# Rotor solidity.

#### JETLAG03

##### Active Member
Hi folks.

I have read somewhere that rotor solidity is important regards security and handling etc, I think I read that 0.035 (ammended) was the magic number. If not exactly there is it better to have a solidity number higher or lower than this??
But, when doing the calculations does one take into account the total area swept by the blades or the area swept by the blades minus the area swept by the rotor bar.
This question then got me thinking as I have one set of blades and two rotor hubs giving me the option of a total diameter of 7.7m (25.3ft) or 8.3m (27.2ft).
Then further questions raised their ugly heads, the rotor disc loading, is this calculated to include or exclude the rotor bar swept area, and same question for the blade loadings, so, I played with an excel spreadsheet out of curiosity to see what magnitude of difference with and without the hub bar made.

here is what I discovered using kg 330(728lbs) and 450kg (992lbs) as for most of us here in France these are our limits. 500kg for the latest machines but not sure if that is also for we home builders??

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#### Tyger

##### Active Member
"Solidity" is the nominal surface area of the blades divided by the area of the disc?

#### WaspAir

##### Supreme Allied Gyro CFI
I have read somewhere that rotor solidity is important regards security and handling etc, I think I read that 0.35 was the magic number.
Is there a typographical error here? 0.35 sounds impractically huge to me.

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#### JETLAG03

##### Active Member
Is there a typographical error here? 0.35 sounds impractically huge to me.
oops

0.0345 TO 0.0386 in the attached spreadsheet. Just one little 0 makes such a difference eh

phil

#### C. Beaty

##### Gold Supporter
With modern power plants, efficiency is less important in the case of hobbycopters which often go as high as 70 lb/ft²

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#### JETLAG03

##### Active Member
With modern power plants, efficiency is less important in the case of hobbycopters which often go as high as 70 lb/ft²

Hi.

With regards to the calculations, one of the questions I posed in the OP " do these calculations when taking into account the length of the blades include or exclude the part of the hub bar.

Similar, I asked, when calculating the disc loading does this calculation use or exclude the m² taken up by the rotor bar.

p.s. In my calculations attached I have included both blade and disc loading (best this thicky can)

phil

#### C. Beaty

##### Gold Supporter
The effect of the hub bar is generally ignored since it accounts for such a small portion of rotor disc area and air velocity is so low at inner portions of the rotor that little lift can be generated.

#### Jazzenjohn

##### Gold Supporter
What effect would there be in reducing blade loading to lower than 35 lbs/sq. ft? Slower RRPM leading to increased retreating blade stalled area and a lower practical forward speed limit? If speed were limited to 40 mph (or some arbitrary number) wouldn't you see increased efficiency? Or would it be too small to make much of a practical difference?

#### C. Beaty

##### Gold Supporter
A rotor’s best lift/drag ratio occurs at a tip to forward speed ratio of 0.35; if forward speed is 40 mph then best rotor tip speed is 40/.35 = 114 mph. That provides the best compromise between stalled area tip speed loss.

#### C. Beaty

##### Gold Supporter
This is beginning to feel a bit like an “advice to the lovelorn” column in a newspaper. What ever became of Ann Landers? I suppose Ann is long gone as are most newspapers.

#### JETLAG03

##### Active Member
This is beginning to feel a bit like an “advice to the lovelorn” column in a newspaper. What ever became of Ann Landers? I suppose Ann is long gone as are most newspapers.

Not sure who "Ann Flanders" is/was.
If I seem a little pedantic is it simply because I asked specific questions and in common with so many postings the replies either did not address the question or only in part. If you feel you are giving out the equivelant of "advice to the lovelorn" then please feel free not to respond to my enquiries, I do not post questions to be mocked for my lack of understanding.
The OP poses the question is the area M² and swept M² taken into account or not when calculating loadings for blades and disc m².
With the long rotorbar (1.1m) on my craft my total disc (8.3m) m² = 54.1055 m² (0-4.15 metre radius) and excluding the rotor bar 52.2380M² = (0.55 - 4.15 metre radius).

I remain respectfully

phil (de fer)

#### Jean Claude

##### Junior Member
With our usely rectangular twoblades, the aspect ratio A is Radius/ Chord A = R/c
and multiplicating by R/R gives R^2 / R.c ie S disc /π S blade

So, the solidity ratio σ is like the opposed of the blades aspect ratio.
As long the speed tip is not approaching the Mach .7, the best solidity is the smallest.

