Question about thrust lines & gyro designs.

Hi all,

Another dumb newbie question.

I know about thrust lines and the rule-of-thumb that the thrust line should be somewhere close to going through the pilot's navel.

(I also know that it's not as simple as that and that the weight of the aircraft, weight of the pilot, centre of gravity and centre of mass all play a calculated part in determining where the centreline actually is.)

I know that having a dropped-keel (like the Dominator/Butterfly/Ultrawhite/etc) allows you to lower the level of the engine, and thus the thrust-line.

I know that a reduction drive pointed downwards can lower this thrust line even further.

I know that a larger, slower prop is actually better that a smaller, faster prop.

I have heard a number of people claim that the standard Benson/Brock design is actually close to Centreline Thrust.

How can this be if other designs require a significantly dropped keel to allow achieve such a low thrust line?

I saw a video on Youtube by a guy named "Gyroflight" (I'm guessing that it's one of you guys) entitled "Mangnolia Pt 1".
http://www.youtube.com/user/Gyroflight#p/a/u/0/mQJFAAJZe6w

At approximately the 6:11 mark, there is visible, what appears to be a regular Bensen/Brock design without any form of drop-keel arrangement yet still adheres to the "thrust line through the pilot's navel" rule.

How is this achieved?
Is the prop smaller?
I'm guessing that it's direct drive with no reduction drive?
At what cost does this come?

Also, there is a rather interesting Tractor machine at the 2:12 mark supposedly owned by "Virgil". Is this a one-off or a kit design?
Is this an effective design that is worth replicating should one wish to go with a tractor model?

Thanks in advance.

You answered your own question when you observed that a larger, slower prop is more efficient than a smaller, faster one. The Bensen-Brock machines were designed for props of only about 4 ft. diameter. That's because the standard power plant for both was the 4000 RPM, direct-drive McCulloch 4318. AT 4,000 RPM, a prop can't be more than about 4 ft. in diameter without exceeding a sensible tip speed (and soon exceeding the sound barrier!).

The really serious high-thrustline problems appeared in the 80's, with the application of the redrive-equipped Rotax to gyros. Suddenly, it was a bolt-on job to have a 5-ft. prop. This required raising the engine at least six inches. The result was -- of course -- a high thrustline.

Some designers made it worse by putting the redrive up and the engine down, or even upside-down. 80's Rotax gyros had a thrustline that was at least 5-6 inches high, sometimes more.

A stock Bensen with a Rotax redrive engine is a PPO waiting to happen. In fact, the Brock KB-3 is just about that. It's a proven Bunt-O-Matic in its original form.

The Ken Brock version of the Bensen is quite close to CLT. The original Bensen B-8M, not so much. The Bensen featured a steel boat gas tank mounted on angle-iron brackets below the seat. It also had very heavy trailer-style wheels with the usual heavy iron hubs in the middle and a massive plate-steel nosewheel fork. Brock moved the fuel up to seat level with his seat tank, used light Azusa go-kart wheels and lightened up the nose fork quite a bit.

Hi Qouj...

Another dumb newbie question.

Click to expand...

There are never any dumb questions, only those to dumb to ask<smile>.
I'd be happy to try and help with your questions.

having a dropped-keel allows you to lower the level of the engine, and thus the thrust-line.

Click to expand...

It allows you to swing a larger prop (like a Warp)...not possible on a Bensen.

a reduction drive pointed downwards can lower this thrust line even further.

Click to expand...

...or it can be flipped in the upward position to allow the mass of the engine to be lower in relation to the TL

a larger, slower prop is actually better that a smaller, faster prop.

Click to expand...

I think in reading that Ya' got it backwards: smaller/lower speed works great on direct drive, larger faster is used on engines w/reduction units (belt or gear).
Bensens fitted w/ a Mac can only turn around a 54" without hitting the keel...with a different <heavier> engine and the need to reduce the higher crank RPM's through a reduction unit, you spin a larger prop and gain more thrust but require the clearance at the keel.

the standard Benson/Brock design is actually close to Centreline Thrust.

Click to expand...

True. Forum Admin GyroMike has a Bensen w/Brock style seat tank...nearly perfect CLT using a McCullough 90 Hp.(don't forget to add a HS..!)

