If a plane were weightless...

[SixPapaCharlie]

If a plane weighed nothing, would it slow down in a dive?

Being that I am on the "Newton is doing a lot more work than Bernoulli" side, I got to thinking.
If Bernoulli was doing the lifting, I wouldn't need to rotate to get off the ground, I would just be sucked up into the sky at a level attitude.
So I rotate and Newton goes "Hell Yeah! Let's do this!!!!"

If the plane was weightless and I pitched down, then I should expect the same via Newton now pushing against the top of the wing. I should go down but also slow down.

Can anyone tell me if a blimp is moving forward at full speed and pushes the nose down, does it see a decrease in speed?

 

[Mikey52]

It's not weight but mass. You might become weightless, but never massless.

 

[NACA 0012]

I agree that being weightless and being massless are different things... I also remember that there's an additional inertia associated with lighter-than-air craft that results from the mass/momentum of the air moving around the craft. There are also changes in buoyancy as the gas expands/contracts with altitude and pressure. The more I think about it the more complicated it seems!

 

[Checkout_my_Six]

So if I drop a pound of feathers....or a 2 ton block of the same size and shape from the same altitude.....which one lands first?

 

[RussR]

If a plane weighed nothing, would it slow down in a dive?

Being that I am on the "Newton is doing a lot more work than Bernoulli" side, I got to thinking.
If Bernoulli was doing the lifting, I wouldn't need to rotate to get off the ground, I would just be sucked up into the sky at a level attitude.
So I rotate and Newton goes "Hell Yeah! Let's do this!!!!"

If the plane was weightless and I pitched down, then I should expect the same via Newton now pushing against the top of the wing. I should go down but also slow down.

Can anyone tell me if a blimp is moving forward at full speed and pushes the nose down, does it see a decrease in speed?

Well if you were weightless, in other words gravity had no effect, then you'd have a temporary decrease in speed due to increased drag during the pitching down (or up). But once you're established in the new attitude, you'd have the same drag as before and therefore would resume your previous speed, since gravity wouldn't be assisting you in your descent.

I am not a certified blimp pilot (but am willing to be if I get any sponsors), but I'd guess that yes, they probably do slow down a bit on pitching down.

This has been running around your brain since at least last Monday, hasn't it?

 

[Dana]

Bernoulli and Newton aren't in conflict; they're just different math to describe the same thing.

If the aircraft is weightless but not massless (if gravity was switched off), then it moves according to the net aerodynamic force... trim for zero lift for level flight, lose speed in a dive because air drag's slowing you down.

If the plane was massless it would also be weightless, and any force would cause an instantaneous infinite acceleration.

Blimps usually trim a few hundred pounds heavy, and use pitch to compensate and maintain level flight. So if you pitch down just a bit to level the blimps, you'll speed up because you're reducing induced drag.

So if I drop a pound of feathers....or a 2 ton block of the same size and shape from the same altitude.....which one lands first?

In a vacuum, the same time. My high school physics teacher had a long glass tube with a feather and weight inside and the air evacuated, to demonstrate this.

 

[NACA 0012]

Bernoulli and Newton aren't in conflict; they're just different math to describe the same thing.

Thanks, Dana... that definitely needed to be said!

 

[Salty]

I'm struggling to see how Bernoulli or Newton (in the frame of reference of lift of an aircraft) have any relevance to a lighter than air craft.

 

[Dana]

I'm struggling to see how Bernoulli or Newton (in the frame of reference of lift of an aircraft) have any relevance to a lighter than air craft.

Bernoulli's theorem describes the pressure changes as a fluid (air) flows over a curved surface. That happens any time the blimp is moving. If the blimp has a nonzero AOA, there is a side (or up/down) force created. As I said earlier, blimps usually fly a bit heavy and use aerodynamic lift to maintain trim.

Newton's laws say (among another things) that an object in motion will keep moving at the same velocity unless acted on by an external force. If drag exceeds lift or vice versa, the blimp will slow down or speed up, in accordance with Newton's laws.

 

[Zeldman]

So if I loaded an airplane to max capacity with filled helium balloons, will the plane go faster at cruise settings.??

 

[dmspilot]

If the aircraft is weightless then there is no "down" as far as it is concerned. It's just a change in direction. Imo the speed would not change.

 

[Jay Urish]

The earth has no gravity... The world sucks.

 

[GoFly]

So if I loaded an airplane to max capacity with filled helium balloons, will the plane go faster at cruise settings.??

You may also set altitude records….until they pop. And you could use one to talk really funny to ATC

 

[Capt. Geoffrey Thorpe]

If an airplane weighed nothing, gravity would have no effect. In a straight down dive it would accelerate until thrust equaled drag. Of course, to make it go straight down, you would have to push on the yoke / stick to operate at a negative angle of attack to get zero lift and make your trajectory straight down.

