robin-1 Some very rough ideas for a small blimp

4 March 2002

The following only makes whatever sense it does in the context of the parent directory and the discussion on the Small Blimps mailing list.

I did these sketches late at night.  But proper engineering for real flying machines requires a lot of sleep and clear vision.

Scope - Overall mechanical structure, prop, horizontal and vertical steering

Not shown here is how the keel is attached to the envelope.  Maybe there are two helium filled envelopes together in one outer net or blimp-shaped envelope. That is another department.

The key points proposed here are:
Carbon fibre windsurfer masts are available in lengths up to about 5.6 metres, and weigh about 2 kg.  Longer masts can probably be obtained for small sailboats. . 4.9 metre mast weighs 1.75 kg (3.9 pounds).   The envelope is likely to be longer than two such masts end-to-end, but that's fine, I think, with appropriate suspension to points forward and aft of the ends of the spars.
This has obvious advantages!  Rather than let the chair hang by gravity, which would require the frame to bend forwards over the pilot's head, I propose some kind of spring arrangement pushing the frame forward.  Then movement can be controlled by pulling on a lever, wheel or whatever to tighten a cable going to the rear keel spar to overcome the spring tension and move the frame backwards.  (In a dual keel approach, the frame can have a left and a right beam, with the pilot reclining between them, so this spring idea becomes redundant, since it would be fine to have the pilot's centre of gravity underneath the hinge point.  Still, I think some kind of spring return to centre would be a good idea.)

This system is bound to have complications. Here are a few I can see.
  1. Forward thrust at the prop tends to push the pilot seat frame forwards, which is the opposite of what is desired when climbing.  This is especially a problem with Plan B (diagram 3), because the full prop force pushes at the base of the seat frame.  In Plan A, only half the prop force acts on the bottom of the seat frame.  An intermediate Plan C is not illustrated - it has chain or belt drive to a horizontal axis pulley behind the pilot's seat and half-way up the seat frame.  From there, via a CV joint, a carbon fibre tube takes the drive to the prop, as in Plan B.
  2. Unless the seat frame angle is rigidly controlled, pilot movement fore and aft will affect prop elevation.
  3. Likewise any changes in ballast, which makes the positioning of ballast tanks trickier, if they are on the seat frame.
This tilting propeller arrangement is intended to give the blimp considerable vertical manoeuverability, without the need for any tailplane or horizontal fins behind the prop.   For this to work well, the propeller should be as close to the rear of the blimp as possible.  

It also would have the characteristic of keeping the pilot relatively vertical to the rest of the world, which is very different from what happens in flying machines other than ballooons.  To the extent that this occurs, then it would mean the pilot would tend to feel that the envelope is a device for being pointed upwards and downwards at will, in order that he or she may move up or down in space.  While the angular momentum of the rest of the craft is likely to be high, and likewise its air-resistance to being moved, the rest of the craft beyond the pilot, seat-frame and ballast there, is likely to be lighter than the pilot and seat-frame.  To the extent this is true, then this is not the pilot steering a larger craft, but the "craft" being a prosthesis - lighter than the pilot, with the unique function of providing mobility in three dimensions in free air, with silence, stability, efficiency and freedom even birds might dream of. 

Here are the diagrams:

Diagram 1 Overall plan 

Two carbon-fibre windsurfer masts to form a keel above the pilot, who sits on a frame which can hinge forwards and aft.
Pilot chair frame moves forwards and aft

Diagram 2 Pilot seat frame movement links to prop thrust vertical angle 

This shows the in-principle linkage - the next diagram shows two of the three ways of achieving the linkage and the power transmission from pedals to prop.

Prop thrust vertical angle aligns with pilot weight movement to cause blimp to dive or climb


Diagram 3 Two of three ways of linking to the prop 

Plan A shows a separation of the prop angle and the prop rotational linkage.  The angle is controlled by a horizontal carbon-fibre spar (blue) and the rotational linkage is by a belt, such as a toothed timing belt, which traverses and admittedly tortuous path from the pedals to the pulley on the prop shaft.  Careful attention is needed to the placement of guide pulleys so belt tension is unaffected by all possible movements.  This approach has some additional challenges with the need to rotate the prop left - right for steering.  Also, the prop will be light and fairly large, and we don't want it hitting the belts.  Maybe put the pulley forward of the prop spar instead of where I show it here, between the prop spar and the prop.  

Plan B is more BMWish than Jap-bikish!  To hell with the chains and belts!  Here, we need two constant velocity joints, because ordinary universal joints will cause trouble when the prop shaft and the pinion shaft (under the pilot's seat) are not aligned.  They may never be aligned, and the need to steer the prop left and right means there will be many times when they are not aligned.   We would like quite acute steering for the prop, but this is a challenge for any CV joint, or other flexible drive arrangement.   CV joints, at least like those found in the rear axles of Volkswagen vans, are like the opposite of ball bearings - the inner race will not rotate with respect to the outer, but it can swivel and slide forwards and backwards.  Yet in Plan B, we are relying on the joints not having any forwards or backwards movement.  These are rough plans of the "barely warming the oven" form, not even half-baked ideas yet.  Plan B is obviously more elegant, since we have the single long carbon fibre spar doing double duty - torsional drive for rotating the prop and longitudinal drive to control its vertical angle.  But this double duty means we need the spar to be stronger than otherwise.  On the other hand, in Plan A, the spar there needs to take the full force of the belt tension with which we are driving the prop - which will be a lot higher than the force the prop sends back to the pilot chair.

Plan C is a combination of the two, and is not illustrated.  Here, there is a belt and pulley arrangement to drive a horizontal shaft behind the pilot's seat. about level with the prop shaft.  Then there is a CV joint, and the rest is as per Plan B.  The advantage of this is that the full force of the prop's thrust meets the pilot chair halfway up, rather than at the bottom, as in Plan B.

I have not attempted to illustrate how the prop and rudder might twist on a vertical axis for steering.  One way to make the link at the top of the prop spar where it meets the rear keel spar is to join the two with a small piece of industrial rubber with fabric internal webbing.  This will be simple, strong and give enough flex for steering and tilting the prop.

Two ways of driving the prop - rotiationally and in terms of vertical thrust angle

Copyright 2002 Robin Whittle, but if you want to use it, let me know.  For the purposes of patent law, this should constitute publication.