How can this be if other designs require a significantly dropped keel to allow achieve such a low thrust line?

Click to expand...

It depends on the engine: Lets say a Soob EA-8x vs a Mac.

For really good performance, a Soob EA-8x needs a reduction unit (it cannot produce full HP at slower prop RPM's) hence needing a larger Prop. thus needing more clearance to the keel for the prop.

A McCullough (since it's lighter than Soob) can run direct drive & smaller prop. because the engine can spin the prop at slower RPM's and still produce it's full HP at those speeds.

Ideally you'd want your CG and the engine CG & TL all to align at your navel.
But on a Soob (heavier than Mac) when you put a reduction unit on it (because it you can install the unit either UP or DOWN) you don't want the prop. TL below the engine CG point. So instead you install the Unit UP & lower the engine down a bit in relation to YOU (so as to split the difference) so the TL goes between the two..equaling CL orientation.
This also helps the machine from becoming too top heavy for ground handling.
(Belt reductions have a wide offset in relation to the crank, gear boxes less so making the engine offset in relation to you less dramatic...and Gear units are more reliable too).

See the diagram<below>: this example uses arbitrary weights for illustration).

In short, start with the design you want (Tractor or Pusher)...then determine your engine.
Every design has pros vs cons.
Engines too...more money/less...reliability, cost to maintain, etc.
Rotax is a fine engine, so is VW & Soob.
Others are using Weber, Yamaha, etc.

This may not be a perfect series of answers, but I hope it helps.

M-M

quoj said:

Also, there is a rather interesting Tractor machine at the 2:12 mark supposedly owned by "Virgil". Is this a one-off or a kit design?
Is this an effective design that is worth replicating should one wish to go with a tractor model?

Click to expand...

The tractor gyro is a one-off using rotor components from the certified mcculloch j2 gyroplane (I believe) and so not easily or cheaply replicateable. I'm not even sure it's a successful design as I've never seen an pictures of it in flight.

.

Minnesota_Mike said:

Hi Qouj...

There are never any dumb questions, only those to dumb to ask<smile>.
I'd be happy to try and help with your questions.


It allows you to swing a larger prop (like a Warp)...not possible on a Bensen.


...or it can be flipped in the upward position to allow the mass of the engine to be lower in relation to the TL


Ideally you'd want your CG and the engine CG & TL all to align at your navel.
But on a Soob (heavier than Mac) when you put a reduction unit on it (because it you can install the unit either UP or DOWN) you don't want the prop. TL below the engine CG point. So instead you install the Unit UP & lower the engine down a bit in relation to YOU (so as to split the difference) so the TL goes between the two..equaling CL orientation.
This also helps the machine from becoming too top heavy for ground handling.

See the diagram<below>: this example uses arbitrary weights for illustration).

M-M

Click to expand...

Hello Mike,

I am not familiar with this principle; why is that an advantage to have the thrust lline above the center of gravity of the engine?

Thank you, Vance

With an EA-8x (my heavy engine) I have two choices when mounting the reduction unit (belt or otherwise).

The keel limits the prop location.

Mounting prop thrust line down forces One to move the engine too high on the airframe..creating a very top heavy install...bad for ground handling.
Mounting prop thrust line above forces the engine down on the airframe (as with many RAF style installs)...much better ground handling.
True...one can mount the reduction unit UP or DOWN...and then install the engine higher or lower to bring the CG of both pilot & engine & prop thrust into alignment.
The choice either way will affect your ground handling...lower mass being better with an engine as heavy as a Soob.

M-M.

This paper from the CAA tells everything you need to know about thrustlines:

http://www.caa.co.uk/docs/33/Paper2009_02red.pdf

Thank You Chuck...
That is an amazing body of knowlege (at 350+ pages..!)
Looks like I have my reading cut out for me with that one.

M-M.

Still confused

Still confused

Minnesota_Mike said:

With an EA-8x (my heavy engine) I have two choices when mounting the reduction unit (belt or otherwise).

