Aviation brain teasers

[3WE]

Are you sure about that Brian?

Newton says energy must be conserved and the energy of moving is m*v^2.

To make numbers simpler- you're flying east at 5 mph in a 5 MPH north wind in a 1 kg airplane...You have 25 perverted newton units of energy.

Turn south, you have 100 PNU's where'd that energy come from?

Newton cannot be wrong!

[brianw999]

Buggered if I know ! I'm only quoting those who should know. !!

To my simple mind...... You are flying towards the east in a block of air. That block of air is travelling over the ground at 20 knots towards the south. You make the turn to the south. The only speed that changes is your ground speed and no aircraft ever stalled as a result of a change in groundspeed.
Now, I've seen a Tiger Moth flying backwards relative to the ground when the pilot turned into a headwind without stalling so it follows that turning into a tailwind won't affect the aircrafts' flying capability.

So.... To answer the question. No, it won't stall because of the wind....

... Provided nothing changes to alter the characteristics of the wind during the turn and it remains a block of air travelling at 20 knots in a southerly direction.

A stall brought about by an excessively steep turn is however an entirely different dragon !

[3WE]

You are no fun Brian so one final question.

Does anything...anything at all...tell you intuitively that since you have picked up a tail wind, that was not there before that you should have a tendency to loose airspeed as you pick up that tail wind. Don't bring math in to the discussion...can you FEEL what I'm talking about.

You turn into a tailwind. My mind tells me that you really should be losing some airspeed!

[brianw999]

>3WE,
Intuition certainly says that you should lose some speed and yes, you do lose speed.....but it's groundspeed, not airspeed.

And don't worry, it doesn't "feel" right to me either !

[3WE]

Well Gabe...Brian just totally ruined this attempt by answering most everything correctly- INCLUDING pointing out some reasons why it's dangerous to turn downwind.

But there's one unaddressed question...do you think someone is turning in their grave?

Ok there's one other unaddressed question- what are some true reasons why it can be dangerous to turn downwind...

1) Turning (especially turning steeply) increases wing loading and tends to decrease airspeed.

2)???

3)???

[Gabriel]

Are you sure about that Brian?

Newton says energy must be conserved and the energy of moving is m*v^2.

To make numbers simpler- you're flying east at 5 mph in a 5 MPH north wind in a 1 kg airplane...You have 25 perverted newton units of energy.

Turn south, you have 100 PNU's where'd that energy come from?

Newton cannot be wrong!

I beg you pardon. When you say a wind of 5 MPH, I understand that 5 MPH is the speed of the air about the ground (that's the traditional definition of wind).

But when you say that you're flying at 5 MPH, that's the speed of the plane about what?

Because I do the math and, if I am flying at 5 MPH before the turn and at 5 MPH after the turn, then my energy didn't change a hair.

[Gabriel]

Well Gabe...Brian just totally ruined this attempt by answering most everything correctly

Most, yes, because this part is wrong:

yes, you do lose speed.....but it's groundspeed

In fact, if you turn downwind of course you gain groundspeed. Is that effect of thinking that, since groundspeed increased, that's because of the tailwind "pushing" from behind what, I thin, intuitively seems to be loosing airspeed.

Ok there's one other unaddressed question- what are some true reasons why it can be dangerous to turn downwind...

1) Turning (especially turning steeply) increases wing loading and tends to decrease airspeed.

2)???

3)???

Ok,

1) Turning downwind increases the chances of stalling because it's still a turn, and in any turn:

- The load factor increases, and hence the stall speed increases too.
- The load factor increases, and hence the induced drag increases too, what will make your plane loose airspeed if the pilot fails to compensate adding more power.

