Poll: How buoyant should our airship be?

We should take the cheapest existing flying machine for 1-2 people as a reference.
I suppose this is a powered paraglider or a powered hang glider
Can you make it cheaper using a balloon? Maybe, I'm not sure.
Helium is very expensive, hydrogen is expensive too, methane requires a double volume for the same lift, hot air requires power to be heated and the lift is very poor, steam is difficult to be obtained and maintained as steam. Helium and hydrogen have higher loss rates because the molecules are small and the balloon has to be often refilled.
What do you achieve by using a balloon? The ability to hover with no power. If the airship is enough buoyant, you can hover with low power using vertical propellers.
I think the best option is methane, 75% buoyancy and, if you can manage with 200W at most, human powered. If you need more than 200W, you need an engine.

The way I'm designing the "Mary Poppins" is to design it to statically lift its own deadweight plus a crew of two, plus 10 - 15% more pounds, just for a little design cushion. This computation alone is not really easy, no way could I design into it, at this early stage, how many lift pounds will come from the "aerodynamicness" of its aerobody, no way whatsoever.

   I'll build in some lift by shaping the body nicely, here and there, and see how much aerodynamic lift I get after I fly the prototype. That bonus lift will be a gift and it will be the payload capability.

    DL, Dynamic LIft is a function of speed and power and the Poppins will have little of either so I know it's not worth the effort to calculate how much lift 5 kilowatts will produce in an experimental aerobody shape. Besides, there's no software for calcing aerobody lift, is there? Just for wing airfoil sections.

    One of my personal design priorities is to be able to turn the motor[s] off and spend the night in the sky, drifting over oceans and continents under the stars. That means I must have a guaranteed 100% static lift capability plus.

    On flights where I want to deliver cargo and land heavy, I can vent the lift bags to lose static lift and hopefully, replace it with dynamic lift, if I buy enough horsepower! But hp is something I no longer want to spend good money on.

So, for the present, I'm expecting I'll only get 10 % dynamic lift and 90 static.

Plan on flying, make a flight plan now.

Pasted from another post:

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So, perhapses the question is the wrong one. Maybe the question should be about the relationship between empty weight and buoyancy. 

I may have to take another shot at this one. 

this whole entire forum is a learning experiece to me...

not to sound like a complete idiot, but how do you achieve aerodynamic lift on an airship? i thought the whole thing was supposed to be lifted by bouyancy, but, apparantly, you don't have to lift your entire ship's weight on lifting gas alone. which is nice. Didn't know you had options...

This is a learning experience for me as well, as I hope it is for everyone. 

Anything that moves through the air can generate aerodynamic lift. All that is needed is a shape that is flatter on the bottom than on top, or a long shape that moves with its nose raised at an angle. 

There is actually another way to generate lift known as the magnus effect. A sphere moving through the air with a backspin also creates lift. There was an airship design based on this concept.

How much do you think the blimp will weight deflated? Think about a blimp for 1-2 people. Take into account the balloon, the strength elements, the seat(s), the mooring stuff, the engine if needed, the propulsion and steering devices.
Much of the stuff weights more if a bigger balloon is needed.
Ok, you want to lift 150kg =  330 lb.
Methane lifts 0.57kg/m^3 = 0.57oz/ft^3. (1kg/m^3=0.998847oz/ft^3)
You need 263m^3 = 9287 ft^3 of methane only for the payload
How heavy do you think a blimp containing ONLY (I know, I seem to be wrong) the gas needed to lift the payload would be? Think about a balloon of 16m = 53ft length and 5m = 16-17ft diameter. Deflate it and lift it by hand. How heavy is it? Can you? Think about the curtain inside the blimp you need to spread along the balloon the weight of the gondola. Think about the net you have over the balloon with the same purpose, to spread the weight. Think about the gondola, the engine, the propulsion and steering devices. Think about things you forgot about. This is not a joke. I'm afraid it would weight 60-100kg = 130-220 lb. That means a bigger and heavier blimp. No time to compute, just guessing: the blimp will reach 150kg = 330 lb, I mean the payload value. The balloon I guess will be 25m = 82 ft length and 6m = 19-20 ft diameter, about 590 m^3 = 20800 ft^3, lifting 336kg = 742 lb, the payload and its weight.
How buoyant should our airship be?
How much power do I accept to use to keep it in place? 1000W? I don't know how much you can lift using 1000W and 2 propellers limited to, let's say, 2m = 6-7 ft diameter? I have no idea if you can lift 4-5 kg = 10 lb or 45kg = 100 lb using 2 propellers of 2m = 6-7 ft diameter and 1000W.
The acceptable buoyancy depends on the dynamic lift you can get using helicopter-like propellers and a reasonable power. Think also about how annoying could be the noise and the airflow generated by a hovering helicopter, this limits somewhat the power too. Would you like small things and dust fly away when you hover?
I'm afraid if you want to dynamically lift 45kg = 100 lb with two 2m = 6-7 ft diameter propellers, you get an airflow of about 7.5m/s = 24-25 ft/s, rather annoying and you might need 6000-7000W I guess.

