Forces in the air

[FenderManDan]

Can sombody explain what is a difference between Bob Hoover's 8 point roll and not spilling his ice t and slight turbulence that allways spills my coffee on a commercial flight. It is kinda weird, however learning what is going on inthe air as far as forces go

Cheers Dan

Edit: if the above does not make any sense, no worries i am in the bed with the nasty clown, i mean cold (damn auto correct) and without meds it just sucks

[lownslow]

Bob Hoover is smooth and coordinated and is in full control of his roll because it is something he wants to do. I also don't think it was a point roll that he did while pouring tea.

The best you can do in turbulence is react to the surprise movements of the airplane. Not smooth, not coordinated, and each movement is unplanned.

Get well soon.

LnS.

[old_man]

Pretty sure that if Bob Hoover was doing a roll and he hit some turbulance...he to would spill his drink.

[trey kule]

I am pretty certain, that soon you will get graphs, charts and pictures from some posters, but I like to keep it simple...

bob hoover keeps the plane in a positive g situation throughout the roll...thus everything acts as you are used to it. Same as if the plane was flying upright, straight and level in unaccelerated flight.
When you hit turbulence in the airliner you get a bit of negative g and violent change...your coffee reacts to the negative g by first seeking the safety of the ceiling and then when it goes positive again, your lap...

BTW...bob does it so smoothly that you dont see the water or coffee flow being interrupted at all.

Hope that simple explanation helps.

[cgzro]

In positive G flight, the water spills because of sudden changes in the G not the actual value of the positive G being experienced. Since both Bobs roll and straight and level flight are positive G flight, there is no difference in the effect of turbulence and both would experience sloshing and spilling in turbulence.

So Bob did not spill water because a) there was no turbulene and b) he superbly kept the G positive the entire way around.

[cgzro]

Here is perhas an even simpler way to think of it.

Take a bucket filled with water and attach a rope to the handle.
Hold rope in hand and don't swing. No water spills.
Now jerk rope up down a bit .. water spills.
Now spin the bucket smoothly around your head, no water spills.
Now, while spinning it, jerk the rope a bit ... water spills.

As long as the accelleration is in the right direction no water will spill, but if you introduce sudden changes in that accelleration you get slosh and spillage.

[FenderManDan]

Thanks guys, that is awesome. CGRZO the bucket example is exactly what triggered me to look back at the school days and refresh on some physics. Next step is to compare forces that are acting upon the roller coaster and a similarly executed aerobatic manouvers.

[cgzro]

Not a huge fan of roller coasters. I don't trust the operators ..

Concerning G forces .. there are to my knowledge only two ways to create them.

a) a bloody huge mass that pulls you toward it, i.e. Earth/Sun/Black Hole .. gravity.

b) having your velocity (i.e. speed and or direction of motion) changed.

To figure out what G you experience at any given time, you have to add the contribution from a) to the contribution from b).

So if you are in deep space then its mostly b) that you'll feel, and sitting in your lazy boy on earth its mosly a) that you will feel.

Roller coaster, aerobatic plane, or even an ultra high speed train in a lovely banked turn .. all the same. Mostly the difference is in the smoothness of the ride and the maximum G. The Train would give you 1 + a "noticible bit" when it banks around a turn at 300Kph, the roller coaster probalbly +4 perhaps, especially those that loop. And an aerobatic plane .. well more $ = more G. By my calculations you get about +/-6G per $100,000 .. so yes .. those 1/2 million dollar things can pull some outrageous G's .. so much so that the weak area now are the cast points on the engine where the motor mounts attach.

One of the reasons my aircraft has registration C-GZRO is because I love that zero G sensation .. and because C-GBUM , C-GSPT and a few other less politically correct registrations were taken

[Colonel Sanders]

What you feel is simple vector summation.

There is always going to be one G pulling you to the center of the earth.

Plus what the aircraft does. The first thing that you have to realize is that pitch attitude is no longer a proxy for angle of attack or what you're going to feel - indicated G on the accelerometer.

For example, you can be inverted an pulling lots of positive G.

You can be straight and level and pushing enormous negative G, right on the edge of a negative AOA stall.

You can be vertically straight up with a zero AOA.

I think all of the above are pretty neat, even if they upset straight and level pilots, whom rarely exceed 30 degrees of bank.

[ch135146]

I think all of the above are pretty neat, even if they upset straight and level pilots, whom rarely exceed 30 degrees of bank.

[FenderManDan]

Plus what the aircraft does. The first thing that you have to realize is that pitch attitude is no longer a proxy for angle of attack or what you're going to feel - indicated G on the accelerometer.

