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Old Nov 05, 2013, 03:59 PM
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OK so far so good , so staying with pitch .....and within the context of an elevator controlled model , if rotor flap back results in a TPP three degrees or so greater than the mast angle, does the flap back result in an application of forward cyclic when the model is at normal flying speed? and would it be true to say that this effect does not occur during a power off decent ?
If so would a pitch up tendency be observed , and is there a solution other than pilot input to achieve a good nose down attitude when the power is cut ?
I ask as some models I have seem to naturally adopt a nose down , and others need some pilot help. I guess the ones that need help have a misalignment between lift and COG which is aggravated by small horizontal stabs?

Tom.
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Old Nov 05, 2013, 04:22 PM
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Sorry to interrupt again... but this is the link to "Mast Bumping" referred to earlier.
Mast Bumping in Helicopters - US Army Training Film (20 min 18 sec)
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Old Nov 05, 2013, 04:25 PM
Mickey from Orlando. Really.
Joined Nov 2004
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Quote:
Originally Posted by Tom Wright View Post
if rotor flap back results in a TPP three degrees or so greater than the mast angle, does the flap back result in an application of forward cyclic when the model is at normal flying speed?.
Yes, the mast is 3 nose down with respect to the TPP, so this is applying 3 of nose down cyclic.


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Originally Posted by Tom Wright View Post
and would it be true to say that this effect does not occur during a power off decent ?
Why would this be true? The rotor doesn't care what causes its forward motion to cause it to flap back. It may flap back less because of the lower airspeed, but as long as there is forward airspeed the rotor will flap back until everything balances out.


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Originally Posted by Tom Wright View Post
If so would a pitch up tendency be observed , and is there a solution other than pilot input to achieve a good nose down attitude when the power is cut ?
I ask as some models I have seem to naturally adopt a nose down , and others need some pilot help. I guess the ones that need help have a misalignment between lift and COG which is aggravated by small horizontal stabs?
The answer is not simple. If you have up elevator trim to cope with a forward CG then when power is lost you may get a nose down. If you have nose down trim in the elevator you might have a tail heavy condition than when stab effectiveness goes away with loss of prop blast, you get a pitch up. There are lots of factors such as prop blast and rotor/COG alignment and motor thrust line. For each model you have to think through the various effects to modify something to get what you want. For the model to naturally adopt a nose down power off condition I'd think you would want the rotor thrust to be behind the COG in trimmed level flight with a download on the tail (up elevator trim). When the prop blast terminates and the tail loses effectiveness you want the the rotor thrust behind the CG so it will adopt a nose down attitude. Of course a very small or very large stab will change this behavior as the large stab will have more effectiveness in the glide....
Sorry it's not a hard answer, but the answer is that you have to use all this knowledge to make the right adjustments in the right direction.

Kinda underscores the advice to build a proven design exactly as designed, doesn't it?
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Old Nov 05, 2013, 05:44 PM
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Thanks Mickey your reply makes absolute sense . This was leading to thoughts about pitch stability margins achieved by some built in rotor thrust to COG misalignment, and compensating with elevator trim.

Buzz

Thanks for posting the video I found it interesting an informative .

Tom.
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Old Nov 06, 2013, 06:02 AM
Brisbane, Australia
Joined Apr 2013
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Buzz, thanks, the video highlighted the coning angle at around 5:30 5:40, and also got me to thinking about loss of control with unloaded blades.
__________________________________________________ ___

Quote:
Tom: if rotor flap back results in a TPP three degrees or so greater than the mast angle, does the flap back result in an application of forward cyclic when the model is at normal flying speed?

Mick: Yes, the mast is 3 nose down with respect to the TPP, so this is applying 3 of nose down cyclic.
__________________________________________________ ___

I had to read those quotes many times before understanding, thought I'd lost the plot there for a while.
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Old Nov 06, 2013, 08:42 AM
Mickey from Orlando. Really.
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Originally Posted by Ian444 View Post
and also got me to thinking about loss of control with unloaded blades..
When unloaded a teetering rotor has no control power. The cause of a lot of fatal accidents in full size gyrocopters. As the teetering rotor has very little resistance to flapping, other bad conditions can happen, like mast bump.


