I’ll try again. What is it in the engines that determine an airplanes maximum speed? If the plane is moving at that speed backwards on a treadmill (at this point the wheels aren’t turning of course) then, when the engines are started, could the plane take off? Lets change the wheels to pontoons and the treadmill to a strong river current with the plane trying to take off upstream. That way we get away from the sidetrack of rolling resistance.
Right - a seaplane (aka floatplane). Yes, it would probably still take off.
The “probably” deals with the case of a seaplane whose engine is barely strong enough to achieve flight on still water. If on a river with a strong current, hydrodynamic drag from the floats would be somewhat greater, so it might have trouble achieving the airspeed necessary for flight.
But if we’re talking about a normal river (instead of a magic one whose current varies with the speed of the seaplane), taking off against the current has a significant practical difficulty: before power is increased, the plane would be moving backward with respect to the air. Until it reaches some positive airspeed, it will be tricky to control. Aircraft - especially those on a low-friction surface (e.g. water) do not much like to face away from a substantial wind.
You could perhaps deal with this by taxiing downstream, then attempting a high-speed 180-degree turn to face upstream. The practical safe limit for this would probably not deal with a strong current.
There’s more than one kind of maximum speed. One is the fastest the aircraft can fly in level flight with the throttle set to 100%. Another is the speed beyond which the aircraft becomes unstable and/or may suffer a structural failure, which makes diving dangerous. In either case, those speeds are based on the plane’s speed through the air (or the air’s speed around the plane) and have nothing to do with the ground at all. What you’re describing here is equivalent to attempting to take off with a very strong tailwind.
Can a plane take off with a strong tail wind? Yes, but it increases the length of runway required. Is there such a thing as a tailwind which is so strong that the plane cannot take off? That’s a tough question. A very strong tailwind would have a tendency to make the prop want to spin backwards, before the engine was started. I suppose it’s possible that this force could be strong enough to prevent the starter motor from spinning up the shaft.
But there’s a puzzle here. If the tailwind keeps increasing, at some point the wind would be so strong that the plane would start rolling forward even though the engine is off. Let’s call this wind speed W1. And there is probably some wind speed at which the prop would be unable to turn and the engine could not start. Let’s call that W2. Personally, I think W1<W2, which means the plane would start rolling before you tried to start the engine and the wind blowing on the prop from behind would never reach W2. But this assumes that the wheel brakes aren’t on. If the brakes are on, then there’s W3, which is the wind speed at which the wind would be strong enough to push the plane forward even though the brakes are on. I think W2<W3, which means you’d be unable to start the plane’s engine if the tailwind is faster than W2 and the brakes are on.
Sorry but that just raises the specter of rolling resistance (or should that be pontooning resistance?) even more. The problem that trips up so many people is the counter intuitive fact that spinning the wheels faster does NOT increase the drag. But with pontoons, the drag DOES increase as the speed through the water increases. Therefore replacing the treadmill with pontoons and a river current is not equivalent. In the treadmill case, there is no such thing as a treadmill speed which is so fast that can prevent the plane from moving forward. In the river and pontoon case, there IS such a thing as a water current speed which is so fast that it can prevent the plane from moving forward at all.
Drag on a float **decreases **considerably once the seaplane gets to speed and goes “on the step.” That’s why floats are designed like boat hulls.
I’m not familiar with the terminology: That would mean that a significant upwards force (i.e., what’s supporting the plane against gravity) is coming from hydrodynamic lift from the water, rather than from buoyancy?
Though I suppose that air lift over the wings would also be relevant. Anything that provides any upwards force would decrease the needed buoyancy, resulting in the plane floating higher, with less of the pontoon submerged.
On the step, or the “planning” position is a brisk taxi (or takeoff) whereby the nose is lifted and the center of buoyancy, and much of the weight of the seaplane, shifts to aft of the normal COB of the float. - with the back half of the float skimming through the water. In fact, I understand that once you’re on step, you can reduce power to maintain that speed (drag from the water is less than when ploughing) Whereas Ploughing is similar attitude wise, but the COB is still forward and the float is 3/4 - 2/3 in the water, so more drag. You have to transition through ploughing to get to planning, then reduce angle of attack enough to create lift. My guess would be that lift really doesn’t reduce drag until the pilot reduces the angle of attack to actually lift off. A guess though.
Planing position, not planning.
(Love, your autocorrect)
That’s just one part of the story. Yes, the shape of the floats and the way they can skip across the top of the water allows them to have less drag than if they were fully submerged. But the drag still increases with speed, especially below 30 knots. There’s a diagram to explain it here. AIUI, the water drag begins to decrease above 30 knots partly because the lift from the plane’s wings partially counteracts the weight of the plane, allowing the pontoons to come up out of the water because less buoyancy is needed.
Here’s a more relevant graph, which shows how drag increases as speed increases, for semi-displacement and planing hulls. Yes, planing offers less resistance than displacement or semi-displacement, but the drag still increases as the speed increases. What changes is when the airplane begins to generate significant lift from the air rushing past the wings, lifting the plane up slightly, so that less and less weight is on the pontoons and they displace less and less water.
