - Vi) / t
a = 28m/s / 17s
a = 1.6m/s2

So the rough average accelleration for a small airplane is 1.6m/s2.

Lets assume that the runway you "eat up" by not using the full runway, is about 50 ft (assuming you high speed taxi with a wide turn): the issue becomes: what will your speed be, starting from zero, 15 meters down the runway?
Assume: 15m = Vi*4.33s+0.5*(1.6m/s * 4.33s^2)
Then we get: V = Vi - a*t
V = 0 - +1.6m/s^2 * 4.33s
V = 6.9m/s

So, in this rough calculation, your speed 50ft down the runway will be 13 knots.

If you can taxi faster than that, my best guess would be it would give you more momentum and higher RPM sooner, giving more thrust sooner (fixed pitch).

But I'd also guess this issue is abit more complicated. It takes time for the engines to catch up to full RPM, and if you're on the roll while the engine is coughing, you're wasting runway. By holding the brakes you also get a chance to assure you have full static RPM before you attempt the takeoff.

Why dont you go out and try it?

 

[dumblonde]

If it is for Private Pilot checkride, number 1.

 

[Offwidth]

Aerodynamically, the prop requires a certain amount of forward motion to operate efficiently.

(Try not to drag a wingtip swinging it around with power!)

 

[Turkey Shoot]

How fast can you taxi?

Vr = 28m/s (55knots)
Average time it takes for a Cessna or Cherokee to get airborne from Vi = 17 seconds.
a = (Vr - Vi) / t
a = 28m/s / 17s
a = 1.6m/s2

So the rough average accelleration for a small airplane is 1.6m/s2.

Lets assume that the runway you "eat up" by not using the full runway, is about 50 ft (assuming you high speed taxi with a wide turn): the issue becomes: what will your speed be, starting from zero, 15 meters down the runway?
Assume: 15m = Vi*4.33s+0.5*(1.6m/s * 4.33s^2)
Then we get: V = Vi - a*t
V = 0 - +1.6m/s^2 * 4.33s
V = 6.9m/s

So, in this rough calculation, your speed 50ft down the runway will be 13 knots.

If you can taxi faster than that, my best guess would be it would give you more momentum and higher RPM sooner, giving more thrust sooner (fixed pitch).

But I'd also guess this issue is abit more complicated. It takes time for the engines to catch up to full RPM, and if you're on the roll while the engine is coughing, you're wasting runway. By holding the brakes you also get a chance to assure you have full static RPM before you attempt the takeoff.

Why dont you go out and try it?

Damn.... Took the words and calculations right off my keyboard.

 

[aerodriver69]

If you look in your private pilot text book on how to preform a short field take off it describes #1, as you have indicated here.
1) Stop at the end of the runway and apply full throttle with the brakes set before rolling out.


#2 is a soft field take off technique.
2) Turning onto the runway without stopping and going full throttle while keeping forward momentum.
This maneuver is to keep you from getting stuck in the soft surface.

For a private pilot check ride I would make sure to perform these take offs correctly for your examiner.
After you have that new private pilot certificate in your pocket you can take off however you like.

 

[midlifeflyer]

Part of the reason for holding the brakes at the end of the runway while going to full power is to make sure that you are in fact generating full power before you release the brakes and get going on that too-short runway..

 

[Say Again Over]

Didn't see anything in your question about a checkride, my technique would be Aerodriver's #1 response, not a great technique if gravel is involved though. :beer:

 

[Murdoughnut]

Thanks for the all the responses guys - it's not for a checkride, I was just practicing short/soft fields the other day and was curious which would be the better method. Thanks!