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Thread: Why seaplanes doesnīt fly on ground effect for long range?

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    Registered User oriol's Avatar
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    Why seaplanes doesnīt fly on ground effect for long range?

    Hi everyone!


    My curiosity led me through the net to find about the actual status of the Ekranoplane program and also a bit about ground effect vehicles etc...

    I discovered Alexander Lippisch and his very unconventional designs. A company based on singapore (Wigetworks) seems to have acquire his pattent for the Lippisch 114.

    Orion complained on a previous thread that there is very limited reliable data about ground effect, WIG...
    As from what I red on the net the proximity of the ground helps to reduce the creation of tip vortices, drastically reducing the induced drag of the main wing/aerodynamic surface.

    On the other hand the momentum generated is far bigger than the one normally produced by a wing on flight. To counteract this momentum you have to put a big elevator, wich adds induced drag.



    Otherwise than that I canīt see why seaplanes donīt fly like WIGs on ground effect for long range?


    The vast majority of ground effect vehicle are designed to operate at uncompressible flow, same as most homebuilders, leisure aviation... If we can manage to use this technology we would fly safer on a single engine into open waters.


    Any information on the subject is welcomed.


    Oriol



    Why seaplanes doesnīt fly on ground effect for long range?-lippisch-x114.jpgWhy seaplanes doesnīt fly on ground effect for long range?-20080913172712_sea_plane.jpgWhy seaplanes doesnīt fly on ground effect for long range?-ebp-04.jpgWhy seaplanes doesnīt fly on ground effect for long range?-ultralight-amphibious-seaplane.jpg

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    Registered User Rick McWilliams's Avatar
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    Re: Why seaplanes doesnīt fly on ground effect for long range?

    I fly a seaplane. I get better efficiency flying at higher altitude. My seaplane has only 32 ft wing span. I need to fly at less than half a span to get any useful effect. It is very scary to fly low over water when the sea state is too high for landing on that heading.

    The WIG aircraft that I know seem to need monsterous engines compared to conventional airplanes. The wings and floats need to be very stout and heavy for operation near water at high speeds. It does not make a statement of efficiency to me.

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    Registered User bmcj's Avatar
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    Re: Why seaplanes doesnīt fly on ground effect for long range?

    I've heard of WW-2 pilots flying in ground (water?) effect to stretch range, but the ground (water?) speed is not even close to what you can get by flying at altitude. Add to that, rough seas tend to make staying close to the water only an intermittent event and, I'm sure (as Rick pointed out) is a very scary thing... clipping a wave could be disastrous. It also requires constant attention and diligence to maintain your minimal altitude at all times without skipping off the surface or wandering higher; at high cruise altitudes, the pilot can relax a bit. And last, but not least, good height perception over the water's surface is very tricky.

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    Registered User oriol's Avatar
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    Re: Why seaplanes doesnīt fly on ground effect for long range?

    Thanks for sharing your first hand experience Rick!


    I haveīnt any experience on seaplanes. In my country the only seaplanes you can see are military canadairs for fire fighting. The ecologist are a strong lobby and it is forbiden to land on lakes etc...

    Seems that the Ekranoplane used only half of his engines for cruise, They just switched all for take off because of the water resistance.


    I canīt find detailed information about performances of WIG vehicles on the net. Wigetworks sells a two seater wig with a motorcycle engine.
    One of the big pros associated with WIGS is that theyīre supposed to get a lot of lift using ground effect from low aspect ratio configurations with low power.



    As I told you I donīt have first hand experience with seaplanes. I know landing in open waters can cause big stresses on the aircraft even if itīs a seaplane. It seems that most most seaplanes just land on lakes. From what I know only heavy duty seaplanes land in open waters: Catalina, canadair CL215...



    From the images of WIGs I saw on the internet I thought, from your words it seems that I was wrong, that to land on waters at moderated speeds (car velocities like) wasnīt a major problem with the proper configuration.




    Oriol

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    Registered User oriol's Avatar
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    Re: Why seaplanes doesnīt fly on ground effect for long range?

    I was thinking the possibilities that ground effect can bring for low perfos slow airplanes to increase their range.
    Particularly an amphibious pendular.


    Why seaplanes doesnīt fly on ground effect for long range?-ramphos.jpg


    In spain Max ceiling for ULM is 300m, you canīt climb very much to increase your performances. So I was wondering to increase the performances doing the opposite of climbing but flying at almost stall speed at sea level. That way if you accidentally hit the sea youīll get a soft landing.


    Anyway I understand your point: flying at higher speeds close to the sea is quite risky.
    Wigs like albatros donīt fly at half their wingspan of the sea but very close to it. The maximum ceiling of the monster Ekranoplane was only 3 meters high, above that ground effect started to disapear.

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    Registered User bmcj's Avatar
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    Re: Why seaplanes doesnīt fly on ground effect for long range?