But we’re trying to imagine a strange situation where the water speed and the air speed are not the same thing, either because it’s floating down a river or there is a very strong tailwind. Let’s consider a 25 knot tailwind scenario. Suppose you are able to start the engine and begin to taxi forward at 2 knots (relative to the water). There’s a certain amount of drag caused by the floats pushing their way through the water. To get up to 5 knots requires more power, so you throttle up. You need even more power to get up to 10 knots. The hulls are still displacing at this point, not planing. The faster you wish to go, the more power is required. Drag is increasing with your speed through the water. Increase power to get to 20 knots. Then 25 knots. Around this point, you’d normally expect to start planing because the wings would generate some lift. But your air speed is zero! So you stay displacing longer than usual. You need more power to get to 30. And even more power to get to 35. Finally around 50 knots (relative to the water) you get 25 knots of air speed and the wings lift starts to pull you up on top of the water instead of in the water.
The question is, does your engine have enough power to push you through the water all the way up to 50 knots? What if the tail wind was 60 knots? Could your engine push you through the water all the way up 85 knots water speed so that you can get 25 knots of air speed? Is there a tailwind speed which is too strong for your engine to push through the water that fast?
These are relevant questions when dealing with floats. The questions are irrelevant when discussing wheels, because rolling resistance does NOT increase with speed. But hull drag though water DOES increase with speed. Therefore a seaplane on floats in a river is not equivalent to a standard airplane on a treadmill.
It remains to be proven whether you’d even be able to get to this point at all. If your plane was in a swiftly flowing river and you wanted to go upstream, would you have enough power to get “on step”? Once you get there, yes you could throttle back. But can you get there for every possible water flow speed? Does there exist a water speed so fast that your seaplane simply can’t overcome it? Maybe.
Does there exist a treadmill speed so fast that your plane’s engine can’t overcome it? Definitely not.
Of course, if you’re in a seaplane on a fast-flowing river, and you want to get upstream, you could always just turn around, take off downstream, and then fly above the river (where the current doesn’t matter) to your destination. This is equivalent to the catapult version of the treadmill.
Do you have any idea what pitch, angle of attack, and power curve have to do with an airplane taking off? It doesn’t appear that you understand the physics of flight in that regard.
Just had to make it clear that the pontoon suggestion is of course, the missing option #10 in sbunny’s list (and one of the four actually done regularly).
Ad hominem attacks don’t move the conversation forward. If you have something to say about the physics question we are discussing, then let’s hear it.
I have another way to visualise the problem.
Instead of using propellers, jets or rockets to apply force to the airplane, use a bungee cord. This technique is used for model planes and even full sized gliders.
Same setup, plane on treadmill etc. except that there is a bungee cord stretched to 100 meters in length attached to a hook on the plane. On take off, that cord will contract to its starting length of 10 meters and in so doing accelerate the plane. Now again, this is a real technique for launching planes. What happens?
The plane accelerates and takes off.
Those who believe a treadmill can prevent a conventional take off must also believe it to be true in this case. I.e. the plane is stationary on the treadmill as the cord contracts. What does that look like? What terrible things must be done to the universe for this scenario to be true?
I think that the assumption is that the friction from the treadmill would cause the cord to remain stretched. It wouldn’t, of course, but I think that’s the assumption.
Just to clarify, isn’t there some kind of anchor which holds the plane stationary while the bungee cord is being stretched? I assume the plane doesn’t move until the anchor is released.
If rolling resistance increased with wheel speed (which it doesn’t) then it would be possible to increase the conveyor belt speed until the increasing rolling resistance balanced the force being applied by the bungee cord. When the anchor was released, the forward pull of the bungee cord would be exactly equal to the backward pull of the rolling resistance between the wheels and the treadmill. Hence the plane would not move when the anchor was released. This is what could happen in an alternate universe where rolling resistance increases with speed. Here in this universe, rolling resistance is a constant.
To recap, we have the following facts.
#1 Airplane wheels (unlike automobile wheels) do not drive the vehicle forward; they spin freely.
#2 Airplane wheels can be used to slow down the plane, or hold it in place, by applying the brakes.
#3 There is a limit to how much the brakes can be used. They essentially convert kinetic energy into heat and the heat needs time to dissipate.
#4 Even when the brakes are not applied, the wheels have rolling resistance. In order to taxi the aircraft, some force must be applied which is greater than or equal to the rolling resistance.
#5 Rolling resistance does NOT increase with speed. It is a constant.
#6 Airplanes engines produce thrust. This thrust MUST be greater than the rolling resistance or else the plane could not even taxi, let alone take off, or fly.
#7 As the plane accelerates down the runway, it experiences aerodynamic drag, which increases according to the square of the air speed. As aerodynamic drag increases, the thrust required to keep the airplane accelerating also increases. Eventually, there comes a point where the aerodynamic drag balances the maximum thrust from the engines and the plane reaches a maximum speed for level flight.
