Thrust line vs center of mass

[Aviator168]

Hi,
There a lot of dicussion on this matter, like if the thrust line is above the center of mass, it is going to cause PPO. But I have never seen any discussion on what is going to happen if the issue of if the thrust line is below the center of mass. Is this going to hurt performance or it is out right dangerous? Your takes.

 

[Vance]

The gyroplane I fly and train in (The Predator) has a slightly low thrust line in relation to the center of gravity.

One of the effects of this is that I need less trim at full power than at cruise power to maintain the same airspeed for a given cyclic pressure.

A Sparrow Hawk and a Dominator have a lower thrust line in relation to the center of gravity than The Predator.

One of the effects of this is that they pitch down when power is reduced and pitch nose up when power is added.

This is considered by many to be a stable response.

I prefer near centerline thrust to minimize the power-pitch coupling.

Many people imagine that raising the center of gravity makes a gyroplane unstable on the ground.

I feel there is very little reason to be going fast around corners in a gyroplane and question this line of thinking.

Most of the new crop of gyroplanes has the thrust line well above the center of gravity causing them to pitch up when power is reduced and pitch down when power is added.

This is considered by many to be an unstable response.

Combined with a short rudder they have power-pitch-yaw coupling.

The American Ranger is the exception with very little power-pitch-yaw coupling.

Most of the new crop of gyroplanes uses a horizontal stabilizer with a lot of volume to make the power-pitch coupling less intrusive and a power push over less likely.

It is my observation that most of the accidents in these new style low slung gyroplane are tip overs on the runway.

In my opinion there are no simple answers as to why a gyroplane behaves as it does. It is a system that works together to govern how a gyroplane responds to pilot inputs.

I feel debating a single aspect of gyroplane design is pointless because everything interacts.

Pilots appear capable of wrecking all gyroplanes.

 

[Aviator168]

I observation also. When I saw most of the moden gyroplanes, I said to myself, "there is no way it is center thrust" Coming from FW, I like my thrust line to be a little below center mass, when god for bid that engine power is lost, the plane pitches down to maintain speed.

 

[Vance]

I am not able to tell if a gyroplane is near centerline thrust by looking at it.

What works with a fixed wing may not be applicable to a gyroplane.

Very few gyroplanes have a downloaded horizontal stabilizer.

Gyroplanes don’t have elevators.

Most gyroplanes are pushers.

A gyroplane is a system, not a collection of isolated concepts.

You don't need a gyroplane to pitch down when the engine goes quiet.

You have lots of time to adjust the airspeed because gyroplanes don't stall or spin in the fixed wing sense.

 

[Abid]

We designed AR-1 to be slightly high thrust line. It’s thrust line is 4 inches higher than its CG with a representative 200 pound pilot. Filling gas tank up will make it closer thrust line to its vertical CG by a little. It’s response when power is applied is to slow down slight by a few knots (3 or so) with slight nose up and when power is taken out to speed up a few knots with slight nose down. It requires less messing with its throttle when in cruise than most European machines I have flown and less messing with rudder with power changes as well. So that behavior has got nothing to do with it’s center of gravity versus it’s thrust line as you are thinking. If what you said always held true, it would not respond that way. It’s not an isolated system. It all works together to create a response that’s desirable. The over emphasis on center line thrust singularly is not well placed in my opinion just for the sake of this response. Center line thrust or close to it simply makes a more efficient aircraft. That’s a better reason to try to achieve it. I’d rather have a slight higher thrust line than a lower thrust line if I have a properly volumed and airfoiled horizontal stabilizer

 

[Aviator168]

We designed AR-1 to be slightly high thrust line. It’s thrust line is 4 inches higher than its CG with a representative 200 pound pilot. Filling gas tank up will make it closer thrust line to its vertical CG by a little. It’s response when power is applied is to slow down slight by a few knots (3 or so) with slight nose up and when power is taken out to speed up a few knots with slight nose down. It requires less messing with its throttle when in cruise than most European machines I have flown and less messing with rudder with power changes as well. So that behavior has got nothing to do with it’s center of gravity versus it’s thrust line as you are thinking. If what you said always held true, it would not respond that way. It’s not an isolated system. It all works together to create a response that’s desirable. The over emphasis on center line thrust singularly is not well placed in my opinion just for the sake of this response. Center line thrust or close to it simply makes a more efficient aircraft. That’s a better reason to try to achieve it. I’d rather have a slight higher thrust line than a lower thrust line if I have a properly volumed and airfoiled horizontal stabilizer

Your HS must have a downward moment and the high the power setting, the bigger the moment.