    You can still get some drag reduction effect even when flying a little higher (out of perceived ground effect), just because the tip vortex is interrupted as it drops below the plane to the water, but the benefit diminishes rapidly with each extra foot of altitude.

    DISCLAIMER: I say this based on various "things' I have heard over the years, but have not seen any hard research to back it up (unless you count "Myth Busters" )

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    Super Moderator orion's Avatar
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    Re: Why seaplanes doesnīt fly on ground effect for long range?

    This is a pretty vast subject for this limited medium but we can highlight some key issues: First off, flying near the ground for extended periods requires a more complex stability analysis since you have to be concerned not only with the aircraft's basic stability, you also need to develop the height stability. This is rather complex since several variables change drastically (as a function of height) in ground proximity so design and flight constraints are narrow and have to be formulated specifically for that realm of flight. But unfortunately, optimizing those characteristics then makes the vehicle extremely poor at any other phase of flight (higher and out of ground effect).

    Optimum ground effect for cruise is very low to the surface and is often expressed both, as a function of span and chord. Ideal heights are below 10% of MAC. This in a typical WIG configuration is then 5% or less of span - much less for a conventional seaplane. Yes, there is some induced drag benefit starting at about half span but it is minimal. Also understand that at cruise the induced drag is relatively low so the efficiency gained is a relative term. Tests have shown that at ideal heights these vehicles can achieve relatively high L/D ratios but there are a lot of details that have to be accounted for so simply flying low will see only a marginal gain. But it is true that bombers low on fuel, returning to base in the Pacific, have at times used ground effect to improve their range.

    Notice that WIGs use very short spans (so they can turn) so most do have high induced drag numbers, so in those cases the ground proximity does help. But again, in general you have to design for this - a conventional airplane will see only minimal benefit.

    The high power numbers are required for very large GE craft since accelerating and getting off the water in a large craft that has very short wings is not easy. The HM-1 Ekranoplan (Caspian Sea Monster) has twelve engines for take-off, but uses only two for cruise. The rest are at idle (too much drag if they were turned off). And since a turbine at idle burns about half the fuel of that same engine at full power, The HM-1 is far from an efficient craft, regardless of how you look at it.

    And of course then you have issues of other craft on the water. A typical cruise for a smaller WIG will be in the 100 to 150 mph range. At that speed, especially if maneuvering around islands or other obstacles, things can happen very quickly and it it foreseeable that collisions could occur. Given its cruise height, even a kayak could be in harms way.

    The Singapore company does not own anything by Lippish - the design is based on Dr. Hanno Fischer's work in Germany, specifically the Airfish-8. The development actually started in Australia probably twenty years ago and has been through at least four owners before it finally settled in Singapore. The particular design was undergoing certification trials in Europe but as far as I've been able to track down it never achieved the goals. It was bought by an investment group in singapore and given certification there for operation in those waters. By all accounts (one of my contacts has actually flown in one and piloted it) though the craft does not seem all that stable and requires a lot of pilot input to maintain height stability. Not good. I don't think it's actually operational yet, although I guess that they are building several more.
    "To live is to learn; to learn is to live" (author unknown)

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    Registered User oriol's Avatar
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    Re: Why seaplanes doesnīt fly on ground effect for long range?

    Thanks for your words on the subject Orion, Bmcj, Rick!


    At first I was a bit surprised of the inverted diehedral (Harrier like) configuration of Lippisch/Hanno designs. It seemed pretty unstable, I guess they do that to send the vortices closer to the ground to keep the induced drag to the minimum.
    One other thing with that kind of dihedral would be to touch the water to stop banking.

    Why seaplanes doesnīt fly on ground effect for long range?-wig00186.jpg

    I guess an hovercraft would be easy to domesticate if maneuvering closer to other ships, they probably can put prop on reverse and deccelerate to keep it hovering for low speed maneuvering.


    About the issue of keeping the WIG at constant height, well thatīs something I canīt quite figure out. If the pilot pulls to much the elevator the WIG may hop. If the jump is too high the landing would be at least hard and scary.

    I thought that if the WIG climbed a bit it automatically loosed dramatically its lift so it tended, more or less gently, to come back to a natural neutral cruise position by itself.

    Is it really that hard to achieve that kind of dynamic stability?


    Oriol

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    Registered User ultralajt's Avatar
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    Re: Why seaplanes doesnīt fly on ground effect for long range?

    You see a sailing boat on horizont floating from right towards your low level flying path... what to do?
    How marine and flying rules find theirself in that situation? Sailing boat under sails has advantage!
    Mitja
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    Registered User oriol's Avatar
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    Re: Why seaplanes doesnīt fly on ground effect for long range?

