Floatplane question

gkainz

gkainz

Final Approach
Joined
Feb 23, 2005
Messages
8,401
Location
Arvada, CO
Display Name
Display name:
Greg Kainz
So, If I'm on floats and need, just for argument sake, 40 knots to take off, and again, just for argument sake, I'm pointed upstream on a river running 40 knots, must I achieve 80 knots to take off? :dunno: :D
 
gkainz said:
So, If I'm on floats and need, just for argument sake, 40 knots to take off, and again, just for argument sake, I'm pointed upstream on a river running 40 knots, must I achieve 80 knots to take off? :dunno: :D
Click to expand...
Absolutely yes! Your WSI will have to read 80 knots for you to fly.

-Skip
 
gkainz said:
So, If I'm on floats and need, just for argument sake, 40 knots to take off, and again, just for argument sake, I'm pointed upstream on a river running 40 knots, must I achieve 80 knots to take off? :dunno: :D
Click to expand...
No, it's the airspeed that matters, just like taking off from a hard surface. When your airspeed reaches 40 knots in your hypothetical example, you'll be airborne.

That's a fast river! Be careful, don't let the plane settle back on to the water and keep that nose as high as you can without stalling or your float will dig in. :D
 
Last edited:
I don't know about flying boats but I've never seen a WSI in any of the planes on floats that I've ever flown. We only use the ASI for takeoffs and landings.
 
My WSI was a fiction because I, too, have never seen one (although I am not that familiar with float and amphib planes). My post addressed indirectly the biggest fallacy in the whole conveyor belt fiasco - that most of the responders (and Greg's questions) all reference speeds which are by their very nature relative measurements, without defining what the reference is.

Yes, in Greg's example you will need an 80 knot WSI speed and a 40kt ASI indication to take off.

-Skip
 
Skip caught the reference - this was in response to the conveyor belt thread ... the flowing river is a direct correlation to the theoretical conveyor belt.
 
... and your prop will need to overcome the drag of pulling the floats through the water at 80 knots, and aerodynamic drag of 40 knots.
-harry
 
Don't forget the downstream wind created by all rivers at the ground effect levels, fairly high for a 40 knot stream. And it will also matter if the water is smooth or choppy, the latter will facilitate getting up on the step for take off speed.

A bush pilot was able to use these factors on a swift river to land, remain stationary abeam a rock in a swift flowing river, pick up raft accident victims from the rock and then take off repeatedly until all were saved.
 
mantakos said:
... and your prop will need to overcome the drag of pulling the floats through the water at 80 knots, and aerodynamic drag of 40 knots.
-harry
Click to expand...

Bingo... Which means that this is a really bad analogy to the conveyor because most seaplanes will *not* take off in this scenario. Hydrodynamic drag is much greater than the rolling resistance you see on a landplane. When I fly the Super Cub on skis, it's a hot rod, with a takeoff roll of only a couple hundred feet. The exact same plane on floats barely staggers into the air, and when the floats finally lift out of the water it surges forward, in a very similar fashion to the way a towplane surges forward when a glider releases.

Incidentally, the way you determine which way to take off in a floatplane is too consider the relative motion of the air. For example, if you're on a river that flows north to south at 10 knots, and there's a 5-knot north wind, you'd take off to to the south. That must make operations interesting at riverside airports with seaplane bases such as SFF or AUW. The landplanes would be taking off to the north and the seaplanes to the south!
 
Last edited:
and if you dont have floats......
 
So Kent, if I understand you correctly, trying to take off on floats against the current may me much more difficult than taking off with a tailwind? That never even crossed my land-only mind. I might have to look into this floatplane thing while up here.
 
I only have a little float time (~ 10 hrs), and none of it on rivers, but it is amazing how the plane surges forward when the floats leave the water.

One of these days I'm going to actually go for the check ride (I'm saving it for BFR time).

~ Christopher
 
AWACSEng said:
So Kent, if I understand you correctly, trying to take off on floats against the current may me much more difficult than taking off with a tailwind?
Click to expand...
I'm not Kent but... not exactly. Generally, takeoff into the wind and against the current is preferred when the wind speed is greater than the speed of the current. On the other hand, when the speed of the current is greater than the wind speed, the beneficial effect of taking off into the wind is cancelled. There are other things to consider too.

Here's a good reference: FAA H-8083-23, Chapter 4.
 
Last edited:
AWACSEng said:
So Kent, if I understand you correctly, trying to take off on floats against the current may me much more difficult than taking off with a tailwind? That never even crossed my land-only mind. I might have to look into this floatplane thing while up here.
Click to expand...

TeenDoc said:
I'm not Kent but... not exactly. Generally, takeoff into the wind and against the current is preferred when the wind speed is greater than the speed of the current. On the other hand, when the speed of the current is greater than the wind speed, the beneficial effect of taking off into the wind is cancelled. There are other things to consider too.
Click to expand...

