Main Gear distance back from CG?

Brian Jackson

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GyroBee Variant - Under Construction
When looking in top view, is there a generally accepted distance that the main gear be from the craft's CG? Most single-place gyros rock back on their tails without pilot, others like the Sport Copter do not. When watching someone balance on the mains during takeoff it might be assumed that the gear should be located directly under the CG, but as with many things gyro related appearances may be deceiving.

Regards,
Brian Jackson
 
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A round about way of answering your question Brian.

When beginning the takeoff roll once some reasonable rotor rpm is achieved and I move the cyclic fully aft the rotor thrust vector (right angle to the center of the rotor disk) is pointed at the front wheel.

As rotor rpm increases and lift increases the nose lifts and I begin to move the cyclic forward to balance on the mains and my rotor thrust vector moves aft. This is a gyroplanes way of telling the pilot when she is ready to lift off.

At some point the rotor thrust vector will get close enough to the center of gravity and the gyroplane will lift off. If I have too much aft cyclic or the mains are too far forward I will lift off at less than the target airspeed with the nose high. If the mains are too far aft I may lift off at higher than my target air speed.

A heavy person in a single seater likely need the mains further forward and a light person may need the mins further aft because the pilot is well ahead of the center of gravity. In a single seater the pilot is a larger percentage of the all up weight than a tandem.

Part of the reason for the hang test is to find out where the rotor head needs to be.

In this picture of The Predator the horizontal center of gravity is about eight inches ahead of the rear black fuel tank strap and the mains are about two feet behind that center of gravity.

If I have the nose tire about an inch above the ground on my takeoff roll she will lift off at just about 50kts. With a heavy passenger and full fuel she will lift off closer to 60kts because the center of gravity has moved forward in relation to the mains.
 

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Using Alan’s method of measurement The Predator has the rotor head over the mains at six degrees.

This is the same picture rotated six degrees.
 

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Brian you could have the mains under the tail if you wanted to,with the wheels back that far when the rotor develops lift the gyro would take off

at a level attitude.I watched a video of a Chinese guy with the wheels at the very rear and he got off just fine.

On my RAF I moved the gear aft 6" to help keep the nose wheel on the ground during the take off run,it made things more manageable for me

and it shortened my distance by quite a lot,by allowing me to achieve a better angle of attack on the rotor,rotor RPM was achieved faster.
 
With a non-castering NW, moving the RAF main gear back would seem to make the takeoff roll a bit more "darty".
Perhaps eddie or others can comment on that.
I don't know, however, because I've yet to modify mine, though I'm considering it.

Regards, Kolibri
 
Brian I would comment further on this subject as I have more than 600 hrs with my mains moved back,however if I do the Kolibri is lurking around

waiting for my comments so he can jump on the safety band wagon and tell everyone that I am surely going to kill myself because I have

moved my mains back, because the placement of the gear is causing excessive forces on my rodends
 
eddie, it was a sincere question, with no ulterior motive. Geez.
 
Alright guys let's cool this animosity. Thank you all for the great info as I continue to learn and expand my understanding of why things are done the way they are. Regardless of personal opinions, preference or argument styles, good information still finds itself here.
 
Somewhere I heard(or read) you should have 15% of the all up flying weight on the nose wheel when pilot is in the seat. Moving the rear gear would determine how much weight is distributed between the two points.
Could the gyro elders confirm this?
 
Although RAF takeoff roll procedure (when solo) is to balance the mains, with a passenger I was taught to lightly keep the NW on the runway.
When I do so, I notice the "dartiness" of my non-castering NW controlled by the rudder pedals. I do not care for it, and consider it a bad design.

My question merely asked if this dartiness increases once the mains are moved back 5-6".
Personally, I'd like to move back my mains.

Regards, Kolibri
 
Kolibri;n1131454 said:
Although RAF takeoff roll procedure (when solo) is to balance the mains, with a passenger I was taught to lightly keep the NW on the runway.
When I do so, I notice the "dartiness" of my non-castering NW controlled by the rudder pedals. I do not care for it, and consider it a bad design.

My question merely asked if this dartiness increases once the mains are moved back 5-6".
Personally, I'd like to move back my mains.

Regards, Kolibri

In my opinion until you learn to take off properly in an RAF your takeoffs will be afflicted with "dartiness".

I strongly suggest that you find a competent RAF flight instructor and take a few hours of dual instruction.

The RAFs, Groen Brothers Modified RAFs and SparrowHawks I have flown were stable in takeoff when the correct procedures were followed.