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I had to read those quotes many times before understanding, thought I'd lost the plot there for a while.
It's worth getting this fine point. Any time the TPP is not perpendicular to the hub, there is some cyclic pitch taking place because the blade/hub connection is stiff in torsion. Any time the rotor is moving there is cyclic pitch. Any time the rotor is moving and there is coning (there is always a little) there is cyclic pitch. As you probably have concluded by now, the gyrocopter in forward flight is a complex balance of motion based cyclic and cyclic pitch.
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Old Nov 06, 2013, 08:55 AM
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If you guys don't mind I would like to continue to look at stuff that applies to the simple roll and elevator machines. I think it is generally agreed that this configuration is the easier one to start with as a first time Auto gyro experience, as demonstrated by the G1.
The G1 is possibly a good example that meets Mickey's suggestion to build a proven design without deviation from the instructions , this should result in successful flight experience and help with learning about trimming and Auto gyro flight characteristics.
Looking at this machine from the design point of view, and how it may help the constructor to go on to design their own reveals some pretty standard numbers that when referenced to the boom line look something like this ......hang angle15degrees . Horizontal stab incidence zero. Motor 3-5 degrees down 2 degrees side . Mast angle 9 degrees . Ground sit angle 6 degrees which when added to mast angle and flap back results in the 18-20 degrees needed for rog.
Now assuming that the rotor and flex plate design is correct it would seem that the horizontal stab area / fin area and rotor shaft angle become the major factors that determine how well the model flies , and as the model has a generous horizontal stab area my question to Mickey is ......For a given set of rotors and known gross weight what is the best practical way to determine what rotor shaft back angle should be set at ?
This is not so much of a problem with a dual function head providing the mast height is sufficient to allow rotor tip clearance to rear mounted components but for a roll only head it would appear that if the shaft angle is not optimized during the design stage then adjusting horizontal stab area or incidence would be needed as an alternative to a re design . This also raises another question about the large HS on the G1 ....does this suggest less than perfect rotor shaft / cog alignment or a design feature to increase the pitch stability margin?
Sorry for the thinking aloud , and rambling, I am trying to align the theory to observation.

Tom.
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Old Nov 06, 2013, 10:03 AM
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Quote:
Originally Posted by Tom Wright View Post
Thanks Mickey your reply makes absolute sense . This was leading to thoughts about pitch stability margins achieved by some built in rotor thrust to COG misalignment, and compensating with elevator trim.

Buzz

Thanks for posting the video I found it interesting an informative .

Tom.
I'm sorry I just interrupted an ongoing discussion... but when I found that video, things started to make more sense. At least to me.

Buzz
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Old Nov 06, 2013, 11:57 AM
Mickey from Orlando. Really.
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Originally Posted by Tom Wright View Post
.For a given set of rotors and known gross weight what is the best practical way to determine what rotor shaft back angle should be set at ?
The way I did it is messy. I first looked at the estimated weight (from experience) the estimated RPM (from measurement), and the designed rotor system. I guessed at a Lift Coefficient of something like 0.5, that when put into the lift equation for a rotor matched my estimated weight. I then found the Cl/Cd curves for a an airfoil at the reynolds number that we were operating at and determined what angle of attack was needed to get that lift coefficient. I then picked a desired airspeed and figured out what angle that the rotor had to have so that the retreating blade was well below the stall angle somewhere about 2/3 of the way out the blade and the advancing blade was operating at something close to the design lift coefficient.
This set both the blade rigging angle and the mast angle.
To do this you have to have all the numbers, weight, blade area, rpm, desired airspeed and airfoil characteristics at that reynolds number.
For G3PO this turned about to be something like -2 blade angle (measured from zero lift line, not the chord line) and 18 mast angle.
From there it's basically build, test, and tweak.
If you look at G3PO it basically taxis, flies and lands with about the same attitude with it's 72 mast angle.
The un-messy way is to kinda accept that for these size models, these numbers aren't going to vary much and start with the standard numbers, build, test and tweak.