Your math looks about right. I use excel to do most of my math these days. 

I don't intend to lift any weight in a manner similar to a helicopter. I propose an aerostat, not a a helistat. The lift will be created by the movement of the airship's body through the air. I think buoyancy should only carry a large fraction of the empty weight

I estimate total mass at about 350 kg with 7.8 kw of generator capacity. Empty, that is about 191 kg. That is 350 kg minus 109 kg for people and 50 kg cargo. 

Envelope mass: 50 kg
Engine mass: 95 kg
Tail and tarp: 19 kg
gondola: 27 kg

We certainly need a complete weight roster. 

Here is my excel file. Balloon weight is based on .2 kg per m^2 of surface area. There is no direct calculation for a curtain or net. But, there is a tarp included in the calculation which is designed to shield the craft from the elements when it is staked down away from home.  

Attachment: project excel aero test3.xls (120.0KB)

I believe we've all acknowledged that it would be good for the ship, when it is landing, to be heavy and to land like an HTA ship so that no ground crew has to grab ropes to hold it down.

However, I personally want to be able to turn off the motor and float when I am on a pleasure cruise. That means that I often will want 100% buoyancy. But I don't want to pay a ground crew when I land. I will have to vent 25% of my lift gas and get heavy in order to land without help. I will be using hydrogen, even if I have to leave the country to fly it. So I won't have to buy helium, I can electrolyze H2 anywhere overnight with the output of my electrical generator.

When I go on a day trip, I will want to save my H2 and use it the next day. I will want to lift 25% of my total weight for that flight with aerodynamic lift. If my total weight [LOW, or Lift Off Weight, as nasa calls it], is 800 pounds, then 200 pounds must be lifted by the aerodynamics of my particular aerobody. How can I calculate if I will get that much lift from my aerobody design? 

I can't. A college aerospace department with high-end software could make a ballpark estimate, if they wanted to, but they don't want to spend the time on my design, they have their own designs they can't figure out ;-]

So, for the prototype SkyBoat, I'll make it 100% buoyant and after it flies I'll find out how much extra weight, payload, it can take-off with. Then I will know how much power-to-static lift I can afford. If I'm unhappy with that percentage I might increase the generator-size to get more power-lift. It'll be heavier but every ship is designed with extra lift for research error, etc.

In short, aerodynamic-to-static-lift ratio is answerable by test flying the prototype  8-]  Here's to successful, and soon, First Flights!

How will you maintain the ships shape after blowing off 25% of your gas? Will you use balloonettes, and if so, are you OK with hydrogen and air being held in the same envelope?

Airships are so big compared to their weight, getting aerodynamic lift is easy. You can get as much lift as you want at almost any speed. The problem is drag. If you want to learn something about your design, assume that you get a specified amount of lift and then calculate the induced drag that results. This will be in addition to the normal drag any airship will experience. 

To do this you need your wing area (as seen from above), your wing span, your speed, air density, and the lift you want (in newtons). 

