Untwisted Autototation>

[bugdevheli]

Has anyone had experience doing autorotations in a machine that has blades that are parallel with no twist. If so are there any notable diferences compared to twisted blades. Thanks Bug

[zeeoo]

Hi Bug,

Quite all the gyros use untwisted blades and autorotate fairly well ;) (www.rotaryforum.com)

the question could be "what is the better for autorotation : flat or twisted ?"

cheers

Victor

[Rotorbee]

If I remember right Prouty's Helicopter Aerodynamics a twist inverse of that of a normal helicopter would be best for the gyrocopter or for autorotation. Seems logic to me, because the rotor would be optimised for the air comming from below.

[jellycopter]

Bug,

The Rotorways that I fly have blades that have no washout. The autorotation characteristics are good - and mere mortals like me can't tell any significant difference in auto characteristics.

Blade inertia and aerofoil section play a huge part in the auto characteristics.

J

[CRAN]

Bug,

Providing the rotor is sized correctly, an untwisted rotor will autorotate acceptably. To understand the detailed aerodynamics of the particular geometry you have in mind I think you will need to do some blade element analysis. Have a look at Leishman's book.

However, with a low cost homebuilt helicopter, I assume that you are considering non-twisted blades for manufacturing simplicity. If this is the case then there are two points that are far more important than the aerodynamics of untwisted blades in autorotation.

Firstly, twisted blades will create the same thrust on about 90% of the power of an equivalent untwisted blade. Therefore, by not twisting the blades you are creating a serious rod for your own back in terms of the amount, size and weight of the power you will need to install to get the machine of the ground.

Secondly, providing the rotor is acceptably designed then the aircraft will autorotate...providing you can get it into autorotation before rotor stall in the first place. If you are trying to design a machine with 'good-autorotation characteristics' then my suggestion is to focus on installing adequate rotor inertia, to give the pilot plenty of time to enter autorotation in the first place and then the required energy to tidy it up at the bottom. This involves the use of tip weights and a sufficiently strong rotor system to carry the additional radial loads.

PM me if you need some more help.

CRAN
:ok:

[Dave_Jackson]

-12-deg (or greater negativity) twist is best for hover.
-6-deg (+/-) twist is best for fast forward flight.
+1-deg (or greater) twist is best for autorotation.

Independent Root and Tip Control (http://www.synchrolite.com/B436.html) is best for everything. :ok:


Edit
I goofed :ouch:
The signs, which preceed the values, have now been corrected.

Twist is the change in the pitch of a blade when viewing from the root out to the tip. Negative is downward and positive is upward.

[zeeoo]

Dave ,
MIMS (Maybe i miss something)

How can you reduce the reverse flow if you twist positively at the root ? doing so, the airfoil gererates a reverse thrust, less drag, more reverse flow.

BTW (by the way) , the common pitch used in gyro blades is from 1 to 3 degrees...

or DOIMSA ?? (do i miss something again)

TY (thank you)

[NickLappos]

Dave has it right (as usual)

The twist on a powered rotor is mostly for hover and mostly to get the entire blade involved in the lift. The tip has less angle by about 10 to 16 degrees, so the tip (which operates at much higher speed) is producing less lift and the root is producing more than if there was no twist. Getting the whole blade working is a way to make the whole disk produce moving air, so the average downwash velocity can be lower, and the mass of air moved casn be greater. This makes the power needed less, so the engine can lift more payload. In other words, blade twist is like making the rotor act larger, and making the downwash more uniform and more efficient.

That is for powered flight. In autorotation, the blade is now in reverse flow, the so-called windmill brake state. In effect, each piece of blade is acting to slow the airstream down (the windmill is braking the flow). That means each piece is producing a negative downwash, and a reverse twist would be best. If the autorotation was purely vertical, the twist should be the exact reverse of the twist for a powered hover, but the autorotation is always made in forward flight, where the flow is much more complex, and twist much less useful. That is because the blade is going upstream and downstream, and varying its collective angle with cyclic, so the actual angle of attack is not an easy thing to optimize.