 

[Illini85]

Another consideration, that doesn't get discussed much, to add to the complexity engineers/designers have to take into account is the variations of thrust at differing air speeds. There is a lot of discussion and testing and fiddling of engine/prop configurations focused on static thrust performance. While this is meaningful and appropriate the considerations get more complicated pretty rapidly. Let me state here that static thrust testing is important also in the context of engine management issues. Differential equations gave me nightmares so I'm no authority and won't tread very deep, but in layman's terms as I understand, as airspeed increases the thrust from a prop decreases. So a setup providing 450lbs max. thrust while tied down on the ground won't have that available while in flight and that thrust diminishes more as airspeed increases.

A low thrust line configuration will have a nose up bias but as airspeed increases the diminishing thrust vector will correspondingly reduce the nose up forces thereby causing the nose to want to lower. Lowering the nose causes airspeed to rise. Higher airspeed then results is lessening nose up forces which further drops the nose and further increases airspeed. Seems like a tendency for airspeed instability that the pilot must correct for. In short high thrust line machines will have a nose up bias (or lessening of the nose down moment) as that higher airspeed/lower thrust condition progresses thus creating tendency for airspeed to slow until there is a balance of airspeed and thrust. Its seems to be a more airspeed stable configuration in my mind that is less demanding of the pilot.

Airspeed and drag forces and their location relative to CG also must be taken into consideration by designers. How the center of pressure is oriented (and can change) on an aircraft and how that pressure increases as airspeed increases is massively important in the design considerations. Typically the slower airspeeds of gyro's, especially in the early years, made it less of a concern. But as you start powering up a design balancing these forces across a wider speed range becomes more critical.

The amazing thing in my mind is how well a gyro can balance thrust pushing it forward, drag pulling it back, lift from the rotor and gravity pulling it down.

Kudos to all you engineers who can master all these variables and the math that goes with it.

What say you?

 

[C. Beaty]

Juan de la Cierva patented CLT in 1929 but what would he have known?
US1948514A
This will work better:

US1948514A - Aircraft with rotative wings - Google Patents

 

[C. Beaty]

 

[Aviator168]

people don't want to buy gyros looking like the tour eiffel, it is not sexy ...

when you compare this :

View attachment 1146211

to this

View attachment 1146212

Hahahaha. But, you can still make the orange one center line thrust by making the rotor head and blade heavier or use a higher mast. However, it is difficult to do so given the weight of the craft. For an UL, it is a different story.

 

[XXavier]

or like this ... ctl, stable, looking like an aircraft and not like a flying fridge (or a tour eiffel), fridges must stay in the kitchens


View attachment 1146214

So far as I know, it remains a prototype. There are probably already too many gyro manufacturers, and the market is small...

 

[XXavier]

Spanish gyro ! , viva de la Cierva my friend !

Ya lo sé, pero no ha pasado de prototipo. Es una pena, pero así son las cosas...

 

[Abid]

people don't want to buy gyros looking like the tour eiffel, it is not sexy ...

when you compare this :

View attachment 1146211

to this

View attachment 1146212

it is clear that people ready to pay will choose the orange one and not the tour eiffel ...

but all the makers ( and not only fara) will write pages and pages trying to tell you that a cg placed 4 inches under the thrust line is not a problem, it won't change the reality , low profile gyro-planes are not as stable as high profile one

you just have to read this (written by a space engineer ...)

http://kekstas.darbastalis.lt/uploads/rotorius/stabilumas.pdf

all is explained ...

the rest is only marketing ... and marketing is no longer necessary coz appart from aviomania and maybe aircommand kits you won't find true high profile gyros anymore ... so the competition is already dead


plus I have never seen a stock low profil gyroplane double hang test to be sure of how down is their center of gravity ... double hang test would maybe show surprises

A lot of assumptions there.
4 inches thrust to CG offset without fuel reduces further down when there is any fuel present. Coming close to a couple of inches. For all intensive purposes that is a center line thrust sir. For a contrast a Magni M16 is about 10 inches high thrust line. For making a point a Magni M16 has a fair number of units to sample and field data shows no PIO or PPO. Flying 50 to 70 mph, its easy to hide many things which then show up as you start going constantly at 100 mph. The new gyroplanes are very capable of cruising at 90, 95 or 100 mph for hours.
This paper has basic pitching moment explained with CP relation to CG. After that it makes a jump to a strong opinion of a personal nature. There was a researcher here that countered the thoughts that a high thrust line gyroplane with a proper horizontal tail even in unloaded condition will be any worse. He presented his study in a paper. I think his name was Raghu. Why don't you read his research.