Bernoulli's theorem describes the pressure changes as a fluid (air) flows over a curved surface.

Bernoulli's theorem states that the sum of pressure, kinetic energy, and gravitational potential energy is constant along a streamline in an ideal fluid - basically conservation of energy. It has nothing to do with curves.

Pressure + (1/2 * Density * Velocity squared) + (Density * Acceleration of gravity * Height) = Constant

 

[Checkout_my_Six]

and....so it begins....maths.

 

[brcase]

Bernoulli's theorem describes the pressure changes as a fluid (air) flows over a curved surface. That happens any time the blimp is moving. If the blimp has a nonzero AOA, there is a side (or up/down) force created. As I said earlier, blimps usually fly a bit heavy and use aerodynamic lift to maintain trim.

Newton's laws say (among another things) that an object in motion will keep moving at the same velocity unless acted on by an external force. If drag exceeds lift or vice versa, the blimp will slow down or speed up, in accordance with Newton's laws.

So short version

Newtons says if you push on it, it will move.
Bernoulli says it will push on it, if air is flowing over a curved surface.

That sound right?

 

[Capt. Geoffrey Thorpe]

More fun thought experiments.

You are flying along straight and level, and Congress repeals the law of gravity. What happens?
The wing is still generating lift, but the downward acceleration of gravity is gone, so the aircraft accelerates upwards at 1G. Sitting inside, you would feel no change in the forces holding your butt to the seat, but you would notice the change in trajectory. I suspect that the aircraft would tend to pitch up to maintain the angle of attack resulting in going around in a big loop.

Alternately, you are again flying along straight and level, and this time Washington sucks all the air out of the atmosphere. What happens (I am ignoring the fact that you (and the engine) need air). Lift (and thrust (and drag)) disappear leaving you in free fall with no force pressing your butt to the seat - just like if you were in the International Space Station. Of course, your speed is not high enough to complete an orbit and you will impact the earth in just a bit.

Conclusions: The forces you experience as you sit in the seat are reactions to aerodynamic forces not gravity. Gravity just affects your trajectory.

 

[Arm3]

Can this weightless plane takeoff from a treadmill moving in the opposite direction?

 

[keen9]

It is always fun to see amateurs talk about engineering!

 

[Dana]

So if I loaded an airplane to max capacity with filled helium balloons, will the plane go faster at cruise settings.??

Yes

Bernoulli's theorem states that the sum of pressure, kinetic energy, and gravitational potential energy is constant along a streamline in an ideal fluid - basically conservation of energy. It has nothing to do with curves.

True, but rounding that curved surface makes the air speed up, causing a pressure drop... according to Bernoulli. It's only one specific example of Bernoulli's theorem, of course, but it's the one most people are thinking of when trying to explain wing lift.

Next, we can discuss whether a fly flying in the passenger compartment affects performance...

 

[Capt. Geoffrey Thorpe]

True, but rounding that curved surface makes the air speed up, causing a pressure drop... according to Bernoulli. It's only one specific example of Bernoulli's theorem, of course, but it's the one most people are thinking of when trying to explain wing lift.

Next, we can discuss whether a fly flying in the passenger compartment affects performance...

Except that the "rounded top causes squeezing / faster flow vs. a flat bottom" thing is basically a fairy tale. If you want to invoke Bernoulli W.R.T. lift, you really have to start with the concept of circulation. The effects of airfoil thickness and forcing the air to accelerate around the thick part are the same on the top and the bottom.

And, it doesn't matter if the fly, or 10,000 parakeets, inside the airplane are flying or sitting on something. The net force on the airplane is the same.

 

[sansoneservices]

If a plane weighed nothing, would it slow down in a dive?

Being that I am on the "Newton is doing a lot more work than Bernoulli" side, I got to thinking.
If Bernoulli was doing the lifting, I wouldn't need to rotate to get off the ground, I would just be sucked up into the sky at a level attitude.
So I rotate and Newton goes "Hell Yeah! Let's do this!!!!"

If the plane was weightless and I pitched down, then I should expect the same via Newton now pushing against the top of the wing. I should go down but also slow down.

Can anyone tell me if a blimp is moving forward at full speed and pushes the nose down, does it see a decrease in speed?

Blimps are usually near neutral in buoyancy, so ascending or descending should not see a change in speed

 

[Dana]

Except that the "rounded top causes squeezing / faster flow vs. a flat bottom" thing is basically a fairy tale. If you want to invoke Bernoulli W.R.T. lift, you really have to start with the concept of circulation. The effects of airfoil thickness and forcing the air to accelerate around the thick part are the same on the top and the bottom.I

I didn't say people understood Bernoulli when they mentioned the word. The problem is people conflate Bernoulli with the incorrect "equal transit time" theory.

