REFLECTOR: speed brake ramblings

[Scott Derrick]

this is a classic example of complexity is bad, simple is good.

You make the brake so it can withstand a certain speed no matter what 
percentage of deployment is used.  You obey the speed limit just as you 
do for gear, flaps, etc...  Ideally the brake speed limit would be your 
airframe redline!

Anybody know the limit on the Mooney brakes..

I just thought of a simpler way of handling deployment at a speed above 
its limit.  Thinking can be bad!! :-)

Just have a spring loaded actuator that if the pressure on the 
brake(belly board) went above a certain set point it would compress the 
spring and close the brake to relieve the pressure.  If you deployed at 
a good speed and then accelerated it would just swing back in place as 
you went faster and faster.  Getting the spring adjusted right would be 
the tricky part, but their are very few failure modes that would be bad.

Scott


michalk wrote:

> Right.
> But in my last paragraph I addressed your point.  A DPDT switch would 
> be needed, increasing the complexity.  The speedbrake system might 
> behave something like this, using a DPDT switch, wired such that one 
> end reversed the polarity:
>
> case1: 0 to 75lbf: switch is engaged such that correct polarity is 
> applied.
>
> case2: 75 to 100lbf: switch disconnects power to speedbrake.  Of 
> course what happens if you want to retract?
>
> case3: over 100lbf, the switch reverses the polarity.  But what 
> happens if you are commanding a retraction?  It would go the wrong way.
>
> Like I said, the SPST is easiest, the DPDT auto-retract would take 
> more thought.
>
> Jim Sower wrote:
>
>> But it wouldn't prevent you from over speeding the speed brake if it 
>> were already deployed, or retract it automatically when limits were 
>> exceeded?
>>
>> michalk wrote:
>>
>>> The diode would allow you to retract the speedbreak if the cutout 
>>> switch were engaged.
>>>
>>> Jim Sower wrote:
>>>
>>>> <<flip the switch to extend the board, and let the cutout switch 
>>>> decide when to extend it some more What would happen if you were 
>>>> cruising at low IAS (say at 15k'), extended the board, dumped the 
>>>> nose and accelerated to some point beyond the limit?  Sounds like 
>>>> your scheme would prevent you from extending at excessive speed, 
>>>> but not be able to retract it if the speed became excessive after 
>>>> it was already extended.
>>>> Am I reading you right? ... Jim S.
>>>>
>>>> michalk wrote:
>>>>
>>>>> Incrementally extending...
>>>>>
>>>>> I was just thinking about this on your first post.
>>>>>
>>>>> It seems it would be quite simple to add a force actuated switch 
>>>>> that would not exceed some limit.
>>>>>
>>>>> I do not remember what the current actuators are rated for, but 
>>>>> let's assume they are 100lbs.
>>>>>
>>>>> Make a collar that fits around the keel end of the actuator, with 
>>>>> a spring with the appropriate K factor to move .25 inches when 
>>>>> 75lbs is applied.  Put a switch in series with the actuator, with 
>>>>> a diode so that the switch is bypassed if you want to retract 
>>>>> while the cutout switch is engaged.  Normal operation would bypass 
>>>>> the diode (no voltage drop), when the cutout switch is not engaged.
>>>>>
>>>>> Want to go down? just flip the switch to extend the board, and let 
>>>>> the cutout switch decide when to extend it some more.  Just don't 
>>>>> exceed the limits by increasing your airspeed.
>>>>>
>>>>> I suppose one could get fancier with a DPDT switch that would 
>>>>> retract it if the forces get too high, but I haven't put enough 
>>>>> brain cells into that problem.
>>>>>
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