I am still in the fixed pitch prop world so I believe in IAS as the AoA indicator at the low altitudes that I fly in. I don't know if that that is right to go about but seems accurate .

For example, you can be inverted an pulling lots of positive G.

I am having a hard time imagining the attitude of that. Is that the exact opposite of the straight and level with negative AoA with tail wanting to flip over you?

I think all of the above are pretty neat, even if they upset straight and level pilots, whom rarely exceed 30 degrees of bank.

I guess you agree with Bob H. that taking off without immediately performing a roll or a loop is not a normal t/o. I never met Bob or seen him fly live, just watched some VHS in the past and YT videos of current and I wonder if he knows how to fly straight and level. JK His flying skills really inspire me even though I am not planning to do aerobatics (yet).

[FenderManDan]

Not a huge fan of roller coasters. I don't trust the operators ..

Concerning G forces .. there are to my knowledge only two ways to create them.

a) a bloody huge mass that pulls you toward it, i.e. Earth/Sun/Black Hole .. gravity.

b) having your velocity (i.e. speed and or direction of motion) changed.

To figure out what G you experience at any given time, you have to add the contribution from a) to the contribution from b).

So if you are in deep space then its mostly b) that you'll feel, and sitting in your lazy boy on earth its mosly a) that you will feel.

Roller coaster, aerobatic plane, or even an ultra high speed train in a lovely banked turn .. all the same. Mostly the difference is in the smoothness of the ride and the maximum G. The Train would give you 1 + a "noticible bit" when it banks around a turn at 300Kph, the roller coaster probalbly +4 perhaps, especially those that loop. And an aerobatic plane .. well more $ = more G. By my calculations you get about +/-6G per $100,000 .. so yes .. those 1/2 million dollar things can pull some outrageous G's .. so much so that the weak area now are the cast points on the engine where the motor mounts attach.

One of the reasons my aircraft has registration C-GZRO is because I love that zero G sensation .. and because C-GBUM , C-GSPT and a few other less politically correct registrations were taken

I have seen some of your YTs, beautiful plane and videos btw. Have you tried a HD GoPro camera?

[Colonel Sanders]

Re: inverted with lots of positive G

I'm glad you're starting to think about it! Don't do this but ...

From cruise flight, half roll inverted and pull the stick or
Control column all the way back, to your belly button.

You will be upside down, pulling enormous positive G
As you complete a half inside loop downwards. Don't
Be surprised if you see 10 or 12 G's on the meter. Most
Pilots and airplanes don't like that.

Yes, a normal takeoff for me is a half roll. And I like
To land out of a surface loop or half reverse cuban 8.
Beats tennis.

BTW, Bob Hoover is one heluva nice guy - go to OSH
And meet him sometime.

PS pardon the spelling - I am on my blackberry in part
Of the world that does not have great internet.

[trampbike]

For example, you can be inverted an pulling lots of positive G.

I am having a hard time imagining the attitude of that.

Top of a tight loop, the upper corners of of square loop and a tight barrel roll come to mind.

Any curved flight path with a short positive radius will make you feel a lot "G forces", or more accurately, it will accelerate you toward the center of the curve, hence pushing you toward the exterior of the turn. As CS said, doesn't matter all that much where the earth is, it's only adding 1G toward it's center, no matter what is your attitude in relation to it.

If you want to know what acceleration, or G force that will be induced by a turn (remember thought that this acceleration is oriented toward the center of the turn, not necessarily toward the floor or the top of the airplane), the formula is pretty simple: Acceleration = (speed^2)/radius
For each 9.8 meters/second^2 of acceleration, it's one more G.

[Beefitarian]

I am pretty certain, that soon you will get graphs, charts and pictures from some posters, but I like to keep it simple...

Hello!?! I'm waiting for charts and graphs here people.

[cgzro]

I have seen some of your YTs, beautiful plane and videos btw. Have you tried a HD GoPro camera?

I've been meaning too. My Samsung died a while back .. too much G I suppose

They look awesome. I'll probably get one at some point.

Hello!?! I'm waiting for charts and graphs here people.

Careful what you wish for

Peter

[trampbike]

EDIT: allright then, here is a graph, but it is simple.

Ever wondered why during a constant altitude turn of 45 degrees of bank you experienced 2^0.5 G, 60 degrees 2G, 80 degrees 5,76 G, and 90 would be infinite?

Picture an airplane with the important forces represented like this:


First airplane, total lift = weight

If you want something to stay at a constant altitude, it's not an option, weight must be balanced by an equal an opposite force, that is, an arrow parralel to the weight arrow but pointing at the opposite direction.

