Thrust Lines. How Far Above is Okay??

Chopper Reid

Senior Member
Joined
Jun 25, 2004
Messages
1,326
Location
Pooncarie. Australia
Aircraft
Rosco built, Suby 2.2, pod
Total Flight Time
4600 gyro hours
We all know that Magni /ELA & MTO's have HTL 's yet those three are considered 'stable' and have a very good safety record. Now, I am not a technical expert here but I am aware that the big stabalizer way out the back and the rotors are two reasons why they are 'stable'.
There will be more reasons, however, I was wondering if the big three actually decided it was ADVANTAGEOUS to have thrust line somewhere in the 4 inches above CLT range??
I have spent quite a bit of time talking here in Australia to difernt people as the governing body here [ASRA] have adapted the Proffessor Houston study which says thrust lines should be with 2 inches of CLT for optiom safety AND a number of peoplel argue that the best /safest thrust line is actually in the 4 inch range above CLT.
I know thrust lines seem to cause people to get very agro at times and this is not intended to insult anyone, I was looking for the likes of Mr Beaty etc, who are very knowledgeble to have have think about this if they will please
 
Illustrative experiment..

Illustrative experiment..

A little illustrative experiment to see what happens when a force is added to an object hanging in the air.

I took a plastic cover for a cd with a little instruction booklet inside.
I hold it over a chair – gave it a little throw up and let go – just as it hangs there I gave it a push with my finger.
I did that several times. Pushing it or knocking it with my finger in COG – above COG – and below COG.

To me it was very convincing how far away from COG one should push.
Not very scientific, but a visible and hands on experience :yo:
 

Attachments

  • CIMG1513-640.jpg
    CIMG1513-640.jpg
    33.1 KB · Views: 0
  • CIMG1514-640.jpg
    CIMG1514-640.jpg
    24 KB · Views: 0
Reid,
Tell me just one good reason why the thrust line is better to be even at one inch above VCG.
 
Last edited:
Im not CB, but ill throw my 2 cents in anyway.

The reason why Magnis and the like feel so solid and fly streight as an arrow is coz they are always loaded.
By loaded i mean that there is alway the TL offset correcting force of the HS acting on the frame.
The slightest divergence from trimmed attitude will have the balanced forces [ the HTL and the down force of the HS] quickly rebalance, thus maken the frame follow the air zactly.
This is a safe setup, but not very efficiant, and has a strong damping effect.
You can still have a safe machine without the big HS, but it needs to be as close to CLT as possable. Itll be more efficiant, with a higher control rate, and CANT PPO.
 
Things to consider:
Positioning of tanks.
Handling of students.
Atittude of pilots.
A specific machine´s safety record has to do more with the people that drive them . . .
As for design built safety, follow the bible to the letter.
The question why X inches above or bellow is a good discussion, lets have it?
thanks
Heron
 
Hi Heron

I like your question, it brings up the point regarding Horizontal stabilizers. When you have a high thrust line, you have the design potential for PPO. When you add a horizontal stabilizer of SUFFICIENT size you compensate for this high thrust line factor, you do not eliminate the design factor. A number of HS out there are really not large enough, mainly on the small single place machines, if they are not in the prop blast.

The Magni and others of similar design appear to have sufficienct size HS.

Tony
 
Tony
A HS is not part of the design if it was introduced as fix for a design problem.
But a machine with less safety can be used in a safe way, training is another way to help with the problem.
If the student goes bold, problems will arise, we have seen that with RAFs and high time FW pilots, they get in trouble.
I don´t know if Magni will ever create another machine, maybe they like that sillouete too much and don´t care for changes. The machine is not bad but the fangs are there.
Maybe we should discuss how to make Magnis better.
thanks
Heron
 
It is my opinion that you do not have to have a thrust line that is within 2 inches of the center of gravity to have a stable gyroplane. There are several designs out there that have a track record that shows a reasonable safety record can be achieved even with a high thrust line. Over-balanced Roters and Powerful Horizontal Stabilizers (Large and on a long moment arm) can compensate for the high thrust line.