And, it doesn't matter if the fly, or 10,000 parakeets, inside the airplane are flying or sitting on something. The net force on the airplane is the same.

C'mon, you know you're not supposed to give the correct answer right away...

 

[eman1200]

Bernoulli.

 

[Sac Arrow]

Bernoulli.

Are you crazy???

Newton. Fig Newton. Bernoulli doesn't give us squat for yummies.

 

[Rene]

So if I loaded an airplane to max capacity with filled helium balloons, will the plane go faster at cruise settings.??

I have thought about this in designing a Air Vehicle compliant with Part 103. Old fashioned control cables outside of the wings and fuselage would allow the fabric to be sealed and that space to be filled with helium. But I just can't figure out how to seal the fabric without adding enough weight to offset the buoyancy of the helium.

 

[Checkout_my_Six]

That's no fun.....use hydrogen.

 

[Rene]

That's no fun.....use hydrogen.

Eh, some wiseacre would say I was violating the 5 gallon rule.

 

[catmandu]

If a plane weighed nothing, would it slow down in a dive?

Only if you failed to rotate the treadmill to vertical.

 

[Rene]

Are you crazy???

Newton. Fig Newton. Bernoulli doesn't give us squat for yummies.

Well, I can tell that you've never had a big plate of al dente Bernoulli with sweet Italian sausage and Arrabbiata sauce.

 

[Sac Arrow]

Well, I can tell that you've never had a big plate of al dente Bernoulli with sweet Italian sausage and Arrabbiata sauce.

Hell, I can't even pronounce it.

 

[flyingcheesehead]

If a plane weighed nothing, would it slow down in a dive?

I don't think it would slow down - It would go the same speed.

If the plane was weightless and I pitched down, then I should expect the same via Newton now pushing against the top of the wing. I should go down but also slow down.

Why would you slow down? When you pitch down there would be a slight additional force on the wing that would momentarily slow you down (probably by fractions of a knot), but once you reach the desired descent pitch, in the steady state descent you'd go the same speed as in cruise.

So if I loaded an airplane to max capacity with filled helium balloons, will the plane go faster at cruise settings.??

Yes.

Eh, some wiseacre would say I was violating the 5 gallon rule.

5 gallon rule??

 

[Capt. Geoffrey Thorpe]

5 gallon rule??

Part 103 limit.

 

[RussR]

5 gallon rule??

@Rene is proposing a Part 103 (ultralight) solution. Part 103 is limited to 5 gallons of fuel. Using Helium would be no problem. If hydrogen was used instead for buoyancy, there would be more than 5 gallons of it. Since hydrogen is usable as a fuel (probably not in this implementation, but that's the joke), that would "violate" Part 103.

 

[Clip4]

For an aircraft in flight to have a gross weight of zero, the BEW would have to be a negative value equal to the weight of the pilot, and useable fuel. As soon as the aircraft took off and used an ounce of fuel, the aircraft GW would be <0 amd you would have to use thrust to pull you to the surface and the aircraft would slow.

 

[Rene]

For an aircraft in flight to have a gross weight of zero, the BEW would have to be a negative value equal to the weight of the pilot, and useable fuel. As soon as the aircraft took off and used an ounce of fuel, the aircraft GW would be <0 amd you would have to use thrust to pull you to the surface and the aircraft would slow.

While the OP did say "weightless", I wasn't thinking about a weight of zero or even close to zero. That would be a dirigible. I would just like to build a 350 lb ultralight. But I'm weight-limited at 254, 278, or some other higher but still inadequate number if it's a floatplane.

If I had an empty 250 pound legal Part 103, fueled it and climbed in, that's 430 pounds. So, if I could offset some of that weight with helium, it might not matter because the legally permitted 5 gallons of fuel could make the legally permitted 20 to 30 or so hp (depending on how draggy it is) perform like a bigger engine with more fuel . . . at least that was my idea.

Of course it takes about 16 cu ft of He to lift a pound. So, besides having to seal the fabric really well, it's not a terribly practical idea.

 

[Sac Arrow]

You might be better off at the equator. (Think Eotvos effect)

 

[Arm3]

6PC, this is probably the closest thing to what you’re asking about that actually exists and flies:

 

[NACA 0012]

Are you crazy???

Newton. Fig Newton. Bernoulli doesn't give us squat for yummies.

While the cross-section of a Fig Newton does have some camber, the trailing edge isn't sharp enough for us to apply the Kutta Condition. Without the resulting circulation, the Joukowski formula would predict no lift and as a result, I have to side with the Bernoulli Brothers on the matter.

Since you brought it up, a Fig Newton would go down pretty good right now!