Short Field Takeoffs

[dmccormack]

Correct, but I do enter the runway with speed - Dan made the 20kt to 3kt comment so I used that as my entering speed since he used that number earlier.

I tossed those numbers out to compare the radius of a turn -- I can park a car in a much tighter space at 3 MPH than I can at 20.

 

[EdFred]

I tossed those numbers out to compare the radius of a turn -- I can park a car in a much tighter space at 3 MPH than I can at 20.

I drive like Bo and Luke Duke.

Like I said, the 20 was only used because you threw it out there. I do know that I taxi normally around 15kts.

 

[mantakos]

... Dan made the 20kt to 3kt comment so I used that as my entering speed ...

Which one? 20kt or 3kt?
-harry

 

[EdFred]

Which one? 20kt or 3kt?
-harry

Entering at 20kts results in 11% less takeoff roll.
Entering at 15kts results in 6% less takeoff roll.

I used 20 for the calculations shown.

 

[Dan Thomas]

What extensive testing did you do without making abrupt power changes, shows that turns at high RPM affect the engine negatively?

Oh, let's have a few references:
http://www.airspacemag.com/flight-today/Tumbling-with-the-Stars.html?c=y&page=2

http://www.hartzellprop.com/pdfs/KitplaneBroch.pdf

http://www.lycoming.com/support/publications/service-instructions/pdfs/SI1009AT.pdf

This last one is Lycomings TBO requirements, and you'll see that AE (aerobatic) engines have a shorter life.

I addition, as a former flight instructor, I taught taildragger fliying. There are four forces involved in pulling the airplane to the left on takeoff:
1. Propeller gyroscopic force: When the tail is raised, the propeller precession forces the nose left. This is strong in an airplane with a large, heavy prop and the pilot had better be ready for it.
2. Propeller torque reaction; foces the left wheel harder against the surface and increases surface friction on that side.
3. Slipstream effects, causing the fin to be pushed to the right, turning the airplane left.
4. Assymetric thrust (P-factor), where the right side of the prop disc has a higher AoA in the three-point attitude and pulls a little harder.

And then we have steep turns; when turning, the prop's gyroscopic forces will pull the nose up or down. Since precession acts on the disc at 90 degrees to the applied force in the direction of rotation, a left turn will try to raise the nose and a right turn will pull it down. As the rate of turn increases, the differential increases. Much more elevator back pressure is needed in the RH turn than in the left.

From a previous post, you implied that you were a chemist and marvelled at how often you had to correct someone with less education in the subject. Are you also a flight instructor or aeronautical engineer?

Dan

 

[mantakos]

Entering at 20kts results in 11% less takeoff roll.
Entering at 15kts results in 6% less takeoff roll.

Okay. I came up with an advantage of 37' at 15kts, and 66' at 20kts, which seems fairly in sync with your numbers.

So by wheeling around at 20kts in a fairly tight circle (which I'm not certain is even possible), and beginning our takeoff roll from off-center, and rolling onto the runway at 20kts, possibly significantly reducing our opportunity to scan for oncoming traffic, for that I get an extra 66'?
-harry

 

[EdFred]

The first article does not provide evidence that the crankshafts broke solely because of gyroscopic forces. Unless they were of course flying the entire routine at the same MP/RPM for the entire time, which we both know they weren't.

I have no idea what you are pointing out in the second link.

In the Lycoming linkunder number 6 , it does mention repeatedly operating at alternating high and low power settings, and overspeeding the engine, but absolutely nothing about gyro forces.

Again, do you have anything that points out the shortening of an engine life due SOLELY to gyroscopic forces?

I am not a chemist, and I do not have degree in chemistry, which is why I find it ridiculous that I have to give chemistry lessons to 4-plus year engineering graduates. I was marvelling at how much I had to teach someone with MORE classroom education in the subject field.

While your points of gyroscopic forces 1 - 4 are correct, they really have no bearing on your argument of turning at the beginning of the takeoff roll will cause my crankshaft to fail.