In short, the twist is nearly reversed when going from powered flight to autorotation.

[Dave_Jackson]

zeeoo,

Blade twist is the difference in a blade's pitch between its root and tip. On a helicopter with twist, the tip has less pitch and the amount is referred to as a negative value, in degrees.

The blades on a gyrocopter have a small amount of positive twist, because they operate full-time in autorotation.


Dave

[zeeoo]

Nick,
Thanks for explaining such complex things in such simple words.

If the explanation is very clear in the case of the powered rotor, it is still opaque in the case of a gyrocopter.

What should be the ideal blade design ?

Dave , TYFTE ( thank you for the explanation)

[bugdevheli]

Thank you for your replies. In view of your various comments, would it seem reasonable to assume that the time lag between downflow in powered flight, and upflow during autorotation, is increased with a non twisted blade because of a cleaner transition due to the inboard area of a twisted blade producing more drag(which pulls down the mrrpm more quickly) whilst the rotor system is in the equilibrium state prior to the flow reversal and subsequent autorotation.

[Lu Zuckerman]

To: NickLappos

Your explanation sounds reasonable but it addresses specific points in time (flightwise). But what about how the theory of blade twist is taught to pilots and mechanics at most factory schools including the service school at Sikorsky. I was taught along with thousands of “mechanics” , maintenance officers and pilots that have attended countless factory and military schools is that the twist is incorporated to minimize spanwise bending and the attendant stresses on the blades or the rotorheads. In otherwise it tends to equalize lift across the blade span. If I am wrong then you had better talk to the guys at your service school.

10-16 degrees negative twist?

Fire away, I'm in a protected bunker.

:E :E

[zeeoo]

Lu,
Could you give your opinion on an ideal blade design for a gyrocopter please ?.

Bug,
are you working on a helico or gyro project, or is it "top secret" classified ? :D
Thanks

[bugdevheli]

Just messing about as usual Zeooo. You know how it is. Make it, Try it, Break it Make it again. Thanks Bug.

[zeeoo]

Bug,
unfortunately, or fortunately, i haven' break somthing (yet) :D
I am in the phase A : "Build something"
perhaps the phase B will be : "trash it" or "repair" , i hope it will be "fly it".
But i would be curious to read about your broken things..
cheers
Victor

[Dave_Jackson]

Lu,

Before Nick beats the :mad: out of you, you have said exactly what Nick said.

Twist brings some of the thrust inward toward the mast and away from the tip. I.e. a reduced moment on the blade and therefore a reduced out-of-plane bend. :ouch:

Dave

[NickLappos]

Thanks, Dave! The problem with Lu's statment (as he transcribed from that little blue book he always quotes) is that he has it backwards. The twist evens out the lift across the blade's span, and makes the helicopter carry more weight for the power. Twist is secondarily a stress reducer at the tip, but a stress raiser inboard. The max twist on an aluminum blade is about 10 degrees because any more will cause the bending load to be too high. With twist, the inboard sections of the blade loses stiffness since the twist permits buckling which vastly reduces the strength of the blade.

The twist is used in hover primarily, and low speed flight. At higher speeds, a variable twist (recall those piezo-electric tabs) is best, where the lift distribution can be kept perfectly even while the blade whirls around the mast, going upwind and downwind.

[jellycopter]

Nick, Cran, Dave J et al,

What's the typical magnitude of hover efficiency gained by adopting 'washout' in a blade as opposed to a non-twisted blade.

It's a loaded question as the Rotorway needs all the 'lifting' help it can get and I was wondering whether a more efficient, twisted blade design would be a relatively inexpensive performance enhancement.

As it stands, the Rotorway autorotates extremely well with plenty of time to get the lever down and RRPM recovery easy. It might be worth sacrificing some of this for improved hover performance which, lets face it, is where most of these helicopters spend most of their time.

Could the required twist be provided dynamically by a fixed outboard tab on the existing blade (Aluminium extrusion leading edge with bonded/rivetted skins) or could you see structural / stress-related difficulties in such an application.