However, designing a gyroplane that has the thrust line at or very near the center of gravity of the aircraft simply takes away one problem that you have to design around. It makes the machine safer and more stable in various situations. It may not be the only way to provide a stable machine, but it certainly is a good way to do it that has many advantages and no substantial disadvantages. Many people complain about the "looks" or the "height" of machines designed to have centerline thrust. But looks and beauty are all in the eye of the beholder. Personally I like the looks of my center line thrust machine. And I know it is not going to ever be trying to push my nose over and send me tumbling to the ground. That makes it look even better to me.
 
Reid,
Tell me just one good reason why the thrust line is better to be even at one inch above VCG.

I never asked that question, however, we know the ELA for example was designed some time after the Magni and the MTO copied the ELA ?? and all of the above designers/ manufactuers could have designed and made their gyros CLT had they wanted to, yet they chose not to and I'm asking why.

I'm asking cause I'm trying to learn more about thrust lines and stability and already I have upped my hard drive [brain] a great deal.
 
I never asked that question, however, we know the ELA for example was designed some time after the Magni and the MTO copied the ELA ?? and all of the above designers/ manufactuers could have designed and made their gyros CLT had they wanted to, yet they chose not to and I'm asking why.

I'm asking cause I'm trying to learn more about thrust lines and stability and already I have upped my hard drive [brain] a great deal.

Actually, Reid, the question was not posted only for you. Enerybody can read your thread and it would be nice hear from them their opinions. From my experience, unless you fly a CTL gyro with a BIG stab at the right place, you cannot value its importance.
Doug Riley (and not only) wrote many-many times fantastic articles reg this.
I will search to find them and post them. Its nice even for us to reread it.
 
Key words: compensate (?) design around (?) Big (?)
Sex appeal sells.
Cute impresses.
Rethoric convinces.
Math is unforgiving . . .
Heron
 
Can someone explain to me how a large effective horizontal stabiliser could make a gyro stable in roll axis ? (heavy controls in yank and bank?)

Pitch I can understand. I'm learning a lot from this thread...

Art
 
This thread thoroughly dealt with the question of offset thrustlines and downloaded stabs. There’s no point in repeating it all.

http://www.rotaryforum.com/forum/showthread.php?t=10434

The key is to get AOA stability. In some FW configurations you need a stab download to arrive at a AOA stable configuration and sometimes you don't. The downloaded stab is not the driver.

Greg G. please take note, as you continue to use this misconception (about FW) in other threads and countless articles to justify and insist on the inclusion of a downloaded stab in a gyroplane for stability. If this is not clear I would be happy to discuss further and bring clarity but please do not continue to spread this misconception.
 
The notion that "a little HTL, with down-loaded stab, is good" has a couple of sources.

Going back a decade, we have Jean Fourcade's article about thrustlines and stability. This article stated essentially that the rotor thrustline (or RTV) had to be continually behind the gyro's CG in order for the gyro to have stick-fixed AOA stability. This rule actually is correct for a gyro (a) without a HS and (b) whose airfame has no net aerodynamic pitching moments (apart from any such moments induced by rotor thrust and prop thrust). Only if the RTV is behind the CG will an increase in rotor thrust pull the tail up, and the nose down. This reaction is the only correct one for AOA stability.

For the RTV continually to be behind the CG (thereby continually creating a nose-down moment) while the aircraft as a whole is in equilibrium, there must be an equal and opposite nose-UP moment. Where will it come from?

In the no-HS craft, we don't have much choice -- our nose-up moment must come from a prop thrustline BELOW the CG. This arrangement gives us the low-thrustline (LTL) configuration made famous by the Dominator and the high-rider Air Commands.

As educational as this model was when Jean first produced it, it's incomplete and even misleading when applied without modification to gyros with HS's. A HS, by its nature and placement far behind the CG, can make it unnecessary to keep the RTV aft of the CG in equilibrium flight What?? Heresy?? No.