• Then you calculate your aspect ratio ((wing span^2)/wing area) 
• and your lift coefficient (Lift/(.5*density*(speed^2)*wing area)) 
• and then plug those numbers in to find the induced drag coefficient (Lift coefficient^2) / (pi * aspect ratio * e). 
• Then you plug the induced drag coefficient into the drag equation (.5*density*speed^2*induced drag coefficient*wing area)

Remember, this is only induced drag. I have much more complicated estimations of drag in my design excel file. 

http://spreadsheets.google.com/ccc?key=0AiiSZl31BXvNdEFDVlZfdUdLY3J2enBVYWVkT1BTdkE&hl=en

This is an editable docs file.

Attachment: drag.xls (11.0KB)

Good stuff! You got a big thumbs up from me on this post!

Might be a good idea to cross-post it in the xCel topic and perhaps even start a new topic completely for "Airship Math"  or something like that, as a reference place.

That way it'll be locatable for future usage by the hordes of builders who'll flock here for the Smallblimps Prize  {8-O>

Oh, to answer the question about maintaining shape after blowing-off hydrogen, I spose I might incorporate a few larger H2 "buoyancy bags" above the one hundred smaller "lift bags" stuffed into the envelope.

The envelope would be made of semi-rigid, plastic shrink-wrap to keep envelope costs minimal. [$200 for 3,000 square feet!] It's kinda stiff stuff and I'm designing sufficient aluminum spars to support its shape.

Buy a new flight bag, we're going UP

Prospecting, like for precious metals? Interesting.. De Beers uses a Zeppelin NT for searching for diamonds. 

Please refer to my Lifting Gas Article # 54 ( third page). The reason I said to pick a design point at an altitude is that if you are neutrally buoyant on the ground then you won't get much altitude. If you try by aerodynamic lift alone for a reasonably big ship the climb rate will be snails pace. If you have a takeoff obstacle to clear, you're automatically in trouble. Additionally, with a low climb rate your runway requirements are horrific.

The other problems with aerodynamic lift:

1. Increasing alpha (angle of attack with respect to the relative wind) increases side force with respect to the long axis. You already  need to design for column buckling along the long axis, side forces jacks up strength (Structural weight) requirements.

2. Just like with a wing, higher alphas shift the center of pressure and is destabilizing which forces you to have larger fins. At low speeds ( read that - when the fins ain't working in spite of their size)  with gusts near the ground you're going to kiss the barn door.

3. The entire economy of an airship is due to getting rid of  lift and induced drag caused by it. Here's a calculation for you. Calculate the amount of thrust to get a 747 just to move forward at a reference speed assuming no lift. Then do the same calculation with lift and induced drag. Check the change in fuel usage. That's why there are Zeppelin records on cargo that have yet to be broken.

4. Like a wing you can only crank in so much alpha and you get the equivalent of a stall. When the flow separates from the top surface you can't get enough control. All of this introduces bending moments in a relatively long object. This ain't good for the home team.

The Zeppelins used alpha. But just as small trim control. Not as a major lift producer.

The wing loading would be so low that alpha would be very small. I would guess it would be less than 1 degree. The reason airships are more fuel efficient is (mostly) that they are slow, not that they don't produce lift. Increased lift leads to smaller airships with higher cruise speeds. 

I'm not sure a 747 and a blimp comparison makes much sense. They likely have no flight speed overlap.

We are talking about a "hybrid" airship. I believe the aspect ratio we need to be looking at is around .4 

Stall should be impossible to achieve at cruise speed. There won't be enough control authority to get the nose up that high. 

I believe the stability problem of increased alpha is due mostly to the shape of the nose. If you look at terminal ballistics, you will see that flat nose bullets drive in straight while sharp nose bullets always flip over (if they have enough travel length). I believe a flat nosed airship will exhibit more pitch stability than a sharp nosed craft. 