    Quote Originally Posted by ultralajt View Post
    You see a sailing boat on horizont floating from right towards your low level flying path... what to do?
    How marine and flying rules find theirself in that situation? Sailing boat under sails has advantage!
    Mitja

    WIGs have already a legal status defined by both the OACI and its equivalent organism for maritime transport (IMO).

    wikipedia on ground effect vehicle:"
    The International Maritime Organization recognizes three classes of ground effect craft:[16]

    1. Type A: a craft which is certified for operation only in ground effect;
    2. Type B: a craft which is certified to temporarily increase its altitude to a limited height outside the influence of ground effect but not exceeding 150 m above the surface; and
    3. Type C: a craft which is certified for operation outside of ground effect and exceeding 150 m above the surface.
    These classes currently only apply to craft carrying 12 passengers or more."


    As from you can see on the youtube videos (linked in a thread here in the forum about gorund effect vehicles). Most of them look quite maneuverable at low speeds. Maybe modern WIGS can change the pitch of the prop to slow it down then maneuver at minimum speed or just land on the water and wait.


    I guess most fast ships just switch into top speed in open waters not in the proximities of crowded harbours to avoid the risk of collision.



    Oriol

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    Re: Why seaplanes doesnīt fly on ground effect for long range?

    A sailboat is effectively stationary compared to the velocity of a WIG or hydrofoil. I would be much more worried about collisions with isolated posts or other thin hard to see obstructions.



    Vertical stability can be done by computer control easily enough by borrowing some tech from UAVs. Gyros and a millimeter wave radar for height ranging and maybe obstacle detection.

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    Registered User oriol's Avatar
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    Re: Why seaplanes doesnīt fly on ground effect for long range?

    Quote Originally Posted by JamesG View Post

    Vertical stability can be done by computer control easily enough by borrowing some tech from UAVs. Gyros and a millimeter wave radar for height ranging and maybe obstacle detection.
    If Youīre doing really long range flight, say Seattle Hawaii. You better have some avionics IFR and the like should be compulsory.
    Fly by wire itīs possible but complicated, I donīt think the authorities would allow easily a homebuilder to fly on a self designed autopilot/fly by wire system.


    If you do a three hour trip in open waters, Barcelona Mallorca like, VFR would be enough. Of course you have to check the weather before depart, like if you were sailing.
    Lippisch first designs and more today recent designs, particularly one and two seaters, look quite simple no avionics at all.


    Oriol

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    Super Moderator orion's Avatar
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    Re: Why seaplanes doesnīt fly on ground effect for long range?

    Quote Originally Posted by oriol View Post
    At first I was a bit surprised of the inverted diehedral (Harrier like) configuration of Lippisch/Hanno designs. It seemed pretty unstable, I guess they do that to send the vortices closer to the ground to keep the induced drag to the minimum. One other thing with that kind of dihedral would be to touch the water to stop banking.

    About the issue of keeping the WIG at constant height, well thatīs something I canīt quite figure out. If the pilot pulls to much the elevator the WIG may hop. If the jump is too high the landing would be at least hard and scary.

    I thought that if the WIG climbed a bit it automatically loosed dramatically its lift so it tended, more or less gently, to come back to a natural neutral cruise position by itself. Is it really that hard to achieve that kind of dynamic stability?
    The wing actually does not have as much anhedral as you think - the airfoils are at a particular angle of incidence and one of the requirements of the WIG wing is that the trailing edge is parallel to the surface. Given the taper, it makes the wing look like it's pointed down. Yes, thee is a bit of roll instability but it's controllable - it's also one reason for the tip winglets.

    The height control needs to be inherent to the design and automatic - yes, an active flight control system can be used but in the case where it goes out (after all, the marine environment can be problematic in continued service), the craft must still have height stable operation. It is possible but is not intuitively simple since it has to be mixed in with stability requirements of the basic vehicle itself (similar to that of an airplane).
    "To live is to learn; to learn is to live" (author unknown)

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    Super Moderator orion's Avatar
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    Re: Why seaplanes doesnīt fly on ground effect for long range?

    Quote Originally Posted by JamesG View Post
    Vertical stability can be done by computer control easily enough by borrowing some tech from UAVs. Gyros and a millimeter wave radar for height ranging and maybe obstacle detection.
    That's not stability - that's vertical control. For general use or commercial operations the craft must be able to demonstrate inherently stable flight, hands off. The electronic control can be useful for maneuvering and obstacle avoidance, automatic navigation, as well as dealing with issues like rouge waves. But the craft itself needs to be designed in a manner similar to that of an airplane.
    "To live is to learn; to learn is to live" (author unknown)

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    Re: Why seaplanes doesnīt fly on ground effect for long range?

    Quote Originally Posted by orion View Post
    That's not stability - that's vertical control.
    Its both. Yes the craft needs inherent flight stability and a fail-safe manual mode. But in the environment in question, especially in anything besides dead calm waters, it will tend to "hunt around" no matter how stable it is. Having to remain constantly vigilant and make minute control inputs to stay in the narrow "sweet spot" will be extremely exhausting and error prone. An electronic control system is really the only way to make WIG practical and safe IMO.

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