The current method for fine tuning the tracking is to bend the trailing by a few thou'. Therefore, I suppose simple blade twist could be established by bending the trailing edge to achieve reflex at the tip and downwards towards the root......

Your thoughts would be appreciated.

J

[Talk Turn]

As Lu says Blade Twist or Washout is there to minimise longitudinal stresses on the blade.
I guess that the tip of the blade which is going much faster than the root will need less pitch on it to generate the same amount of lift as the inboard part of the blade which is slower and therefore needs more pitch.

[bugdevheli]

Jellycopter. Bending the trailing edges as envisaged will not achieve the extra lift that you need. The blade chord has its centre of lift at the 25% fom the leading edge so as to provide maximum efficiency throughout the range of pitch change. Radically altering the setting of the entire trailing edge would degrade the lift coeficient that the blade already has. My guess is that it would also have a detrimental effect on the pitching moment causing vibrational feedback to your cyclic. Bug.

[zeeoo]

a question for the specialists :

about an ideal blade design for autorotation.

what about a VR7b airfoil

a 90 % taper

a twist from - 7 deg at the root
to +2 deg at 75 % of the span
to +1 deg before the tip.

a sweptback tip at 0 deg with a little 2 deg anhedral

thank you

[Dave_Jackson]

jellycopter,

" What's the typical magnitude of hover efficiency gained by adopting 'washout' in a blade as opposed to a non-twisted blade."

Wow, did you get lucky. :D
Prouty's 'Combined Momentum and Blade Element Theory with Empirical Corrections' from Chapter 1 ' Aerodynamic of Hovering Flight' just happens to be coded into the computer. In addition, the data on Eric Cymbaliuk's Rotorway just happens to be in there. In fact, it's in there twice, one with the NACA 0012 airfoil and the other with the 8-H-12 airfoil.

The input data has not been check and it might have been modified from the actual craft for experimentation, but the comparisons should still have validity. Hover is at GW. A twist of -8-degrees will be used.

0012 airfoil with no twist.
Collective pitch = 8.21-degrees
Power = 117.5 hp

0012 airfoil with -8-degree linear twist.
Collective pitch = 14.18-degrees
Power = 109.3 hp


8-H-12 airfoil with no twist.
Collective pitch = 4.44-degrees
Power = 95.7 hp

8-H-12 airfoil with -8-degree linear twist.
Collective pitch = 10.41-degrees
Power = 91.7 hp

Please remember that these values are theoretical and that Prouty's algorithms are obviously intended for larger helicopters.

Dave

[zeeoo]

Dave,
your numbers show clearly the gain.

but these numbers are for helicopters..have you got similar comparisons made on gyro blades.. in other words, what should be the ideal data for a gyro blade.. if you know the answer, of course.
cheers

[delta3]

Sorry also not for a gyro

First a table (showing hp / descent rates as a function of twist. Bold is optimum.

http://www.e-sign.be/private/heli/Twist_compared.jpg

An example of difference between 0 and 6 degrees of twist when doing an autorot, full fuel, one pilot, 80 knots

+6 degrees (standard R44 -my measurements) :
http://www.e-sign.be/private/heli/Autorot_twist.jpg

0° twist :
http://www.e-sign.be/private/heli/Autorot_no_twist.jpg

The flow is much more linear in the case of zero twist and descent rate seams to be optimum.

delta

[zeeoo]

Delta 3
interesting.
what i see is hat a twisted blade allows an optimal in autorotation appereantly. i can see that in the non twisted blade, apparentlly, the sum of lift is higher.. or is it an impression ?
what i can also see is hat the lift could be re-centered a little .. maybe with a non linear twist ?or am i wrong ?)
Thank you

(how did you get this software ?)

[delta3]

Zeeoo

For optimality the lift distribution should be linear in the radius, since the surface the rotor acts on increases linearly with the radius (tube element surface = 2 * pi * r * dr) . The idea is to have constant induction.

For the given W&B :
In hover this requires a +9 ° twist. I can give the graphs if you whish.
In autorot it looks like 0° gives the most linear lift.