Do not take into account the disappearance of the blades in the center: The center is inactive.

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Ann Landers:

#### Resasi

##### Gold Supporter
I do not post questions to be mocked for my lack of understanding.
You were not being mocked Phil, Chuck was being humorous. It was interpreted by me as a gentle piece of fun only.

The difference in language, lack of knowledge of the person, and our inability to read facial expression in the form of communication does sometime lead to misreading posts.

Of course, I may myself have misread his response...it is posible that he may have replied acerbically.

As for Anne Landers Chuck has supplied her details in the post above.

#### JETLAG03

##### Active Member
@Resasi I thank you for your concern and explanation. I am not a well read man, for example I had to google " acerbically" I pose questions here and elsewhere because I do not understand a subject in the hope that some-one more intelligent than I (there are many) will enlighten me.
Both Chuck and Jean Claude to name but two, are very intelligent and knowledgeable and I admire and respect their input. Both have sometimes posted replies that are complete and complex, and when requested have given deeper and easier to follow explanations. Chapeau

The problem is, when one receives a reply that is perceived as mocking or insulting, it is not only the poster that feels offended, but many other people who may have been of a mind to post will reflect on the risks of receiving a similar response, do not post at all we all loose.

Personally, I am an old fart (64) with skin as thick as a rhino and don't care.
I try to think carefully before posting in case I may cause an offence.
Because Chuck replied simply to supply a link to Ann Landers, some one we in Blighty would call an "agony aunt" does nothing to improve the "miscommunication"

I said my piece, I am not offended, simply disappointed, but, I think I shall live.

phil

#### Doug Riley

##### Platinum Member
Chuck Beaty has a dry sense of humor that would well befit a Yankee. It probably doesn't translate to other languages very well; humor seldom does.

To the OP: the solidity ratio, in my experience, usually is simply the ratio of the blades' total area to the entire calculated area of the rotor "disk" -- the latter equaling simply pi x diameter, with no subtraction of the blade area.

As with any empirical "rule of thumb" that treats the rotor as a "disk," solidity is a simple but useful fiction. The rotor is not a disk, nor is it a cone or a circular wing. It is one or more rectangular wings of quite high aspect ratio, with a specific airfoil section. These wings experience various airspeeds and angles of attack. No more and no less.

#### All_In

##### Gold Supporter
@Resasi I thank you for your concern and explanation. I am not a well read man, for example I had to google " acerbically" I pose questions here and elsewhere because I do not understand a subject in the hope that some-one more intelligent than I (there are many) will enlighten me.
Both Chuck and Jean Claude to name but two, are very intelligent and knowledgeable and I admire and respect their input. Both have sometimes posted replies that are complete and complex, and when requested have given deeper and easier to follow explanations. Chapeau

The problem is, when one receives a reply that is perceived as mocking or insulting, it is not only the poster that feels offended, but many other people who may have been of a mind to post will reflect on the risks of receiving a similar response, do not post at all we all loose.

Personally, I am an old fart (64) with skin as thick as a rhino and don't care.
I try to think carefully before posting in case I may cause an offence.
Because Chuck replied simply to supply a link to Ann Landers, some one we in Blighty would call an "agony aunt" does nothing to improve the "miscommunication"

I said my piece, I am not offended, simply disappointed, but, I think I shall live.

phil
Hi Phil
I was going to respond that Chuck is like Ann's expert advice to the lovelorn only He's the Ann Landers, Guru extraordinaire of rotorcraft aerodynamics, design, experimentation, research, invention, knowledge, and real experience with and of gyros and the only one answering all of our questions most of the time. That is how it is like Ann and I hope explains the humor American's understood who read her column if even only once.

His comment was funny and no slight to your excellent questions, that I'm struggling to learn more about. You are way ahead of me but your questions helped me so much. What helped me the most to catch up, and start to understand the importance, was the excellent link Jon shared!!!

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#### All_In

##### Gold Supporter
I'm not sure you realize how much your questions help all of us learn.

It cannot be misunderstood and is the universal language of truth!
I personally thank you for your posts!!

#### JETLAG03

##### Active Member
@All_In The reason I like math is there is no grey area. It is either right or wrong.

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