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Originally Posted by Tom Wright View Post
Does this suggest less than perfect rotor shaft / cog alignment or a design feature to increase the pitch stability margin?
No way to be in the designers head, but the shaft COG alignment is poor in the G1/2 as far as I'm concerned because of how little elevator power is available when the power is off. Seems like the stab/elevator size is an attempt to recover some control power without redesigning the fuse pod.
I really think if you are going to finely optimize an elevator controlled model it needs to have a flying stab in addition to the correct rotor/COG alignment.
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Old Nov 06, 2013, 01:54 PM
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Mickey
I am glad I asked the questions in my previous post , as your response was very useful.
Your suggestion to try an all flying tail plane is a good idea particularly when experimenting with prototypes, so I will give it a try in conjunction with the COG set advice below which is a quote from your GPO3 PDF.

Quote
To set the CG, pick up the model with two fingers
looped over the fly bar with the fly bar turned
perpendicular to the body (blades will
be fore and aft). Hold the model up so you can see the side. Adjust
the CG until the main mast hangs straight up and
down when the model is hung by the fly bar. You can
sight against a door frame in your house for a vertical
line. Now move the battery
a tiny amount forward
until the nose comes down 1 or 2 degrees from the
previous position. In other words you want the main
mast to just past vertical
in the nose down direction.
End quote

As my current projects are centered on a low mast heights, and the numbers that describe the rotor shaft angle will be similar to the ones suggested in your reply a compromise with mast angle is necessary to avoid boom strikes . My question here is what would be the best approach to minimize the effect of a rotor shaft angle set lower than the calculations suggest ?

Tom.
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Old Nov 06, 2013, 02:36 PM
Mickey from Orlando. Really.
Joined Nov 2004
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Originally Posted by Tom Wright View Post
My question here is what would be the best approach to minimize the effect of a rotor shaft angle set lower than the calculations suggest ?
Don't assume that the tail boom is the 0-0 reference. In other words, suppose you have a mast angle of 5 with respect to the boom. Redraw the model with the boom at a positive 10 angle. Then draw a horizontal line on the redrawn model. Now make the horizontal tail and motor box/thrust line parallel to the horizontal line, not the tail boom.
This gives you all the same clearance but the rotor is at the right attitude.
It's easy to simulate, just remount the horizontal tail at a new angle and put a lot of downthrust in to match the horizontal tail.
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Old Nov 06, 2013, 02:43 PM
Mickey from Orlando. Really.
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Quote:
Originally Posted by Tom Wright View Post
so I will give it a try in conjunction with the COG set advice below which is a quote from your GPO3 PDF.
Keep in mind that G3PO uses a fairly rigid rotor. There is a piece of metal bridging the blade bolts! This means that G3PO rotor basically doesn't flap very much and the nose down cyclic is being applied directly. In G3PO the trim condition has the swashplate a few degrees nose down from the shaft. It's the same amount of cyclic to stop the up flap from motion but is accomplished without having to have the rotor flap back.
Also G3PO steers much more from direct force on the shaft rather than thrust/CG misalignment. The rotor basically never tilts from the nominal position while maneuvering so the rotor thrust is always in the same direction.
Flexy/tilty plate heads are different.
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Old Nov 06, 2013, 04:39 PM
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Quote:
Originally Posted by mnowell129 View Post
Don't assume that the tail boom is the 0-0 reference. In other words, suppose you have a mast angle of 5 with respect to the boom. Redraw the model with the boom at a positive 10 angle. Then draw a horizontal line on the redrawn model. Now make the horizontal tail and motor box/thrust line parallel to the horizontal line, not the tail boom.
This gives you all the same clearance but the rotor is at the right attitude.
It's easy to simulate, just remount the horizontal tail at a new angle and put a lot of down thrust in to match the horizontal tail.
This indeed confirms my current line of thought ,so it will be interesting to do a comparative between the two set ups, my immediate concern would be the power off decent performance?
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Old Nov 06, 2013, 06:32 PM
Mickey from Orlando. Really.
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Originally Posted by Tom Wright View Post
This indeed confirms my current line of thought ,so it will be interesting to do a comparative between the two set ups, my immediate concern would be the power off decent performance?
I assume you mean descent performance....
It should be the same, or better, the stab should be at a better angle.
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Old Nov 06, 2013, 07:31 PM
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I am looking decent performance in the descent

Should have a newly configured model ready for flying in a couple of days
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