Mounting prop thrust line down forces One to move the engine too high on the airframe..creating a very top heavy install...bad for ground handling.
Mounting prop thrust line above forces the engine down on the airframe (as with many RAF style installs)...much better ground handling.
True...one can mount the reduction unit UP or DOWN...and then install the engine higher or lower to bring the CG of both pilot & engine & prop thrust into alignment.
The choice either way will affect your ground handling...lower mass being better with an engine as heavy as a Soob.

M-M.

Click to expand...

Thank you Mike,

At the risk of seeming pedantic, isn’t ground handling more related to the center of gravity of the aircraft rather than the engine?

Thank you, Vance

The Pilot location, Engine location & Prop thrust line...of course are all are related to the final CG within the Airframe.
Assuming One could mount the engine at shoulder height and still have a CL orientation of the Pilot, Engine and prop...would One desire to do this...?
Thanks but no thanks...not on my machine.

Your turn...lets hear your answers to his initial questions.

M-M.

Still puzzled by your reasoning and confused.

Still puzzled by your reasoning and confused.

Minnesota_Mike said:

The Pilot location, Engine location & Prop thrust line...of course are all are related to the final CG within the Airframe.
Assuming One could mount the engine at shoulder height and still have a CL orientation of the Pilot, Engine and prop...would One desire to do this...?
Thanks but no thanks...not on my machine.

Your turn...lets hear your answers to his initial questions.

M-M.

Click to expand...

Thank you Mike,

I feel that Doug answered his initial questions well.

I feel I don’t have enough knowledge to offer opinions.

I would need to ask more questions about his questions to see if I knew enough from my experience to offer opinions.

I felt they were very thoughtful and observant questions.

I was not familiar with your reasoning behind not wanting the prop thrust line below the engine CG so I asked.

I still don’t understand your reasoning.

I have no desire to enter into a debate with you.

I also found your relating engine weight with desirable propeller diameter confusing. I am easily confused.

Thank you, Vance

I view the forum as a place to exchange idea's and knowledge....the debate of idea's is good for our sport as it furthers the collective knowledge of us all.

I do not pretend to be an expert and attempted to answer his questions as simply as I could based upon my understanding of the generally accepted principles many use/have used and my experience applying them.
Through our shared experiences of trial and error our sport has become much safer and enjoyable, and with fewer fatalities as an end result.

Of course one could get into discussing Mac vs Soob vs Rotax... Air cooled vs Water, the different mass of propellers, the tip speeds, different gear boxes/redrives (and their required orientation), location of and mass of radiators, batteries, the effect of their moment arms, etc, etc. for a clearer view and understanding of the impact each makes to the final Airframe CG.

When it comes right down to it though, it IS sometimes difficult to explain simply- since comparisons are difficult when it comes down to "Apples vs Oranges" configurations.

Certainly many, many variables to digest...no doubt.
Much easier to stick to tried and proven flight worthy designs no doubt.

M-M.

It is desirable to have items that can vary in weight such as the pilot, passenger, cargo and fuel, as close to the CG as practical so changes in those weights results in as little change to the aircrafts CG as possible. Items of fixed weight such as engine, landing gear, rotor, airframe, etc. can be arranged as desired by the designer as long as the resulting CG is at the required location.

.

Mike- I also am confused on Vances higher level. True, if you you flop the gearbox up to have a lower engine, you have lowered the CG of the gyro, but you have raised the thrustline more than you have lowered the CG. What ever percent the engine is of the total weight of the gyro, that same percent is multiplied times the distance you moved the engine, and this is how much the CG has moved. If say the engine weighs 250 pounds and the total weight of the gyro is 750 pounds, the engine is 1/3 the total weight. If you lower this engine 3 inches, you only will have lowered the CG 1/3 of that, or just 1 inch. But now you will flip the gearbox up, and every inch you move the gearbox, you move the thrustline one inch up. So, unless you had a too low of a thrustline you needed to raise, you will be raising your thrustline while lowering your CG. True, if you want the best ground handling, the lower the CG the better. But you are creating a worsening HTL by doing so, now we are introducing undesirable flight characteristics. Maybe I am not understanding your meaning like Vance said he didn't understand? Stan

I'll take a shot at your questions. Disclaimer: I'm not a gyro designer, but did OK in physics.

quoj said:

...I know about thrust lines and the rule-of-thumb that the thrust line should be somewhere close to going through the pilot's navel...