Just because you have not seen double hang tests of our gyroplane does not mean they have not been done. Plus the double hang tests done mostly by amateur builders are usually not accurate as in the second hang they generally end up having to balance on the mains for second line and that generally is inaccurate because the mains are never properly high enough to truly balance. So its a bit of a guesswork but its close enough I guess.

 

[Vance]

The American Rangers I have flown did not have any noticeable power-pitch-yaw coupling.

All of the Magni(s) I have flown had noticeable power-pitch-yaw coupling.

All of the MTOs, Calidus(s) and Cavalons I have flown had noticeable power-pitch-yaw coupling.

All of the ELAs I have flown had noticeable power-pitch-yaw coupling.

All of the RAFs I have flown had noticeable power-pitch-yaw coupling.

I don’t feel power-pitch-yaw coupling is dangerous when flown correctly.

I just find power-pitch-yaw coupling annoying when taking off or landing.

A gyroplane is a system of interacting parts and I cannot look at a gyroplane and know how it will fly.

 

[Smack]

Thanks for the patent link, Chuck.
Hmmmm, adjusting the engine tilt with a "rack and pinion" !
Such would seem good for gross movements, but I'd favor a screw-type drive for more precision and less backlash.
Great discussion y'all.
Brian

 

[Abid]

Here is the data to the best of my knowledge that was posted for different machines for High Thrust Line (HTL)


Magni M16 11 inch offset
Magni M24 13 inch
MTOSport 8 inch
Cavalon 12.8 inch
Calidus 5 inch
AR-1 4 inches


Very high thrust line machines (8 - 10 inches +) should have a significant tail volume more than the usual 12%.

 

[Aviator168]

Reduce prop size and use 1 or 2 extra blades. This is going to reduce prop efficiency, but you make a better fairing. Also, you can hang the engine upside down and the mass center can be higher.

 

[Vance]

this message is not aimed at discussing the center of gravity of Fara's giro and is just a piece of information given to the fellows who think about designing and building a LTL giro

taking all possible precautions :

- pilot and passenger at 1 meter above the ground ( the butts of the passenger et pilots are as high as the mid level of the engine)

- 50 kg rotor/rotor.head/electric prespinner at 2.85 m above the ground

- pre spinner 4 kg batt pack at 2.7 m

- fuel tank just under the butt of the passenger,

- 8 kg fairing

my calculated center of gravity is ONLY 10,5 cm (4 inch) above the thrust line ( with pilot and 50 kg of fuel) 5 cm above Thust line with a passenger and 5 kilos lugage

all have been taken in account coz i physically weighted every part of this gyro (which exists) and all entered in the spreadsheet (including tubes etc) ,

a true uggly Tour eiffel ... and even like this the gap is only 2.5 inches LTL ... imagine lowering 260 Kg ( pilots passenger fuel fairing) by 0.5 m

prop is 1.82 m rising the center of thrust by 5 cm

LTL look really horrible , and bathtubes can't be CTL

..the only

solution is a tractor

View attachment 1146238

She looks great to me jm-urbani!

 

[Vance]

Reduce prop size and use 1 or 2 extra blades. This is going to reduce prop efficiency, but you make a better fairing. Also, you can hang the engine upside down and the mass center can be higher.

It appears to me a Rotax 912 or 914 should not be run inverted.

 

[Aviator168]

A big prop is a big prop; no matter where you put it and it has to clear the ground and the center of mass can't be very low from that. Maybe just for the look and easy of entry and exit, you can have the center of mass a few inches lower than the thrust line and compensate it with the HS. Have you noticed that most of those commercial gyro all have fairly large HS?

Edit. You can also use an electric pre-rotator and hang the battery on the mast near the top and/or put the fuel tank high up. In my gyro (verson 2), I am going to use this engine because the gear box can be installed inverted, and it is a 4 stroke engine.

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