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!

 

[bubbers44]

flaps 20, bleeds off, set max power and hold on works for the B757 in Tegucigalpa with a 10 knot tailwind and 6100 foot runway in the mountains. Sorry, just remembering the good old days. But it sure was fun.

 

[AC560]

flaps 20, bleeds off, set max power and hold on works for the B757 in Tegucigalpa with a 10 knot tailwind and 6100 foot runway in the mountains. Sorry, just remembering the good old days. But it sure was fun.

Curious about this from some other conversations. My understanding is you get a performance credit for taking off in this fashion and thus can carry more payload. But I always sort of understood the case to be that you would actually get more performance from a rolling start, but because you couldn't dispatch the numbers you did the hold and start for the credit to dispatch.

I can definitely be mistaken in all of this but sure would like somebody to educate me a bit on it.

 

[bubbers44]

We were not allowed to do a rolling start. It had to be a static runup at the end of the runway. To make this take off the highway behind us had to be closed so we didn't blow anyone over. Usually we were right at max take off weight and sometimes had to wait for the wind to decrease to a 10 knot tailwind to be legal because the terrain taking off to the south prohibited a normal weight take off. I loved that airport and is the only thing I really miss being retired. That 757 was the greatest plane I ever flew. It had performance.

 

[Say Again Over]

sometimes had to wait for the wind to decrease to a 10 knot tailwind to be legal

Ya sure ya did, good answer.

 

[bubbers44]

The only highway was behind our tail with the traffic stopped until we took off so the tower got our 10 knot tailwind most every time or we had to taxi off the runway. It was interesting to see what the windsock was doing half way down the runway to see what it really was though.