CFI myths..Special VFR day/night et al

[Sctt@NJA]

Both sides are right on this.

1. The airplane doesn't care if it is in still air or moving (as long as its a steady-state wind)

2. Downwind turns ARE dangerous (but only when close to the ground)

Concerning point 1: The inertia type arguments don't pan out. Try doing a 180 in 100kt plus winds and you will see- the airplane doesn't care.

Concerning point 2: Its simply a matter of climb gradient. The RATE of climb will be the same with a headwind or tailwind. The ANGLE of climb relative to the ground will change. (clearly a poor ANGLE of climb is what is hazardous when you need to clear obstacles)

So low-level downwind turns are more dangerous than upwind turns due to the increased groundspeed which causes a decreased climb angle (remember angle of climb is the aircrafts ability to climb over distance relative to the ground) and thus reduced performance in regards to clearing obstacles.

Note: When I say "low-level" I mean low enough that nearby obstacles are a concern. 1000 feet over prairy land doesn't count. 10 feet with a tree line ahead does.

I would speculate that the other phenomina observed by some pilots (eg. incleased RATE of climb when turning upwind) are an illusion based on groundspeed and angle of climb.

The same maybe for those who say you lose airspeed... if you were doing a low level climbing turn to an increasing tailwind component the decreasing climb angle (due to increasing groundspeed) would cause the pilot to compensate and get their climb angle back by raising the nose, thus losing airspeed. The proximity of the ground is providing a reference of climb angle that doesn't exist higher up.

 

[bigD]

Does anyone else have a decent explanation for this? I am very interested to see someone else's analysis.

SkyWest's explanation about the downforce is right on. Just remember that air is a fluid in many of the same ways that water is. If you had a fishbowl full of water, and started adding fish to it, you'd kind of expect the weight of the bowl to increase as you add the fish, right? The weight doesn't just suddenly disappear because the fish are swimming around in the water. It's the same kind of thing when dealing with airplanes, birds, or whatever flying around in a closed vessel filled with air.

 

[100LL... Again!]

How 'bout we get together next time its blowing about 30-40 knots. I'll take off and do a rapid downwind turn while maintaining airspeed. If the downwind turn theorem is true, I would have to desend radically to maintain airspeed.

$100 bet. (I'll feel bad taking your money).

 

[bigD]

Wait, a $100 bet that you'll have to radically decend to maintain airspeed?

You're on!

 

[Sctt@NJA]

You wont have to descend to maintain airspeed. But if you need to clear obstacles you would be better off turning upwind.

See? The hazardous downwind turn "myth" is true. Its just the reason for it thats misunderstood.

Fact: For a given pitch and powersetting the airplane will maintain the same airspeed and rate of climb regardless of turning upwind or downwind.

Fact: That same airplane while turning downwind will have a greater groundspeed and lesser angle of climb, so its climb perfomance relative to the ground will be worse.

The people saying the downwind turn is a "myth" are correct in the sense that rate of climb and airspeed are not affected by a steady state wind. They are wrong however in thinking that a low-level downwind turn is not dangerous. Its all about ANGLE of climb. (How high you can climb for a given distance over the ground)

 

[100LL... Again!]

Scott-

Check out my post again, I was in agreement with what you said.

I was saying that IF the downwind turn myth was true, then I would have two choices; attempt to hold altitude and lose arspeed, or maintain airspeed and lose altitude.

I agree with your point about the climb gradient. If you check the previous page, I mentioned it back there.

What I think happens insome instances is that during the downwind turn the pilot subconsciously notices the increased groundspeed, pitches up to 'compensate' for what he perceives as excess speed, and thusly loses speed. In this instance there is a speed loss, but it is self inflicted - not the effect of a changing wind component.

Life is full of optical illusions, and some are very convincing.

For example, I actually thought I would get paid actual money to fly airplanes, but it was an illusion for years.

 

[Sctt@NJA]

Yes, I do believe we are on the same page

 

[avbug]

It goes way beyond an optical illousion. That's part and parcel for the problems that many pilots encounter during a downwind turn, but far too many times even this summer I've encountered the "mythical" downwind turn at work.

Ignoring the ground, a hard turn downwind very often will see a marked airspeed decrease especially when heavy. A hard turn into the wind will see the opposite.

I fully comprehend the physics and aerodynamics and reason. I also know what happens in reality, and the truth is that the fabled downwind turn IS reality.

Most ag pilots don't give this a second thought. We know.

Take an airplane already loaded to the gills, headed on a direct crosswind. Fly it close to minimum airspeed, so there's no margin. Haul it hard downwind, and see what happens. Haul it hard upwind and see what happens. Do that day in and day out, thirty times an hour for eight hours a day, and you'll find religion when it comes to doing downwind turns.

Pilots do get caught by the "illousion." But for those who live in the environment, doing nothing but critical turns in wind all day and see the effects, do you really think we're that stupid? Uh-uh. We know.

I fully appreciate the science. I do. But then I've taken learned engineers into the shop to show them how their unbreakable design snaps off...and hotshot pilots and aerodynamicists into places whence their only reply was first to throw up, and then say, "oh."

Are downwind turns a reality, and the attendant airspeed with them not a myth? Yes, they can be, and are. It is, as they say, where the rubber meets the road.

 

[bigD]

Avbug - I've said this before in an earlier conversation we've had about this same subject - from a physics standpoint I find what you say hard to believe, but I also respect your experience and I'm certainly not calling you a liar, so there must be some explanation that fits your observations into existing physical laws.

Problem is, the owner of the flight school I rent from would likely frown upon me operating the Duchess in the kinds of conditions that you describe. So I can't verify what you say for myself. And the physicist in me NEEDS this verification.

So can I ride along with you? I know it's dangerous, but I'll sign a waiver absolving anyone from responsibility if I get killed!

 

[avbug]

Hop in, but bring a sick sack.

And a helmet. Right at the moment I have just one seat, and you'll be riding in the hopper. No windows and you can't see the instruments, but I'll tell you what happened.

In my day we didn't have windows. And we were grateful. (now I have to close my eyes, instead).