There are more than a few of the above that have been tipped over from poor takeoff and landing procedure and destroyed.

Flight instruction is much cheaper and safer than destroying the aircraft.
 
RAF2000 dual takeoff procedure.png

Vance, as much as you'd like to believe otherwise, there's nothing faulty about my RAF takeoffs.
However, having flown two Sport Copter gyros with free-castering NW, I notice the difference.
I'll stick by my term for the RAF as "darty".

That aside, if I were a newly minted gyro CFI, with -- what, three? -- former students who subsequently damaged their machines,
I'd spend more time contemplating the quality and efficacy of my instruction vs. opining about somebody else with likely more
RAF experience who has never had a takeoff or landing incident, much less damaged or destroyed his gyro.

But that's just me.
 
Kolibri;n1131493 said:


Vance, as much as you'd like to believe otherwise, there's nothing faulty about my RAF takeoffs.
However, having flown two Sport Copter gyros with free-castering NW, I notice the difference.
I'll stick by my term for the RAF as "darty".

That aside, if I were a newly minted gyro CFI, with -- what, three? -- former students who subsequently damaged their machines,
I'd spend more time contemplating the quality and efficacy of my instruction vs. opining about somebody else with likely more
RAF experience who has never had a takeoff or landing incident, much less damaged or destroyed his gyro.

But that's just me.

Exactly the response I expected.

In my opinion based on my experience if the procedure is followed an RAF is not "darty".

I prefer a free castering nose wheel for most gyroplane operations; that is not what this thread or your inquiry is about.
 
I quoted the procedure from the RAF Manual, and that is what I use.
If you believe that RAF's procedure is somehow improper, then go take it up with RAFSA.

With mains moved back, I presume that the NW will be on the runway for even solo takeoffs.
My question was about the sensation of that. Your non-dartiness opinion is noted.
 
the Darty effect happens when after the nose wheel lifts off and the forward speed increases more rudder

is needed which in turn cocks the nose wheel sideways if the nose wheel is allowed to contact the runway at an increases speed the gyro will dart one way or

the other and if the nose wheel comes is in good contact with the runway It will flip you over very rapidly,this can happen even with two on board.

Moving the mains back 6" eliminates the nose wheel from coming off the runway with just one on board,even with the mains moved back you can still

raise the nose whenever you want even with two on board. One of the benefits is that you can achieve a faster rotor speed because with the nose on the ground

you can set the rotor at a better angle that cant be achieved with balancing on the mains,because the rotor is being used to keep the nose wheel off the ground.

Moving the mains forward makes for faster,safer and shorter takeoff runs.



.
 
eddie, thanks for your reply.

the Darty effect happens when after the nose wheel lifts off and the forward speed increases more rudder

is needed which in turn cocks the nose wheel sideways if the nose wheel is allowed to contact the runway at an increases speed the gyro will dart one way or

the other and if the nose wheel comes is in good contact with the runway It will flip you over very rapidly,this can happen even with two on board.
Yes, certainly, and it's something devoutly to be avoided.
This has no doubt been responsible for many tip-overs.
However, I was referring to the takeoff roll with the NW kept on the runway.

I find the RAF's direct pedal link to the NW somewhat coarse, which can more easily lead to overcontrolling.
One must input rudder pedal with enhanced judiciousness (compared to castering NW of my experience).
I've never found it a huge issue, but it's something to watch out for.
This is what I meant by "darty". I was just wondering if that sensation was affected by a longer wheelbase.

btw, I think I mentioned this a long time ago, but it seems good timing to do so again now; that I do admire your tinkering ability with the RAF,
regardless of how I may disagree with you on some things along the way. The RAF needs a lot of tinkering, and I like many of your mods.


_____
Probably the last major mod I'll do to my RAF will be to move back the mains, add suspension, and mod the NW for castering.
To steer the NW during taxi, I'm thinking of a non-pedal mechanism, something like a separate "tiller" (as used in airliners).
I'll start a thread about this if I take the idea further.

Regards, Kolibri
 
Yes its still darty if you over use the rudder pedals,its like some taildraggers I have flown that are sensitive to heavy rudder pedal input on take off. I am very careful

with the use of the pedals as the speed increases,I actually just lock them with equal pressure on both and if I have to make a correction its a very

small and careful one.the one good thing I have going for me is the takeoff roll is very quick and short,when the nose comes off the ground I have to go to

almost full right rudder to keep it straight down the runway after breaking ground. And then I throttle back to reduce torque roll.
 
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