This geometric reasoning is also confirmed by the power math:

9° in hover gives the least power.
Between -3° and 0° in autorot gives the least descent rate.

To be sure: the above graph (tulip shape) is with twist (nl +6° which is the R44 twist according to my measurements), the straight shape below is without twist.

Remark : I don't know about standard sign conventions but with +6° as in the R44, I mean greater angles of attack in powered flight at root diminishing towards the tip, standard Helo mode so to speak

The autorot model should approach autogyro circomstances (I am not a specialist on that). As Nick said in vertical autorot negative twist should be optimal, nl the reverse from powered flight. This is however not a design mode (as opposed to helos where hover could be a design mode), since there should always be forward motion (hope so at least). As soon as there is forward motion the optimum moves to zero.

Software: Own on-going development, pretty close now to full 3D

Delta3

[zeeoo]

Delta3,
thanks for your precious explanations, it enlights me a lot in the path i have to take.

I as wrong thinking "helico" about gyro blades..i must orient my searches through this kind of data.

So, according to you and Nick , and ideal gyro blade should have the inverse twist of a helico one. right ?

of course i'd appreciate the graphs, thanks !
Can i use your pictures on another forum ?

thanks a lot.

Victor

ps : i own a small CAD/real time 3D business..in case you need something :D

[NickLappos]

zeeoo,

I think the twist for forward flight in autorotation is a mystery for me, but it would have to be very much lower than the twist for powered flight. If you were optimizing the twist for a zero knot autorotation, (a vertical autorotational descent) then I think the ideal twist would be the exact inverse of the twist for powered hover.

[zeeoo]

Nick,
thank you for your opinion.
i was not convinced of the need for a neg. twist but, as a newbie, I made my homework and it really makes more sense indeed, Dave had the good numbers.

I consider a gyro rotor flying at about 130 mph with a disk angle of 8 degrees back. I am trying to think about what would be the blade design for that purpose.

One thing ticks me... what about the tip in this case? it cant be positive twist.. high tip speeds, it must have a zero or very small twist.
Could a gyro blade be more (+) twisted in the autorotative zone then have a (-) twist elsewhere ? that means a non linear and non progressive twist.. right ?

that would also mean a different bending moment.

Thank you

[NickLappos]

zeeoo,
The confusion I have is to decide which factor domnates, the speed effect on the advancing and retreating side, ot the desire to balance the lift (thrust?) of all the blade sections. I have a a gut feel that the speed effects truly dominate the issue at 130 knots, especally when one considers the change to the tip between the upsweeping portion of the blades trip around the mast and the downsweeping portion. I think the twist to alleviate the differences between the speed effects dominates, thus I think the high speed autorotation twist (130 knots, 8 degrees of rotor attack) is closer to the powered flight twist, perhaps half that of powered flight. On;ly a gut feel, though.

Can you run three cases, one withlinear twist at 10 degrees positive (conventional helicopter) and one at 3 degrees positive twist and one with 3 degrees of negative twist (all at 130 k and 8 degrees of rotor back tilt). Solving for HP and lift distribution could tell us the answer.

[Dave_Jackson]

zeeoo,

In an earlier post you said " DOIMSA ?? (do i miss something again)"

No. You were right and I was wrong. The posting has now been corrected. Thanks.

____________________


This thread invokes an interesting experiment, assuming that it has not already been done in the past.
The experiment would consist of taking a simplistic rotorcraft and providing it with the following features. The angle of the rotor mast can be incrementally rotated from a 10-degree aft tilt to a 10-degree forward tilt.
A pusher propeller, which has a pitch that can be varied from full feather to flat pitch.
A means of modifying the ratio of the power that goes to the propeller and to the rotor.
Test runs would start with the rotor at the full aft tilt and all the power going to the propeller. [fully gyrocopter mode]
Subsequent runs would consist of incrementally; advancing the angle of the rotor mast, and shifting power from the propeller to the rotor.
On the final run the mast would be tilted fully forward and all the power would be going to the rotor [fully helicopter mode]

The objective is to discover which run (which configuration setting) resulted in the highest forward velocity for a fixed power, or alternatively the lowest power for runs at a fixed speed.