A HS may have no up- or down-load in equilibrium flight and still function. How? Once the aircraft's AOA departs from an equilibrium value, the HS automatically BEGINS to impart a moment to the frame. This moment is in a direction that will tend to restore equilibrium AOA -- if AOA increases, the moment will be nose-down; if it decreases, the moment will be nose-up. The HS makes these moments only when they're needed. You don't have to fly around with a permanent nose-down moment induced by aft RTV (and therefore you don't need a balancing nose-UP moment created by a down-loaded HS or a low thrustline).

The second source of the "we need HTL" theory is various pilots' experience with aircraft designed more or less per the Fourcade model. A LTL gyro, for example, has its RTV aft of CG -- maybe way aft. When power on such a gyro is chopped, if the HS is not powerful enough (or loses power because of its location), the gyro will overcorrect and nose down rather steeply. Why? That aft RTV is hauling up the tail, possible farther than you would like. The weak HS can't stop this effect as promptly as we want, and the gyro overspeeds.

The problem here, however, is the LTL (sometimes amplified by an airframe that has its own negative pitching moment). The answer need not be HTL-and-HS-download. In fact, the better answer is CLT and minimal HS load in either direction.

A little HS download is "erring on the side of caution" when you have a prop thrustline whose location isn't known precisely -- or your are working off plans, such as the Gyrobee's, which have a known HTL. But that doesn't mean you should PURPOSELY build in HTL and then crank in H-stab down-load. That makes no more sense than cutting your car's brake lines just because, after, all, you have a great emergency brake.

The Magni's HS/HTL setup clearly works, but it's not, IMHO, proof of some secret benefit of HTL. It merely proves that an adequate HS will function correctly with a variety of CG locations, both LTL and HTL.
 
I have to observe that the quest to meet a certain set of pitch stability numbers is treated as an end in itself. It's not. The ultimate goal is a safe outcome.

Some LTL machines may not be technically capable of hitting the numbers in a throttle-chop situation (or uncommanded loss of thrust), but if the safety records are good, it suggests the intuitive reactions of pilots to the machine's behaviors are proving to be the correct responses. (There's not much that's more intuitive than pulling back on the stick when the nose dips.)

Obviously, it's nice to have a machine that will not require the pilot in the loop to behave safely in case of medical incapacitation, and if you ever had a catastrophic engine failure that compromised control rods, you'd want a machine that didn't automatically dive. But both those scenarios are pretty remote.
 
While I see several different ideas about CLT,HTL,and LTL, and opinions that vary, I am very glad to see all of it. Had this discussion/debate/argument taken place with this intensity 10 to even 20 years ago there would probably be several gyro pilots who are no longer with us, out there logging hours, and we would not have the reputation of the pregnant highschool cheerleader.

The first one I can remember trying to get this thru everyones head was Chuck Beaty, Chuck it looks like it finally took hold, THANK YOU

Tony
 
I want to thank all the contributors to my question, there is a lot of great knowledge out there and like Chuck Beaty etc.
 
In the UK the requirement is CLT +-2 inch.

The main reason for this is that true CLT is almost impossible to attain in flight.

Unless your pilot and fuel are exactly on the C of G location then different pilot weights and different fuel loads will move the C of G around, more so on a 2 seater.

A little variation from CLT is unavoidable in most types hence the 2 inch limit. If full fuel gives you 2 inch HTL and min fuel gives you 2 inch LTL then your gyro would meet that part of the requirement. Mid flight at half fuel you would of course be in the sweet spot at CLT.
 
In the UK the requirement is CLT +-2 inch.

The main reason for this is that true CLT is almost impossible to attain in flight.

Unless your pilot and fuel are exactly on the C of G location then different pilot weights and different fuel loads will move the C of G around, more so on a 2 seater.

A little variation from CLT is unavoidable in most types hence the 2 inch limit. If full fuel gives you 2 inch HTL and min fuel gives you 2 inch LTL then your gyro would meet that part of the requirement. Mid flight at half fuel you would of course be in the sweet spot at CLT.

The UK adopted Proffessor Houstons report and its now part of their gyro standards. Part of my questions is that I was wondering why the USA hasnt seen fit to adopt that report given the depth of the study or do you already have similair standards now[ within 2 inches of CLT]
 
Top