Drag Reduction in Thrusters

[Guest TOSGcentral]

I have just answered a PM on Thruster Drag reduction. I am frequently asked this question so I am also putting my reply here for general information.

 

Hi,

 

 

Just got back after a couple of days away – hence delay in replying.

 

 

There are a number of things that you can do, most of which are drag reduction devices. None of them compromise the aircraft’s safety per se if the devices are fitted correctly.

 

 

However, whether they are legal or not (ie compliant with CAO 95.25 certification for the type) may be another matter. If the aircraft, either now or potentially in the future, is to be used in a flying school then any work MUST be signed off by an RAAus L2 Maintenance Authority holder. Equally – illegally modifying a certified aircraft may leave you vulnerable to negation of insurance cover and criminal proceeding for flying an illegal aircraft!

 

 

The following is a list of what you can do, along with some additional notes.

 

 

• You require a R582 DCDI engine for best results although the anti drag devices will improve any Thruster.
   
   
  
• The engine should be correctly propped and pitched. Excellent results will be obtained by a 68†Brolga 3 Blade on 16 or 17 degree pitch blocks (R582 motor only).
   
   
  
• Aerofoil section lift struts. These will give you 5-6kts extra straight away. You must preserve the original 95.25 lift strut attachment fittings and on initial assembly the aircraft must be put into a spirit levelled flying attitude and the struts adjusted accurately via water levels prior to drilling (when correct the front and back struts in combination look a little odd to the eye).
   
   
  
• Full cockpit enclosure. This involves fitting the T500 roof panel and rear bulkhead panel. The latter will only fit the T300/T500 pod design and not the original Gemini or TST pods that do not have the lower rear support bulkhead. You then fit doors. This gives you less drag and more comfort. The flat factory doors are a bit ‘squeezy’ across the shoulders for larger pilots. The new certified bulged doors and frames produced by TOSG are more practical.
   
   
  

 

(NOTE – putting on the roof and rear panels but not fitting doors, and if you have no rear fuselage enclosure, adds substantially to the drag and deterioration of both climb rate and glide angle.

 

• Pod side flares. These are GRP mouldings that fit around the strut cluster at the axle carry beam and give you a few extra knots. The latest TOSG pods have these moulded into the shell of the pod itself.
   
   
  
• Anti Interference drag fairing. This fills in the gap between the roof of the cockpit and the underside of the wing. TOSG will have two legal versions of these available in the new year.
   
   
  
• Main Wheel Spats. These reduce drag a little but their main function is to keep the underside of the wing clean and therefore more efficient.
   
   
  
• Engine support strut fairings. These are a total waste of time due to the variable spiralling prop wash according to power setting – do not bother!
   
   
  
• Rear fuselage enclosure. Very desireable and reduces profile drag while increasing skin friction drag. The latter may be reduced by three good coats of ArmorAll. A few wrinkles here though. The only legal rear enclosure is the T500 and you cannot put the stringer assembly on the Gemini, TST or T300 as they do not have long enough forward booms to counter balance the increased aft weight. A lightweight rear enclosure has been trialled successfully and TOSG will be producing kits for these in the coming year.
   
   
  
• Reduced Angle of incidence bracket. Although primarily aimed at taming landing characteristics, we are expecting to also get some high end speed benefits. But these brackets are presently under development and flight testing so nothing can be said just yet.
   
   
  

 

 

SOME WARNINGS. It is not too difficult to make your Thruster go faster and/or more economically. Both the Flying Fox and Eagle Super Thrusters have been clocked on police radar at 85 knots in level flight – which is 5 knots above their Vne. BUT – beware! 70 knots may be expected for cruise on a well optioned Thruster but there are other considerations as well.

 

 

While drag reduction devices may be legal they are doing nothing about changing the design aerodynamics of the aircraft (torsional and bending loads). 70 knots is the max turbulence penetration speed of the Thruster, but the Super Thrusters are capable of exceeding Vne – you have to be more careful of your piloting!

 

 

Also they glide much better and are heavier so they need more runway room than conventional Thrusters and your circuit positioning also has to adjust to compensate.

 

 

To counter some of this TOSG is currently developing a 5 stage flapperon system for them that will slow them up somewhat and return them to normal Thruster operating distances as well as a reflex flap setting which will improve top end cruise.

 

 

In addition the significant increase of airspeed operating range requires additional trimming on what can be a ‘heavy’ aircraft at speed. So we are also developing both rudder and elevator aerodynamic, cockpit controlled trim tabs to make thing a bit easier.

 

 

Aye

 

 

Tony