Roughly speaking, the craft might be thought of as a simplistic compound helicopter, which does not have wings, or as a gyrocopter with full-time partial power at the rotor.

[slowrotor]

I have a 12" four bladed rotor off a toy RC helo.
When dropped from the ceiling the rotor autorotates in the reverse direction. Rotating backwards the blade angle is quite negative, yet it autos pretty well spinning up and the descent rate slows.
I was actually surprised, I thought it would screw itself down faster with what appears to be about 10 degrees negative pitch.
But with vertical inflow the angle could be normal.
Quite interesting this autorotation business.

Negative pitch near the root gets my vote for best auto.
I suspect the gyrocopters use flat pitch as an easy compromise.

[zeeoo]

Nick,
if we are talking about a kind of gyrodyne, the speed factor would be a little more important than in a helo, that's a field where they can compete.
But i can't drop the manoeuvrability factor, that's a field where they can also compete.


It's interesting, you have the feeling that a "high speed gyrotor" could be closer to a helo one.. it makes me think that a compromised helo blade (lessened twist) could fit.

Dave,
i have a proposition for what you want, easy to do, yes i said "easy" :)

Slowrotor,
my bro in law also runs a RC helo and he can fly upside down :D, but i wonder if the behaviour of a scale model can be interpolated from a RC one... In your case, i feel that the rotor drag slows your descent, but it is not real autorotation.
I suspect also that gyro blades are flat to be easily manufactured (extruded blades, straight composites molds).

Thank you

[NickLappos]

"If it looks like a wing, it will fly almost as good as the best wing." K. D. Wood

twist is used to squeeze the last 5% out of the machine, but twist is expensive, hard to make, and weakens the blade structure.


zeeoo, when I use high speed in this thread, I mean the 130 to 150 knot range. Twist is much less helpful there, and might reduce the stall margin, too.

[zeeoo]

Thank you Nick,

The KD Wood quote reminds me "if it looks like gold.. then it is gold.."

i'm inclined to copy the best designs "as i see them" but there are so much little tricks..

My next question would be about the importance of the reverse flow zone in the perfos. La cierva and most of the old gyros neglected this zone or cared more about the lift zone..

How would you rate the importance of thos zone.

I hope i don't abuse of your knowledge, If you reply, i'll mark you on my "pay a beer" list

cheers

[Dave_Jackson]

zeeoo,

You said; " i have a proposition for what you want, easy to do, yes i said "easy" "

Yes! I definitely would like to hear your proposition.

Dave

[zeeoo]

Dave,

i said easy and it is, this is just what i am going to do on my machine. I think always in the "scalability" ;)

Ok let's makea deal : i give you the idea and you help me in some "calculus" :D

the idea is simple : the mast is mounted on a 2 axis structure. one screw holds for-aft, another left-right. you adjust the for-aft and you can obtain a disk angle from 8 to -8 degrees. you could even adjust the lateral angle to counter adv blade torque.

I know the objection you will oppose to a such mechanism and i know the answer i will give you.

cheers

edit : this system is not intended to be changed in flight but adjusted on the ground

[bugdevheli]

Whilst expounding my theory on rotors in autorotation,a friend of mine raised the question. In powered flight does the retreating blade on the helicopter produce any enengy to drive the helicopter foreward due to the rowing action on that side. I am still thinking about a possible answer. Any suggestions. Bug.

Dave Jackson. Im a bit brain dead this evening. Can you run through how the new heligyro maintains level flight during the transition from auto to power driven. Not trying to be a smart ass. Just cant think it through!. Bug.

[Lu Zuckerman]

To: Anyone interested in reading this.

It has been stated that some if not all autogyro or gyrocopter blades have no negative twist. I can't argue the point. However I read on an autogyro forum that when the blades are running at or near flat pitch and the rotor disc is moved to effect a maneuver the blade tips could go negative to the airstream and cause the blades to diverge form their plane of rotation.

This may have happened on the Cheyenne helicopter. I say may as the Cheyenne exprienced two rotor divergent incidents or, it might have been as a result of the blade design.