I am currently a flight instructor.

 

[EdFred]

Okay. I came up with an advantage of 37' at 15kts, and 66' at 20kts, which seems fairly in sync with your numbers.

So by wheeling around at 20kts in a fairly tight circle (which I'm not certain is even possible), and beginning our takeoff roll from off-center, and rolling onto the runway at 20kts, possibly significantly reducing our opportunity to scan for oncoming traffic, for that I get an extra 66'?
-harry

Traffic is 6 or 1/2 dozen. In stopping and holding the brakes, you can't see the traffic that comes around the trees behind you. But like I said, this is paper physics, and on the grass fields I've used this technique on, I can tell the rate of acceleration is not constant, and is less at the beginning of the take off roll (below the curve) from a stop.

When the snow melts, I will retest and post the results.

 

[dmccormack]

Okay. I came up with an advantage of 37' at 15kts, and 66' at 20kts, which seems fairly in sync with your numbers.

So by wheeling around at 20kts in a fairly tight circle (which I'm not certain is even possible), and beginning our takeoff roll from off-center, and rolling onto the runway at 20kts, possibly significantly reducing our opportunity to scan for oncoming traffic, for that I get an extra 66'?
-harry

The advantage assumes absolute use of all available runway.

Unlikely at 20 MPH.

 

[EdFred]

The advantage assumes absolute use of all available runway.

Unlikely at 20 MPH.

No, it doesn't. Read my giant post.

Assuming the same acceleration rate (same engine, same plane, same weight) It will take me 2.78seconds to drive the arc - 80ft radius, and I will have a velocity of 56.8fps at the end of the arc, and I'll even give you 20 foot of wasted runway and call it 100' from the end.

 

[gismo]

You're doing 40 MPH at the end of the arc?

I can pretty much guarantee that the speed is nowhere near 40 mph at the end of an 80 ft radius turn in an airplane on grass because this implies about 1.3 g of centripetal force. Even if you were willing to load the gear that hard, your tires aren't going to grip the grass for more than about 0.25 g under the best conditions and that dictates that your speed at the end of a constant 80 ft radius turn cannot exceed about 25 ft/sec or 17 mph.

You could improve on this by making the turn with constantly increasing radius and having done a few of these turning takeoffs I can attest that an increasing radius is the norm. But IIRC the improvement in speed would only be about 25% taking you to 21 or 22 mph and at the same time you'd have traveled further down the runway by the time you were done turning so the actual gain compared to a non-turning takeoff would be modest. And that's ignoring the fact that you are "wasting" a lot of energy making a high speed turn due to the friction and side loads on the tires. In reality your acceleration will be significantly less in a max performance turn than when going straight ahead although I think the difference decreases with an increase in power.

 

[dmccormack]

I can pretty much guarantee that the speed is nowhere near 40 mph at the end of an 80 ft radius turn in an airplane on grass because this implies about 1.3 g of centripetal force. Even if you were willing to load the gear that hard, your tires aren't going to grip the grass for more than about 0.25 g under the best conditions and that dictates that your speed at the end of a constant 80 ft radius turn cannot exceed about 25 ft/sec or 17 mph.

That would be more realistic. But I can assure you I'm nowhere good enough to do that in a castering tailwheel airplane.

 

[gismo]

That would be more realistic. But I can assure you I'm nowhere good enough to do that in a castering tailwheel airplane.

The only times I've tried a turning takeoff in a taildragger I made sure I had plenty of room because I coudn't be absolutely sure it would go where I intended.

 

[brcase]

I will be at a higher speed 100' down the runway using a rolling start, than I would be from a dead stop. What does being on centerline have anything to do with using the least amount of runway length?

The photo of a friend of mine in a DC3 with the wheels outside the edges of the pavement on his private strip comes to mind. Only the tailwheel would have been on the pavement if he hadn't had it in the air already in the photo.

Sorry for the distraction, now back to your regularly scheduled thread topic.