2) After after take off, turning downwind may increase the risk of stall because you'll likely be climbing, and since the wind speed tends to increase with altitude, especially when close to the ground, you'd be climbing into an increasing downwind (so it's not the block of air that Brian described, but rather layers of air that move one about the other) and that will make you tend to loose airspeed if not compensated by the pilot by reducing the climb rate (I'm assuming he already is at take-off power). But that's called climbing downwind. Turning or not is irrelevant for this effect.
3) Turning downwind in a gusting wind increase your chances of stalling, (compared to no gusts), but that's because a gust that suddenly increases the tailwind will make you loose airspeed. But that is called flying in gusting tailwind. Turning or not is irrelevant for this effect. Moreover, some (including myself) argue that that's in fact called flying in gusting wind, and that whether it's a tailwind or headwind is irrelevant, because gusts happen in both ways. The wind can suddenly increase or it can suddenly diminish. Flying with a headwind that suddenly diminishes 20 kts is the same (for the airspeed) than flying with a tailwind that suddenly increases 20 kts.
4) Turning downwind when flying VFR (especially low) and paying too much attention to the landscape and too little attention to the instruments and other cues (like stick force) can make the pilot feel the visual perception that the plane is accelerating (because the groundspeed is increasing) and intuitively "correct" by slowing down, which will cause a loss in airspeed.
5) Turning from base to final with a tail wind in the base leg will make the turn radius larger and the turn longer, so if that's not anticipated, the plane will tend to overshoot the runway's extended centerline. If the pilot tries to correct by steepening the turn, this can increase the risk of a stall for the reason 1.

Mmmmm. Maybe turning downwind is dangerous. The FAA should pass a new FAR mandating that downwind turns are prohibited.

[3WE]

I beg you pardon. When you say a wind of 5 MPH, I understand that 5 MPH is the speed of the air about the ground (that's the traditional definition of wind).

But when you say that you're flying at 5 MPH, that's the speed of the plane about what?

Because I do the math and, if I am flying at 5 MPH before the turn and at 5 MPH after the turn, then my energy didn't change a hair.

Don't play stupid Gabriel...you know that calculations and assumptions like this almost drove a certain genteman nuts...

I salute Brian for being able to say "it sure SEEMS like you should lose some airspeed"...

It SEEMS it so strongly that the elder mechanic was convinced that his calculations proved it...

[brianw999]

My bad, I should of course have said that when you turn downwind you gain groundspeed. One thing the wind is not doing though is push the aircraft. The only time the wind can do that is if the aircraft is on the ground. When the aircraft is airborne it is in the mass of air. It's airspeed is the speed it travels through that block of air.
People seem to me to be mixing up air and ground speed.
An analogy for you.....

I can run at 20 mph. I am in a train called "The block of air" that is travelling at 100 mph. I run down the corridor toward the front of the train at 20mph. I am doing 20mph in the train called "The block of air".
The train called "The block of air" is doing 100mph over the ground and I am doing 120 mph over the ground within the train called "The block of air"

[Gabriel]

An analogy for you.....

I can run at 20 mph. I am in a train called "The block of air" that is travelling at 100 mph. I run down the corridor toward the front of the train at 20mph. I am doing 20mph in the train called "The block of air".
The train called "The block of air" is doing 100mph over the ground and I am doing 120 mph over the ground within the train called "The block of air"

Another analogy.

Imagine a small balsa plane flying at 5 MPH and turning in circles inside a 747. Would it make any difference if the 747 is parked at the gate or flying at 500 kts with all and the "block of air" inside that for the little balsa plane would be a 500 MPH wind?

[Gabriel]

By the way Brian, 3WE would not say it, but it's your turn.

[brianw999]

A pilot takes off from a local GA airfield. His aircrafts take off speed and landing speed are the same at 70 knots. 70 knots is also the speed at which the flaps are raised and lowered. As he climbs out he goes through his post take off checklist....
1/ Airspeed 70 knots.
2/ Flaps up.
3/ Brakes on.
4/ Gear up.

He continues an uneventful flight until he is on final approach to his destination. He goes through his pre-landing checklist....
1/ Airspeed 70 knots.
2/ Flaps down.
3/ Gear down.
4/ Brakes off.

The approach continues normally but just as he touches down there is a terrible shuddering and rending of metal. The aircraft breaks up and he is killed in the crash.

What did he do wrong to cause this terrible accident.

[3WE]

Are we allowed to Parlour talk / major speculate???

After takeoff he was a bit off from 'tradition'...normally wheels come up first, but he brought the flaps up first, and then oversped with respect to the gear stuff....

So that they did not deploy properly....

....assuming that the gear down and safely locked system somehow ALSO malfunctioned.....

Ummm....errrrrr....never mind.....

I could also play the Evan card- I need to know the make and model of the aircraft to know the particular procedures (to hell with broad, fundamental airmanship concepts).

[brianw999]

Don't get too technical. Yes, you would normally raise the gear first, that was my mistake.
I can say that there was absolutely no malfunction of any aircraft system at any time prior to the crash.

[Petertenthije]

Did he hit a sinkhole? They seem to be all the rage lately.

Failing that, turning the brakes on would be a nice idea.