You can talk amongst yourselves (Mike Meyers)



:E :E

[Dave_Jackson]

zeeoo,

" ...you help me in some "calculus"?".

Sure. Calculus was the Roman founder of Calcutta. Next question. ;)

Upon further thought, the so-called 'experiment' might not be so simple. Variables such as; compensating for rotor induced yaw, twist, type of rotorhub, range of forward speeds, etc., etc. would have to be considered.

The subject is of interest because any 'true rotorcraft' that is intended for future high-speed flight will certainly require forward thrust augmentation. At the opposite end of the spectrum, one or more recreational gyronauts have said that partial power assist to the rotor has resulted in more efficient slow-speed flight.


bugdevheli,

"Can you run through how the new heligyro maintains level flight during the transition from auto to power driven."

I assume that you are concerned about the craft transitioning from level powered gyrocopter flight into level powered helicopter flight. The idea is/was to develop the craft in a number of stages. If the craft is at the final stage of development, the transition would be made by transferring engine power from the propeller to the rotor. while at the same time increasing the rotor's collective. In reality, the craft would probably never be required to make this particular transition, since the flight and power-train control linkages would be optimized for a specific flight mode at the specific forward speed.


Lu

Prouty has three books (now one new book) that have dozens of short and interesting articles, and there is no math. :ok:

One of the articles is called 'Stopping a Rotor in Flight'. It discusses the Lockheed XH-51 and the loss of centrifugal stiffening in a blade when it's rotation is slowed down. He explains the bend-twist coupling by discussing the wings of a plane when swept forward and when swept back. All very interesting.

Dave

[delta3]

bugdevheli

that idea has also occured to me when analysing the data.

My answer would be yes, but than what is rowing ?

I visualise this as follows: rotor drag can be decomposed in two components :

(1) the profile drag, which is the drag component of the aeroforces projected on the hubplane

(2) the induced drag, which is the lift component of the forces projected on the hubplane

If you look closely at the blade flow plot of the retreating blade (Figure A) (90° in my conventions) you see that the net flow (the green arrow) is quited tilted. The lift which is defined perpendicular to that flow is also tilted (to the left in the figure so the forward of the heli, since we are talking about a retreating blade) resulting in a significant induced drag. So for the retreating blade this force is indeed propulsing (=rowing) the heli (since it point forward)

On the other hand the forward blade has a far flatter pitch, the figure shows that the tip is almost autorotating and not producing a lot of lift nor induced drag

The overall drag is also displayed (Figure C) :
(1) the blue mesh is the profile drag,
(2) the green mesh the induced drag
(3) the multicolor plot the total drag : this shows the overal result of rowing

The fourth figure displays the resulting forces ('approx' used by the the dynamic simulation engine, 'precise' also fully calculates stall in the reversed flow area, differences are small)

The quantity DragHPx is the (rowing) forward force as projected to the hub-plane (in my definition the neutral plane of the rotor which is slighted tilted forward in the case of an R44) : for the given parameters it is between 163 and 194 newtons, which is more than half of the forward forced produced by the rotor lift which is 306N (because the rotor plane is tilted forward with respect to the hub-plane, as a result of flapping)

For who is not yet lost the total forward force of 952 N is greater than the som of 163 and 306 because of the 3 degree tilt of the hub-plane with respect to the heli, which gives an extra forward contribution by the main lift.


delta3



Figure A (retreating blade)

http://www.e-sign.be/private/heli/retreating.jpg



Figure B (forward blade)

http://www.e-sign.be/private/heli/forward.jpg



Figure C (total drag)

http://www.e-sign.be/private/heli/drag.jpg



Figure D (forces)

http://www.e-sign.be/private/heli/power.jpg

[bugdevheli]

Dave jackson. Thanks for the explanation. Makes sense now. Delta3 Thank you for exploring the rowing idea. I will tell my friend that he s not entirely mad. Regards Bug.

[zeeoo]

A silly question to all :

is the twist relative to the chord or the zero lift line ? (i think the second one but...)

thank you