Take-off and Landing Requirements

We have decided the take-off and landing surface will be small fields. So I want to try and put some numbers to it. This is pretty much short take-off and landing (STOL). We may be able to add a V (for vertical) in there somewhere at some point, especially for landing. But for now, what does that mean?

"STOL (Short Take 0ff and Landing). STOL performance of an aircraft is the ability of aircraft to take off and clear a 50-foot obstruction in a distance of 1,500 feet from beginning the takeoff run. It must also be able to stop within 1,500 feet after crossing a 50-foot obstacle on landing." Dictionary of Aeronautical Terms

We can compress that distance into 100ft or 500 ft or whatever. We just need to figure out what performance we want. I think the 50 foot clearance height is a good height to use.   

Maybe the STOL specification can wait til we have a FirstFlight take-off run of any length. In other words, that spec might belong in the "Nice to Have" bag. Maybe tweak it for the commercial version.

 We won't know what it is till we get speed results, since power is a major part of what STOL depends on. I know aerodynamic lift figgers into it but less so than power.

Hybrid airships have good STOL anyway due to their static lift portion. Putting in a STOL spec now would force people to think of more power and that points automatically to gasoline propulsion, which we do not favor.

We want a practical airship. We need to have some performance requirements to make it practical, like being able to operate from the type of surface we plan to use.

Is this another item from the "Make a Wish" bag? 

We need to make ANY sort of flyable prototype before we can encumber the design with unknown parameters.

Except for the secret LockMart P-791, there's never been an HTA/LTA craft before. Where is the math formula gonna come from? Fairy dust?  Until after we have flown a prototype, or wind tunnel-tested a professionally-done model, specifications are just an exercise in creative fairy tales.

I don't agree. As I have said so many times, virtually all airships operate HTA. 

But sure this can be a wish list. Just because it is a "requirement" doesn't mean we are going to get it. 

Airships operate at HTA so they can land. A hot air (or free hydrogen) balloon starts off LTA, tied down. Occupants get on, it's still LTA, they cast off and fly away. To come down, they let hot air (lift) go and become HTA.
Airships operate at HTA so they don't rise higher than desired, use the motor and control surfaces to fly up and down. If an airship desires to free balloon, without power, and not descend, it can adjust ballast for neutral lift, or buoyancy. For a good controlled landing, getting heavy is recommended.

One of the problems the Zeppelins has was adiabatic heating. As they came down, the compression of the lift gas heated it up which increases lift just when you're trying to land. Not a big problem but one to remember.

Any thoughts on the number? We talked about the issue a good bit, but we didn't talk about what take-off length we are looking for.

So many of these questions get back to airplane design. If you want an aircraft that takes off and clears a 50 foot object in X number of feet, you just specified two things:

1. Engine acceleration for the mass of the airship - you have to get up to the takeoff speed to get off the ground in time to clear the object.

2. Coefficient of lift required to be generated by the lifting body at speed V. Generation of lift causes an acceleration upward!  The amount you're heavy by is the mass equivalent you have to accelerate upward by, before you get to the 50 object by your sideways motion.

This is a simple differential equation simulation. Once you decide X, then you size the hardware to make that happen.

Indeed. We need some parameter to size the engine, why not this one? If we want to operate from open fields, take-off distance is more important than cruising speed.

I got a D in differential equations so...

Any opinion on what would be a good value of X to shoot for?

You need to start by setting a percent HLA. If your craft weighs 245 lbs and you set as a goal that 80% of the lift comes from buoyancy, then you design to that.

Here comes the circular part of the design process. You don't know how much lift your craft can generate until you build an R/C model or a test article. ( You'll remember the Wrights built a glider first which allowed them to discover an error in the Smeaton equation, which led them to build a wind tunnel).

Next with known good data or estimates, write a sim ( I can do that) that simulates a takeoff. If it takes the sim takes  5000 feet to clear a 50 object, raise the percentage, and re-fly the sim.

Remember each time you raise the percentage, the envelope gets bigger and to keep the same weight, the engine gets smaller. So its a new problem each time, not just scaling one parameter to guess the endpoint. When you change the envelope to change the percent lift you also changed the envelope lift and drag.

It's more intertwined than a Louisiana family where all the cousins married each other.

It is intertwined, and from my work tonight, I have seen it can be very sensitive. 

We need to start with a reasonable coefficient of lift for an unsophisticated airship hull. I would start below .5. We also need to note that static prop efficiency will be very poor (assuming an unducted prop). The build I am running now is lifting 55kg aerodynamically with a wing area of 80 m^2. It is 65% buoyant. I am running 2.6 kw at 50% prop efficiency. I estimate take-off at under 10 mph in about 50 ft. Parasite drag is negligible at this speed. I have also reduced induced drag by 20% due to ground effect. I don't know whether or not that is a reasonable number. 

My gut tells me we can easily smoke the 500 ft short take-off, even if my numbers are way off.

Induced drag is canceled out in ground effect.

What alpha are you rotating to to achieve takeoff? (sneaky stability question.)

Additionally, you might want to increase your wing loading to take off at 20 mph.
This will save structure weight and will avoid every puff of wind kicking you off the ground.

One other issue, since we're talking takeoff design: Nav said he wanted a fore and aft prop to eliminate fins and control surfaces and use vectored thrust for directional control.

Since on a ultra-lite all weight is minimized, a consequence of this is that power is minimized since you picked the smallest power plant to help with keeping weight down.

If you require more than 5 degrees rotation to take off, your power plant needs to be sized to not only provide the control needed, but at the inclined control angle must have enough thrust left over to continue the climb.

I don't know the angle of attack. I would need to find similarly shaped airfoil data to estimate that. I threw together an airship shape in javafoil and it made .5 Cl at a little under 4 degrees alpha. But that should be regarded as a ballpark figure. 

How did you account for the aspect ratio?

I didn't in getting that AoA number. But ground effect will reduce induced drag. So the 2D AoA will be closer to the real world than usual. 

I found this for estimating maximum prop efficiency. http://www.jefflewis.net/aviation_theory-theo_prop_eff.html

What shape did you base your Cl and Cd on?

BTW, the jeff lewis site is interesting. Maybe we should  ping him for aero info.

I used the Van de Vooren symetrical airfoil on javafoil with increased thickness and trailed edge angle. http://www.mh-aerotools.de/airfoils/jf_applet.htm