Bank your airplane now (other two airplanes in the picture). If the total lift arrow stayed as long as the weight arrow, the airplane would start descending. Why? Because the vertical component of the total lift (the arrow parralel and opposite to the weight arrow) would then be shorter than the weight arrow. How shorter? Lenght of the total lift arrow times cosine of the angle between vertical and the total lift shorter.

We want to keep the altitude constant, hence we want the vertical component of the lift to stay as long as the weight arrow, which doesn't change. To do that, you have to lengthen the total length arrow by pulling on the stick (higher AoA). So how much longer does the total lift need to be if you bank 60 degrees? 1/(cosine of the angle between vertical and the total lift arrow) longer. 1/cosine60=2, hence a constant altitude turn banked 60 degrees makes you feel twice as heavy.

Now it is obvious as to why a 90 degrees of bank constant altitude turn is impossible if the only thing that produce lift are the wings. Cosine of 90 is 0, so 1/cosine90 = destruction of the universe. A 90 degrees of bank turn would not make you explode with infinte Gs thought, you would simply loose altitude. If you don't want to loose any, than apply some top rudder and hope that the fuselage provides enough lift.

[Beefitarian]

Edit: Now we're talking, thanks Trampbike!

Video would be even better!

For example, you can be inverted an pulling lots of positive G.

I am having a hard time imagining the attitude of that. Is that the exact opposite of the straight and level with negative AoA with tail wanting to flip over you?

I think the most basic example of negative G would be if you shove the yoke/stick forward. The plane will go down and you will wan't to keep going straight until you're pulled down by the seat belt. There you'll have it negative G.

[trampbike]

Edit: Now we're talking, thanks Trampbike!

Video would be even better!

Not sure it helps, but at least I tried.

[MIQ]

Since we are already talking about it, I was wondering if you guys have any tips (if there are even any) to practice G tolerance. I don't have any problems with motion sickness or something like that but I noticed, that I find negative G forces quite uncomfortable. I don't ever feel sick or anything but I really don't like the feeling of negative G's. I have no problem with positive G forces or any other kind of motion though. I tried Google already too but couldn't find anything specific. I would like to improve my tolerance for negative G's if this is somehow possible so that spins and other aerobatic maneuvers don't feel uncomfortable anymore. Or is it just something you get used to after a while?

[old_man]

Negative G....I was taught to just relax my body the best I could. Not to tense up. Breath normally and not hold my breathe. I found it actually worked.

[Colonel Sanders]

Positive G tolerance is easy - lift weights

Negative G is weird. Relax and build it
Very slowly - an additional 1/2 neg G at
A time

Start with straight and level inverted for
Longer and longer lengths of time

The start with inverted turns, increasing
The bank angle.

Then do pushes to 45 up, then level off
Inverted again

Then push through the vertical for a
1/2 outside cuban 8 to the upright 45
Down, and pull level to upright

Finally complete the second half of the
Outside Loop - watch the increasing
Airspeed and increasing neg G!

Don't hammer the neg G too fast
Otherwise you will tear up your
Inner ear with the Wobblies.

[rapid602]

First of all when I was about 10 years old I got to sit on the real Colonel Sanders knee, wish I had a picture of that.

Second I got to watch Bob Hoover Fly at Oshkosh, what a treat, saw him fly a yellow P51 as well I think, seemed to rememebr the name Rockwell on the side. Non the less, made me feel inferior as a pilot and still do.

Question here is ... Does anyone ever admit to spilly water, lemonade etc, practicing that manouver. Infact, I would bet Bob Hoover spilled some. If anyone knows him please find out.

I am not a memebr of the wet crotch club as I think that aerobatics are much prettier to watch from on the ground, and I love and appreciate the people who perform them.
I never ever tried the manouver, and do not have the desire to drown myself trying it.

[MIQ]

Negative G....I was taught to just relax my body the best I could. Not to tense up. Breath normally and not hold my breathe. I found it actually worked.

Thank you, that makes sense and I will try to do that next time.

Positive G tolerance is easy - lift weights

I'm lifting weights on a regular basis, I guess that's why I have no problem at all with positive G's. Thank you for the other tips, I'm not doing any aerobatics just yet. I actually respect aerobatics a lot and would also be one of the people who'd rather appreciate it from the ground but I think one day I should try it and take an aerobatic course or something.

[Colonel Sanders]

No one actually likes negative G

Many people hate it

But the aircraft will do it

And either you're the type of pilot who will
Do everything in the Vg diagram, or you're
Not.

Hard to explain.

It's an acquired taste!