#8 As the air flows past the wings (or the wings move through the air, depending on your point of view) the wings generate lift. Lift increases with wind speed.
#9 When the wind speed reaches a certain point, there is enough lift to take off. If (for whatever reason) the wind speed never reaches this point, the airplane cannot take off. This has nothing at all to do with the speed of the wheels.
#10 Airplanes require time to reach takeoff wind speed, which means they must cover some distance while they are accelerating. This is why we need runways to take off.
#11 Replacing the runway with a treadmill spinning the opposite way accomplishes nothing. It has no effect on rolling resistance. It has no effect on acceleration. It has no effect on wind speed. It has no effect on the distance required to reach takeoff speed. It has no effect on the plane’s braking ability. The treadmill would need to be the same length as the runway.
#12 Replacing the runway with a catapult accomplishes quite a lot. It allows the airplane to accelerate much more quickly and get up to takeoff speed in a much shorter distance.
#13 Replacing the runway with a treadmill spinning forward instead of backward would act as a weak catapult, shortening the takeoff distance by a small amount.
#14 A tailwind increases the distance required to reach takeoff speed. A headwind shortens it.
#15 Replacing the wheels on the runway with floats on a river introduces different variables because hull resistance of the floats moving through the water is not constant; it changes with the water speed.
It seems to me that the place where people get tripped up is #5 because it seems counter intuitive.
Let’s combine the bungee cord and anchor idea with the treadmill idea at the same time (with no headwind or tailwind). Put the airplane on a treadmill. Anchor the plane to the ground next to the treadmill, holding it in place. Stretch the bungee cord. Now spin up the treadmill. The plane’s wheels will spin at the same speed as the treadmill. Spin up the treadmill faster. The plane’s wheels will spin faster. No matter how fast the wheels spin, the air speed remains zero. There is no lift on the wings. Almost everyone agrees up to this point in the story that the airplane cannot take off with no lift. Now, what happens when you release the anchor? Will the airplane move forward? Intuition says “That depends on how fast the treadmill is spinning.” But intuition is wrong. It DOESN’T depend on how fast the treadmill is spinning. The rolling resistance of the wheels is exactly the same at 1 mph as it is at 100 mph. If the bungee cord is strong enough to make the plane move on a runway then it’s strong enough to make the airplane move on a treadmill. And yes the same thing goes for engine thrust as goes for the bungee cord.
Instead of a treadmill running in the same direction, or big fan, here’s something that would be practical: build a runway on the side of a hill. Like this.
No, you’ve actually hit on the nuance that trips most people up. Under the scenario where the conveyer always matches the speed of the wheels, and there is no friction and instantaneous acceleration, you have a paradox and infinities involved. So that’s why the thought experiment proves that it’s not possible.
In the semi real world with axle friction, the conveyer would spin up to the point where the rearward vector from friction on the axle equals thrust from the engine, and then you’d have an equilibrium holding the plane in place - except that the forward thrust line would be above the reverse thrust line and the plane would pitch over onto its nose. (-:
In the real real world, the tires would rapidly spun up until they burst, ruining your whole day. And if the tires were made of obtanium, friction would cause the bearings to fail. And if the bearings were made of unobtanium, the energy would transfer to the gear hardware, heating it until it melted.
In the really really real world, conveyers cannot accelerate like that and would be limited in velocity, and so the aircraft engines would rapidly overpower it and take off, with the wheels turning somewhat faster than they would on takeoff from pavement.
Be very hard to land on, and would be unsafe if you need to abort your take off.
No, that wouldn’t work because rolling resistance is constant and does not increase with wheel speed.
The rearward vector from friction in the axle is exactly the same at 1 mph as it is at 10 mph or 100 mph. If the plane’s engine has enough thrust to overcome the rolling resistance at 1mph wheel speed (and it better, or else that’s a really lousy airplane which is incapable of taxiing) then it has enough thrust to overcome the rolling resistance at any speed you set on the treadmill. There is no way you can reach equilibrium by setting the treadmill’s speed to some really high number. That number does not exist.
Well… unless you assume that the treadmill is magical but the airplane isn’t. Then maybe you can invent some scenarios where the treadmill continues to function but the airplane’s wheels suffer a malfunction, causing the rolling resistance to increase.
Okay, there might be another way to do it. Forget about rolling resistance completely. Assume that both the treadmill and the airplane are magical, with no friction at all. In that case, you could hold back the airplane, not by setting the treadmill to a very high speed, but by ramping up the treadmill’s speed very quickly. Each time you increase the treadmill’s speed, the bottom of the wheels are being pushed rearward and the axle is not being held in place. This imparts momentum to the airplane. Increase the treadmill’s speed some more and it imparts some more momentum. If you keep increasing the speed continuously, it would create a rearward vector. If you did it fast enough, you could match the thrust of the airplane’s engine. But I feel like this is cheating because it requires not only a magical treadmill but also a magical airplane, each capable of unlimited speeds without malfunctioning.
However, this still would not help you to launch the plane in a shorter distance with a treadmill which is shorter than the runway.