To me, hybrid means electric and combustion drive, like the cars have. We describe airships as either lta or hta. For the lta, lift is not necessary, just point in the direction you want to go and hit the throttle. For hta, we are using lifting bodies, like the space shuttle. I consider the blimps and zeps as lta. They float, and altitude is controlled by bouyancy as much as motor power.
For the hta, these are inflated gliders. Both are great, I'm not sure what the project will develop. The hta will require less storage and less helium, it just won't float. As long as you have speed and/or altitude, you are flying.
To withstand the forces and loads of hta flight, an airship would have to be quite sturdy. Inflated airplanes have been done, construction costs probably are not too bad, storage ok, safety is questionable.
Lta is either going to be the blimp type or the zep type. The blimp type stores easy enough, conventional construction, any propulsion, all we have to do is figure out the lifting gas situation.
Then there is the zep. It's always big, and is probably the most expensive to build. I like it because I believe it has the most opportunity to be really different, use new technologies. A plastic monocoque solar powered airship hasn't been done, as far as I know. Once it's built, it's flying, why stop.

Calling a hybrid airship an HTA craft is like calling a hybrid car an electric car. Sure it uses electricity, but not exclusively. Technically all airships operate with less than neutral buoyancy. A hybrid airship just makes that heaviness significant. But it doesn't really matter what we call it. What matters is that everyone knows what were talking about. http://en.wikipedia.org/wiki/Hybrid_airship

Let's look at this point by point.

The wing loading would be so low that alpha would be very small. I would guess it would be less than 1 degree.

-dude6935

I believe that this is highly dependent on the final airship shape. On the net there are some CFD files that show flow profiles at angle of attack.

The reason airships are more fuel efficient is (mostly) that they are slow, not that they don't produce lift. Increased lift leads to smaller airships with higher cruise speeds.

-dude6935

Here I disagree ( respectfully). Please refer to my Lifting Gas Article # 54 ( third page). The entire reason for rigid ships with the additional structural weight penalty is they hold their aerodynamic shape and therefore are faster due to lower profile drag. I understand you're talking about hybrid ships and I am not. If you fly a blimp at high angle of attack (AOA) the deformation will be greater than for a rigid. Therefore the drag will skyrocket.

I'm not sure a 747 and a blimp comparison makes much sense. They likely have no flight speed overlap.

-dude6935

With respect to fuel consumption, the principle is the same. You're original question had to do with finding a operating design point between lift and buoyancy. The more you lean toward lift the more you increase operating costs due to generating that lift.

We are talking about a "hybrid" airship. I believe the aspect ratio we need to be looking at is around .4 
Stall should be impossible to achieve at cruise speed. There won't be enough control authority to get the nose up that high.

-dude6935

Control authority limits or not, the effects of gusts have to be factored in. Several of the early crashes happened because limited control in gusty weather and poor structures design. The assumption made on early airships was if you ( during test flights) slammed the control surface to one extreme and then quickly to the other and designed the structure to handle that load then you could handle all weather loads. Many documented crashes demonstrate that this reasoning is faulty. Additionally gusts could easily put you at an AOA that wasn't intended.

I believe the stability problem of increased alpha is due mostly to the shape of the nose. If you look at terminal ballistics, you will see that flat nose bullets drive in straight while sharp nose bullets always flip over (if they have enough travel length). I believe a flat nosed airship will exhibit more pitch stability than a sharp nosed craft. 

-dude6935

Any shape that gets you lift will have a Cp (center of pressure ) change with AOA. A flat nosed airship will not only have a high coefficient of drag, the sharp edge will cause a flow separation that will augment a Cp shift and increase drag.
Bullets are under a different flow regime due to spinning at a high rate about their long axis, which we hope an airship is not doing.

Thanks,

Swampie777

Even if wing loading is dependent on shape, we get to stipulate the shape.

O.K.
I was just making some general observations. Like I said, I'm developing rigid airships. Not hybrid. I agree with navigaitor, stopping in mid air has it's perks. I see photogrammetry from a still ship with very low vibration as an application plus. ( Not to mention hovering just over a lake with a fishing pole...... " Honey, I caught a catfish, add another 500 cubic feet to the envelope,please.) [ Figure out the size of the catfish from my other posts!]

They tie down airplanes at airports for good reasons. Take that same small plane, increase it's size holding on to the same weight ( change in materials from airplane to hybrid) with the volume change that goes with going to light wing loading.  Then try to take off in a 10 knot crosswind. A hybrid does not just give you the advantages of both, it also gives some of the disavantages of both.

If you say "I'm just going to fly it on calm days"... then you'll be flying about 3 days out of the year.

Cheers!

Swampie777