What Gil said.

Think of it in terms of your runway moving, and the wind relative to the runway. Assume your airplane can stagger into ground effect (floats out of water) at around 40 KIAS. Let's also say, just for the heck of it, that you do have a "WaterSpeed Indicator" like Skip suggests.

Using the example of a river flowing North to South at 10 knots...

1) If the wind is calm, you take off downstream (south). When your airspeed is 40 KIAS, your "WSI" would read 30 knots.
2) If the wind is out of the north at 5 knots, you still take off downstream, with a 5-knot tailwind relative to the shore, but that's a 5-knot headwind relative to your "runway". When your airspeed is 40 KIAS, your WSI would read 35 knots.
3) If the wind is out of the north at 10 knots, the wind and current basically cancel each other out and you can take off either direction, and both your ASI and WSI would read 40 knots at liftoff.
4) If the wind is out of the north at 15 knots (or anything more than 10), you'd take off upstream. At your 40 KIAS liftoff speed, your WSI would read 35 knots.

Notice how your "Waterspeed" is always at or below the 40 knots of airspeed you need to take off when you reach 40 knots of airspeed.

This is kind of the simple view, and there's other stuff to take into consideration, but all else being equal, that's how it works. (Gil, Dave, etc. feel free to correct.)
 
Nice explanation Kent. I would only add that another thing to consider deciding which way to takeoff, with or against the current/wind, is directional control. It's really tough to keep directional control on the takeoff roll as soon as you raise the water rudder. This problem is accentuated if the current is fast and the roll is with the current. If the width of a fast flowing river is marginal, the risk of loosing directional control and capzising outweighs any benefit of taking off with the current. Reluctantly, you may have to choose a much longer and rougher takeoff roll with it's own risk too, but maybe more manageable; in this case, the risk of getting a float caught in a high wave. This event can have disastrous consequences but you can prevent it by keeping that nose as high as possible.
 
Last edited:
flyingcheesehead said:
Bingo... Which means that this is a really bad analogy to the conveyor because most seaplanes will *not* take off in this scenario.
Click to expand...
'scuuuuuuze me? I happen to think it's a darn fine analogy to the conveyor problem! :D Numbers, drag and details be darned, it's a perfect real world implementation of the problem presented with the conveyor belt. :D
 
gkainz said:
'scuuuuuuze me? I happen to think it's a darn fine analogy to the conveyor problem! :D Numbers, drag and details be darned, it's a perfect real world implementation of the problem presented with the conveyor belt. :D
Click to expand...

Yeah, if you're one of the people who believes the plane won't take off from the conveyor. Not a whole lot of small floatplanes, if any, that will take off at a water speed of 80 knots!
 
Reality check: Have any of you ever seen a river moving 40 knots? I don't think I have, either. Maybe the rapids just above Niagra Falls. But I'm quite sure that I would not be out in 40-knot water in any kind of craft.

Regards,
Joe
 
guys guys guys ... it was an "analogy" ... not realistic ... 40 knots, 80 knots, all hypothetical ... just trying to move the image of the "airplane on a conveyor belt" into a visualization that had just a touch more reality ... geez! :)
 
There is a story in Wager about Don Sheldon doing that in a float equipped-cub, I believe it was on the Susitna River somewhere, Devils Canyon maybe? I'll look it up when I get home.

Katherine and I flew into the mouth of Devils Canyon last winter when I took her up in the 170.
 
Last edited:
alaskaflyer said:
There is a story in Wager about Don Sheldon doing that in a float equipped-cub, I believe it was on the Susitna River somewhere, Devils Canyon maybe? I'll look it up when I get home.

Katherine and I flew into the mouth of Devils Canyon last winter when I took her up in the 170.
Click to expand...

I'm wondering what the flow rate of the water was at the rescue rock, maybe even changed since then with earthquakes & whatever.
 
Dave Krall CFII said:
That sounds about average from what I've seen & been in.
Click to expand...

You need to go to Lansing, WV (near Beckley) and go down the Gauley River when they have the dam release.
the water discharges from the lake through three discharge tubes in the powerplant. Average discharge is around 10,000 cubic feet of water per second with maximum flow at 18,000 cubic feet of water per second. We put in in the spray from the discharge. They were three colums of water about 15 feet in diameter, shooting out about 40-60 feet. talking about fast moving water!
 
cherokeeflyboy said:
You need to go to Lansing, WV (near Beckley) and go down the Gauley River when they have the dam release.
the water discharges from the lake through three discharge tubes in the powerplant. Average discharge is around 10,000 cubic feet of water per second with maximum flow at 18,000 cubic feet of water per second. We put in in the spray from the discharge. They were three colums of water about 15 feet in diameter, shooting out about 40-60 feet. talking about fast moving water!
Click to expand...

:needpics:

-Skip
 
You must log in or register to reply here.
Back
Top