Click to expand...

This is a generalization, most applicable to light, single-place machines, in which the pilot is normally the heaviest component. In heavier machines in which the pilot's weight is a smaller portion of the overall flying weight, it may be a less accurate assumption. (For example, some two-place machines have rotors weighing over 100 pounds at the highest point on the gyro, contributing to a higher CG.)

quoj said:

(I also know that...centre of gravity and centre of mass...

Click to expand...

Center of Gravity (CG) and Center of Mas (CoM) are two different terms for the same thing.

quoj said:

I know that having a dropped-keel (like the Dominator/Butterfly/Ultrawhite/etc) allows you to lower the level of the engine, and thus the thrust-line...

Click to expand...

Not always. The goal in stepping the keel was to allow a bigger, slower-turning prop for greater efficiency, not to allow lower engine placement. If the reduction drive is mounted so as to place the prop below the center of the engine, you can move the engine higher and the prop lower with the change, both of which are usually helpful in getting the thrustline to pass through the CG/CoM.

If you mount the redrive so as to place the prop above the center of the engine, you'll need a much larger drop in the rear keel to obtain centerline thrust, since your moving both the CG and the thrustline down as you mount lower on the mast.

quoj said:

...I know that a larger, slower prop is actually better that a smaller, faster prop.

Click to expand...

You are correct. A 72-HP Mac turning its prop at 4,000 RPM will generate significantly less static thrust (static meaning measured at zero airspeed) than a 65-HP Rotax 582 turning its prop at 2500 RPM. The difference between the two will narrow at higher flying speeds, but in a gyro it's more useful to have the greatest advantage at lower airspeeds.

quoj said:

I have heard a number of people claim that the standard Benson/Brock design is actually close to Centreline Thrust.

How can this be if other designs require a significantly dropped keel to allow achieve such a low thrust line? ...Is the prop smaller? I'm guessing that it's direct drive with no reduction drive? At what cost does this come?

Click to expand...

Doug explained the progress Brock made in lightening the wheels and moving the fuel and its tank weight higher. On the props, you are correct again. The Macs (and VW engines) are usually operated direct drive, while the Rotaxes use reduction drives. Regarding the downsides, as others have explained, the Mac engines turning 4,000 RPM are limited to smaller, less-efficient props to avoid excessive prop tip speeds. On the other hand, they're simpler and lighter without the reduction drive.

StanFoster said:

Mike- I also am confused on Vances higher level. True, if you you flop the gearbox up to have a lower engine, you have lowered the CG of the gyro, but you have raised the thrustline more than you have lowered the CG.

...............But now you will flip the gearbox up, and every inch you move the gearbox, you move the thrustline one inch up.

Click to expand...

The assumption here is that no matter if the gearbox is up or down the thrust line is the same as the engine is remounted to keep the propeller at the same position within the frame.

.

Minnesota_Mike said:

With an EA-8x (my heavy engine) I have two choices when mounting the reduction unit (belt or otherwise).

The keel limits the prop location.

Mounting prop thrust line down forces One to move the engine too high on the airframe..creating a very top heavy install...bad for ground handling.

Click to expand...

Vance said:

Thank you Mike,

At the risk of seeming pedantic, isn't ground handling more related to the center of gravity of the aircraft rather than the engine?

Click to expand...

Landing gear geometry is calculated based greatly on the aircrafts CG and moving that CG, especially higher, will likely necessitate changes in the landing gear to maintain acceptable stability.

Now, modifying an existing ship's landing gear runs added cost so that may effect the designers decisions in certain matters.

.

Thanks for all your help guys.
You've certainly answered my questions, and then some!

Being a beginner, I'll be looking for the safest, most stable gyro I can build, so I'll be aiming for 100% straight down the eye-diddle-diddle of the Centreline! (And a whopping, great Hstab to go with it!)

Mike, I'm glad I read your post about trust lines-reduction drives and mac engines, I have a Bensen with 72 hp mac, I want to swing a longer prop and was wondering if any one has come up with a reduction for a mac? really don't want to change engines if I don't have to.

For a 72 Mac the prop to use is a Tennessee Prop 52" x 24".
For a 90 Mac use a 52" x 26".

No one makes a redrive for the Mac.