Thread: Discussion RadJet from the KING!
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Old Sep 06, 2012, 03:21 AM
Simon1013 is offline
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I just bought a Radjet 800 (not flown yet) and have been reading this thread with interest. Lots of really useful suff on this thread although it takes a while to get through.

I did read with interest the discussion on the 'death spin' that the Radjet can get into. As well as being an RC'er I am also a 1:1 flying instructor and an aerospace engineer.

In the 1:1 world, there is a standard recovery technique which works for 95% of aircraft which is:-

Power to idle
FULL opposite rudder
Move control stick forward

Hold until the spin stops, centralise the controls and recover from the dive.

Clearly this doesn't work for the Radjet.

However, it should be noted that there are aircraft that are cleared with a recovery technique that differs from the standard due to unusual spin behaviour. Some aircraft (PA-28 for example) are not cleared to spin at all. All recoveries include the use of rudder.

There are many things that can affect spin behaviour; a couple of dominant characteristics are:-

Center of Gravity - has a significant effect on spin and recovery behaviour. A mor rearward c of g will make an aircraft easier to spin, and more inclined to stay in the spin. I noticed that the c of g being recommended on this formum is substantially further rearward that the designer recommends. Maybe the spin behaviour was a consideration when they were selecting the design c of g for this aircraft.

Wing Planform
Delta type have a center of pressure movement during deep stalls very different to what you get with a conventional 'straight' wing. It is likely that the 'death spin' is caused by the aircraft getting into a very high angle of attack situation, with very low airspeed and some yaw component. For a delta wing aircraft this can be unrecoverable (again exacerbated by rearward c of g).

Distribution of mass.
Aircraft with long noses etc (like the Radjet) have a tendency for any spin to flatten out as the rate of rotation increases which increases the depth of stall and again delays recovery.

Fin surface area and distance from c of g
Aircraft with a small vertical tailplane, only a short distance from the center of gravity (like the Radjet), due to the limited aerodynamic effect and the small lever arm having little direction-restoring effect. the rearward c of g exacerbates this again.

So having read the posts etc, although the Radjet does seem to be a great aircraft, it does have all the design features that may make it prone to an unrecoverable spin.

In the full-size certification world, an aircraft that has difficult recovering from a spin will typically have some additional fin area added as far back as possible (usually below the empenage). there are many example of this in 1:1 world. I noticed that somebody did try this earlier in the thread and reported good results. The other easy change would be to use a more forward c of g, but this would be at the expense of other handling traits that benefit from the more rearward position (catch-22).

There is little you can do about the distribution of mass - maybe remove the tip-tanks and keep the nose nice and light (although I appreciate the desire to beef-up this bit to help with 'arrivals-gone-wrong' may help a little.

One poster mentioned about leaving the controls neutral and using power to get out of the condition. This is very unconventional from a 1:1 point of view but when you have an aircraft with such a huge Thrust:Weight ratio you can pretty much throw conventional aerodynamic theory away. Using power to get out of the 'death spin' has been proven to work well for a very overpowered aircraft.

One thing that I haven't seen discussed in this forum is 2D thrust vectoring. I would imagine that 2D thrust vectoring in the pitch-sense would work well on this aircraft - and would probably provide enough pitch authority at low/zero airspeed to recover from the 'deal spin'.
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Last edited by Simon1013; Sep 06, 2012 at 04:10 AM.
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