Thrustline

Part 7

By Greg Gremminger

HTL, CLT, LTL? (High Prop Thrustline, Centerline Prop Thrust, Low Prop Thrustline)

So, if you have worked your way this far in this paper, I’m not sure you are anxious to start another technical subject.  But if you are a gyro geek like me, always hungry to learn something new, you might want to plunge into the following “blasphemous” subject.  If you have researched gyros very deeply at all, you have probably been exposed to all the emotional arguments and opinions on HTL, CLT and LTL.   You may even understand all the discussions about center of gravity, pitch moments, thrustlines, balance of static moments, etc.   You may have some strong opinions about what is best, CLT, LTL or HTL.  I don’t intend to get into all the technical derivations and customary arguments here.  There are just a couple of points I don’t think you may have been introduced to on the subject of prop thrustlines.

A couple of statements:  Magni gyros are HTL – High Prop Thrustlines – by a number of inches!  So are all the Magni “clones”.   By the popular static sum of moments analysis, this should mean that all these similar gyro configurations should be digging burning holes almost daily from static instability Power Pushovers (PPO, buntovers) and Pilot Induced Oscillations (PIO).  This isn’t happening!  For the dynamic pitch damping reasons discussed above.

Now here’s a second statement:    HTL, CLT and LTL is no longer an issue with “Big Tails Way Back”.   Neither HTL, CLT nor LTL have the important airframe pitch dynamic damping without an effective horizontal stabilizer.    Without a horizontal stabilizer, perfect CLT might be in balance, but it’s an unstable neutral balance, and any slight disturbance can start pitch oscillations that will not be automatically damped – the pilot has to do it.   And, perfect balance with CLT is almost impossible – don’t eat a big lunch or use the restroom; don’t use more than a couple inches of fuel in the tank!   Without an effective horizontal stabilizer, both HTL and LTL would easily diverge in higher or lower airspeed without pilot intervention – skilled and constant pilot intervention – sometimes subconscious, but still muscle and brain work!   With the incorporation of a good horizontal stabilizer, placed “way back” to multiply its effect on dynamic damping, close attention to prop thrustline is no longer a big concern.    Prop thrustline is even less of a concern with a bigger tail further way back.    If it were, certainly all the Magnis and Magni “clones” would be digging smoking holes.    Can we all say “pitch dynamic damping”?   This is what most producers outside the U.S. are doing – and they are breaking safety records and impressing all of aviation.

Here is a statement you will certainly have a hard time believing:  HTL is airspeed stable.  LTL is airspeed unstable!  What you have been led to believe is that HTL is dangerously unstable.   Certainly it is without a good horizontal stabilizer, but so are CLT and LTL without a good horizontal stabilizer – dynamic damper.    That is no longer a concern with a “Big Tail Way Back” – strong dynamic airframe pitch damping makes them all stable and insulated from PPO, buntovers or PIO.   But, aircraft designers would also prefer that aircraft be airspeed stable – if it starts to go faster, it automatically slows down.  If it starts to go slower, it automatically speeds up – to its trimmed condition airspeed.  You don’t always want to be having to reign in airspeed if it starts to change.

With gyros, especially less stable gyros that tend to get less and less stable at higher airspeeds, it is important that they don’t automatically try to go faster and faster at higher airspeeds.  But that is exactly what LTL does.  Here’s why: First, understand that some gyros perceived as “CLT” may actually be LTL.  On a LTL configuration, prop thrust statically holds the nose higher, essentially slowing the aircraft slower than its actual trimmed condition.  But, as airspeed increases, real prop thrust decreases – at high airspeeds prop thrust is much less than at lower airspeeds because of the faster incoming air.   As it goes faster, the reducing prop thrust allows the nose to drop lower, increasing the airspeed.  The faster it goes, the less prop thrust and the faster and faster it keeps going – for this situation, the pilot must actively slow the gyro with cyclic input to keep the airspeed from running away.  The exact opposite is true if the aircraft is slowing down – it tries to go slower and slower as the prop starts to bite harder in the slower air raising the nose and slowing it down further.  This is airspeed unstable; the airspeed does not automatically try to return to the intended trimmed airspeed, but instead continues to diverge from its trimmed airspeed. 

On an HTL configuration, prop thrust statically pushes the nose lower, essentially speeding the aircraft faster than its actual trimmed condition.   But, if the airspeed increases for some reason, the prop thrust decreases and allows the nose to rise, slowing the airspeed.  If the airspeed decreases for some reason, the prop thrust increases and pushes the nose lower to increase the airspeed back to its trimmed airspeed.    Upon increasing airspeed, HTL slows the airspeed by pushing the nose down less.    Upon decreasing airspeed, the increasing HTL prop thrust pushes the nose lower to increase airspeed back to its trimmed condition.  At higher airspeeds, or with pilot inattention, this can be an important attribute – it certainly reduces pilot workload that would otherwise always be having to monitor and correct airspeed constantly.

I would agree that HTL should be avoided if you just don’t have a good horizontal stabilizer – just don’t have good dynamic airframe pitch damping.  But, with a good Big Tail Way Back, HTL is no longer a thing to be avoided.  With a Big Tail Way Back, this benefit of HTL can be exploited, as it is in Magni gyros and all the “clones”.  Not to mention, with a Big Tail Way Back, designers no longer need to provide ladders to climb into the cockpit, or worry about tipping over so easily landing badly in a crosswind.  If you do other things right – can you say Big Tail Way Back? – HTL can be a very good thing! 

Thanks for your attention and diligence.  Fly safe – Greg Gremminger