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b_kimoun

The Speed Trim

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This new topic is intended to respond to 2 messages posted both in "The trim bug...issue topic". The first message posted by :DJ : "Posted 22January 2011 - 04:16 PMDJ, on 30December 2010 - 08:28 PM, said:Afterreading all the comments here I have to jump in. What Ireally don't like in FS is that in real flying a plane is trimmed for a speed,therefore if the plane is trimmed straight and level and you bring the powerback, the nose will drop to maintain the speed. In reverse, if pwr is increasedthe nose will go up. In all versions of FS and other sims the aircraft willtend to accelerate or decelerate vice nose going up or down. It is afondamental flaw in FS. I have beenflying for 25 years (air Force and Airlines) and while I was an Air Forceinstructor this was always a struggle to make student pilots understand thatconcept. I could continue for pages on the aerodynamics lessons but I thinkthis is enough. Happyflying in the New Year,"And the second message posted by audiohavoc supporting the above message :"Exactly! Itook to the controls of a real aircraft for the first time yesterday and thiswas one of the most important differences I noticed between FSX and realflying."-------------------------------------------------------------------------------------In fact, it appears that , as a matter of trim, not all aircrafts respond to speed change the way DJ and audiohavoc are suggesting. Only aircrafts equipped with speed trim system seem to have this ability.The following is what I read about this subject:The speed trimsystem is a speed stability augmentation system designed to improve flightcharacteristics during operations with a low gross weight, aft center ofgravity and high thrust when the autopilot is not engaged. The purpose of theSTS is to return the airplane to a trimmed speed by commanding the stabilizerin a direction opposite the speed change. The STS monitors inputs of stabilizerposition, thrust lever position, airspeed and vertical speed and then trims thestabilizer using the autopilot stabilizer trim. As the airplane speed increasesor decreases from the trimmed speed, the stabilizer is commanded in thedirection to return the airplane to the trimmed speed. This increases controlcolumn forces to force the airplane to return to the trimmed speed. As theairplane returns to the trimmed speed, the STS commanded stabilizer movement isremoved.

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I was flying a Piper Warrior II and it does not have a "speed trim system" yet it if trimmed for level flight, increasing the power will cause the aircraft to climb and decreasing the power will cause the aircraft to descend.

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I was flying a Piper Warrior II and it does not have a "speed trim system" yet it if trimmed for level flight, increasing the power will cause the aircraft to climb and decreasing the power will cause the aircraft to descend.
What else do you expect it to do? A trimmed aircraft (without autopilot or or other system) is trimmed for speed not altitude. It will attempt to hold that speed by climbing or descending if the power is increased or decreased.Let it stabilise in the climb or descent and the spped well be very close to the trimmed speed in level flight.

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What else do you expect it to do? A trimmed aircraft (without autopilot or or other system) is trimmed for speed not altitude. It will attempt to hold that speed by climbing or descending if the power is increased or decreased.Let it stabilise in the climb or descent and the spped well be very close to the trimmed speed in level flight.
That is exactly what I am stating to the original poster and what I expect it to do. There is no "speed trim system" required for this to occur.

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That is exactly what I am stating to the original poster and what I expect it to do. There is no "speed trim system" required for this to occur.
Sorry, I misundersttod your point :(

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Oh Boy... this is going to show my age;I first flight simulator from subLOGIC (before being purchased by MicroSoft) was like that... get the aircraft trimmed for straight and level flight at a power setting and increase power it would start to gain airspeed, then climb with the resultant lose of airspeed. Decrease power and it would start to slow then descend with the resultant increase in airspeed.If memory serves me correctly (which in itself is a rarity there days) this feature was "dumbed" out to make it easier for a great number of people to simulate flying.

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What else do you expect it to do? A trimmed aircraft (without autopilot or or other system) is trimmed for speed not altitude. It will attempt to hold that speed by climbing or descending if the power is increased or decreased.Let it stabilise in the climb or descent and the spped well be very close to the trimmed speed in level flight.
Exactly. Once the trim speed is disturbed (let's say thrust reduced ), a stable aircraft will swing (in altitude, with loss in altitude) about the trim speed. That is, in the descent phase, it accelerates and starts to level off ( with loss of altitude of course) when approachnig the trim speed and at this stage a short ascent phase starts, then again a descent phase commences due to loss of speed etc. This means that one can not have again a trim situation unless he adjusts thrust to have the previous speed (the true trim speed), or adjusts the trimming surface position. The speed stability augmentation system that I wrote about above uses this last alternative.

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And what's about trimming an aircraft neither for speed or altitude but for attitude?Wolfgang

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And what's about trimming an aircraft neither for speed or altitude but for attitude?Wolfgang
Sorry Wolfgang, frankly, I have no answer to your question. But keep in mind, that a trimmed level flight means Sum(Lifts) = 0, and Sum (pitching moments) = 0.In my above post, I mean that reducing power impacts the aircraft speed and consequently makes the dynamic pressure q decrease. q decreased means that the above equations are no more satisfied and the aircraft enters in a hunting process trying to recover the initial equilibrium situation, that is the condition that makes again Sum(Lifts) = 0, and Sum (pitching moments) = 0. If nothing is done, the hunting process continuous until crash (loss of altitude and the equivalent potential energy gained trying unsuccesfully to accelerate and maintain the aircraft at the trim speed).

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