Vol. 5 November 2007
Velocity's Quarterly Whenever-We-Get-Around-To-It Newsletter
   
 
This is a ListBox

The Velocity BLT Special
Scott Baker

Project Hot Rod
Duane Swing & Scott Swing

New Toe-Brake Pedals
Ken Baker

The Mishler Nose-Wheel Lock System
Ken Mishler

 

 

Header

 

Project Hot Rod
Duane Swing and Scott Swing

What can be done to make the Velocity XL compete with the Lancair IV-P in true air speed? The answer is the Velocity Hot Rod in progress at the Service Center. What we are doing is mating an SE wing and canard (as modified for the rocket racing aircraft) to an XL fuselage and pushing the thing forward using a twin turbo, twin intercooled, balanced and blueprinted Continental TSIO-550 engine developing 400 horsepower at 2700 RPM. Performance Engines is putting together this engine for us. This engine will be flow-balanced and internal moving parts will be dynamically balanced to within 1 gram of opposing parts. We actually don’t know exactly how fast it will go but probably faster than the Lancair at any altitude. The shorter wing and consequent higher wing loading will add about 15 knots to our indicated speeds and the additional horsepower will add about another 10 knots at the higher altitudes. We expect to see around 285 knots (327 mph) at around 26,000 to 27,000 feet. This turbo set-up, without the burden of pressurizing the cabin, is capable of cruising at 30,000 feet quite easily. It will take at least another year for the actual performance numbers to be available as the owner is a professor in California and can only be here to work on the airplane from mid May through mid August.

-Duane Swing

Push-Rod Aileron Control System
Scott Swing

All of our initial design considerations in building the Hot Rod revolved around speed. How can we squeeze as much speed as possible out of off-the-shelf parts? Many of you have seen the press that we've been getting lately with our TXL. In a 65% cruise configuration, we've seen true airspeeds at FL250 greater than 250ktas. Despite these blazing speeds, you've got to remember that these numbers are true airspeeds at altitude. We do not exceed our established Vne. So I'm sure you're now asking yourself the question: "So how do you increase the Vne on the Hot Rod?" Before I go on, I need all of you to understand something: We are professionals, and this is an R&D project of the Velocity Skunkworks. Do not try this at home on your Velocity. There is more to raising an aircraft's Vne than just changing around its control system. With that said, let's go on.

Now, establishing the Vne for an aircraft is a complex subject. It involves lots of design theoreticals, and lots of application practicals. The two major factors in determining a Vne are structural load, and aerodynamic flutter. (Well, I guess there's only one really important factor: make sure things don't fall apart.) We've covered the bases in the structural loading department through a series of wing structure and spar design changes that I won't go into here, but the airfoil remains unchanged. Flutter, however, is a more complex subject. It's sort of the Black Art of aerodynamic study. Flutter, in an oversimplified nutshell, is the formation of a destructive feedback oscillation when a lifting surface bends under load. This ugly phenomenon rears its head most prominently on control surfaces. This can be counteracted a number of ways- through proper control system balance, tighter control system tolerances, and specially designed control surfaces. (For those at home: 'specially designed' does not mean just making it stronger. Destructive harmonics can occur in just about any structure, no matter how strong, if the right frequency is struck.)

Those of you building at home know that the aileron system is controlled through a series of torque tubes extending from the control stick, through the keel, and stopping at the keel plate. From here, the firewall is pierced and control is accomplished through push/pull cables that attach, in turn, to another set of torque tubes at the wing roots. The push/pull cables are a great solution for the control system in a normally configured Velocity. These cables are readily available, low in cost, and provide a neat tucked-away solution for aileron control. Unfortunately, even the best cables have a bit of play in them. Control system play is your worst enemy in designing for speed. Because of this deficiency, we decided to design a complete push rod & torque tube system for the ailerons. The tightness and static balance of this new system is incredible.   Except for the actual push rods themselves that substitute the older push/pull cables, we used only off-the-shelf parts from our different models and existing options.  We used bell-cranks from the SUV yoke control system and the big aluminum angle from the new toe brake system.  This prototype system was, of course, built on an RG which made it a little more difficult to configure because of the transverse bulkheads. This is basically how it works:

From the bell-crank at the whale-tail end of the keel there are rod ends at each end of steel push rods extending out to 90 degree bell-cranks mounted to aluminum angles that are mounted to the gear bulkheads.  There is a hole in the gear bulkhead just inboard of the transverse bulkhead and just below the horizontal bulkhead.  The hole in the firewall is also just under the horizontal and inboard of the transverse.  It takes at least a 1” hole but by the time you get the full motion, it is a little elongated as you can see from the pictures.  In order to get the same travel up and down with the aileron, you still need to attach the rear push rod to the aileron bell-crank in the engine compartment 90 degrees to each other.  If you don’t you will get differential movement of the ailerons.  You can actually get more than 2.5 inches of movement each way but you still limit the travel with the stick in the hole.  It would be very easy to install stops in the system at the bel-cranks mounted on the gear bulkhead. 

The first thing you notice when operating the system is how smooth and tight it is.  There is no slop at all  which should lessen the chance for flutter at higher speeds.   Since the pushrods do not isolate or dampen the movements like the cables do, we may have to deal with the inertia of the system and aileron weight.  I think that in flight under aerodynamic load, that feel should go away.  Also, the roll trim system with slightly heavier springs should also reduce the effect.  Unless we retrofit another airplane with the system, we will not be testing the system until this plane flies mid next year. 

Basically we removed the cables, clamps, and the cable brackets for the aileron wells and added 2 bell-cranks, 4 rod ends, 4 push rods with inserts, and two bell-crank brackets.  The push rod system is definitely more expensive than the cable set up but we will see if it is worth it.

Keep in mind that this is the maiden installation of this kind on a Velocity, and even mid-way through we discovered a number of ways to clean-up the installation. We think that in the next installation we will extend the keel-plate bellcrank aft of the firewall through two bearings. This way the push-rod 90 degree bellcranks can be mounted on the firewall and the entire assembly is contained in the engine compartment. The R&D push-rod system, as it exists right now, will force us to make some funky "stealth fighter" style angles on the false-bulkhead baggage deck. Regardless we're confident that this will be a great system for the new Hot Rod.

-Scott Swing