Propwash article

Doug Riley

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Jan 11, 2004
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A few people may have glanced at my article in the latest PRA magazine. There's a typo that's misleading and contradicts what we actually found.

In the diagram of the propwash of Mike Gaspard's gyro, there are two test points labelled #3, but none labelled #4. The one lower in the picture is actually #4. There was fast propwash in this Location #4, as the chart states.

Sorry, guys 'n girls.
 
G'Day Doug,

Any chance it can be posted since it has gone to print?

Cheers,

Mitch.
 
G'Day Doug,

Any chance it can be posted since it has gone to print?

Cheers,

Mitch.
People that are interested in this type of article can join the PRA and get the real thing!

PRA members like Doug, Chuck B., Greg G. and a host of other PRA members are providing a free service here to those that don't feel that they have an obligation to support the only U.S. organization devoted to experimental rotorcraft.

Chuck threatened at one time to quit posting the facts on the Internet because of his thinking that it detracted from the PRA efforts. But I guess his feeling of obligation to the gyro community over rode that. Too bad others don't honor thier obligation to the PRA. And those that say they don't have an obligation are in denial!

Doug, great article! This is going to be another one of those PRA studies, to join those of Chuck's and others thru the years, that people can refer to during theory discussions, some of which will be done here.
 
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G'Day Dean,

That's rather abrupt of you ole Man. But thanks for your opinion.

My obligation is to ASRA. The AUSTRALIAN SPORT ROTORCRAFT ASSOCIATION. I come to this forum and others to learn. Not to be told I need to join your club and buy a magazine.

Fact is I used to be a member of the PRA but the PRA had some major problems (Large numbers of Board members resigning, who owns the airport) and some little problems (web site issues etc) and getting a magazine was like paying for a lottery ticket.

So with all due respect to you and Chuck and Doug or anyone else that writes for the PRA, or who are members of the PRA, I'll just wait till a Mate emails me a scanned copy. All of Greg's stuff is available written sound etc. Doug posts freely as does Chuck.

People who are interested in this type of article and information join forums such as this Dean. Did you buy your copy of the Houston Report on Stability?

As for these guys....." PRA members like Doug, Chuck B., Greg G. and a host of other PRA members are providing a free service here to those that don't feel that they have an obligation to support the only U.S. organization devoted to experimental rotorcraft. "

They provide a free service to the forum members and highly valued and appreciated they are. I hope we get to discuss and learn more about prop wash here.

Cheers.

Mitch.
 
Mitch:

I posted the results of the tests here in a summary form, right after we did them. That was in April, 2007. The bottom line is that, in the vicinity of the tail surfaces, the propwash is a cylinder that's 1/2 - 2/3 of the diameter of the prop itself.

At WOT, the speed of the wash in its fastest zones is a little higher than the gyro's own maximum airspeed.

Important conclusion: a HS on the keel provides zero stabilization at low airspeeds. Something like a T-tail or tall tail would appear to be best.

Once I donate an article to PRA, I feel it's theirs. If they want to post selected articles (or the whole mag), of course they may do so.

Attached is the corrected diagram for Mike Gaspard's machine.
 

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I say . . . sell the magazine apart from membership and watch numbers grow but what do I know?
Sensei is about to loose his title with me.
Heron
 
Thanks Doug.

All such contributions are of course deeply appreciated by all who come here. This Forum is of course one of the big sources of information for all who would understand the sport better.

After all it does say 'to help us all' on this particular part of the forum, quite apart from the PRA's own raison d'etre which I believe is to serve as an instrument of unification of people with a common interest and of advancing the progress of rotorcraft for personal flying.

A voluntary, nonprofit, nonpartisan organization whose members are dedicated to the advancement of knowledge, public education, and safety of privately owned noncommercial rotorcraft.
 
G'Day Dean,

That's rather abrupt of you ole Man. But thanks for your opinion.
In the U.S. you would have said 'blunt' or 'harsh'! But, I understand what you mean. And yes, I was blunt. Didn't see any need to beat around the bush about it.

My obligation is to ASRA. The AUSTRALIAN SPORT ROTORCRAFT ASSOCIATION. I come to this forum and others to learn. Not to be told I need to join your club and buy a magazine.
Of course your obligation is to the ASRA! Do you belong?

Mine is to the PRA and everyone that comes to this forum has an obligation to the PRA whether they want to acknowledge it or not. But it is obvious that the ASRA isn't giving you everything you need so it needs to be supplanted by the PRA members who create articles and participate here. The ASRA has some knowledgeable people like Tim McClure and I appreciate his participating here and on the U.S. ASTM gyro standards committee. But you know what, I haven’t had to go to the ASRA site to get my questions answered.

Fact is I used to be a member of the PRA but the PRA had some major problems (Large numbers of Board members resigning, who owns the airport) and some little problems (web site issues etc) and getting a magazine was like paying for a lottery ticket.
Mitch, that is past, way past history! It isn't relevant any longer. Different PRA leadership, a change (slow but changing!) in BOD inertia and changes in the PRA office have made a substantial difference.

So with all due respect to you and Chuck and Doug or anyone else that writes for the PRA, or who are members of the PRA, I'll just wait till a Mate emails me a scanned copy. All of Greg's stuff is available written sound etc. Doug posts freely as does Chuck.
Mitch, with all due respect I don't care how much respect I get! I'm too old to worry about such trivialities.

And when you say ‘freely’ does that mean non-PRA members get their info free as in freeload?!!

I'm getting into a confrontation with you that I didn't intend when I responded to your initial post. This is what has become increasingly common to call unintended consequences. What that means is, whoops, I didn’t see that coming!

My comments weren’t directed at you as such since you are on the fringe of what I was talking about. But your post did give me an opportunity to point out that PRA members, which are the PRA, are providing a lot of valuable info here that non-PRA Forum members, and lurkers, get without supporting the organization.

People who are interested in this type of article and information join forums such as this Dean. Did you buy your copy of the Houston Report on Stability?
Did I buy? Didn't need to since it was funded by a university, on the Internet and in the public domain. But I do have a fair size aviation library (don’t understand most of it!) that I purchased. I don't have a problem with paying for what I need.

.....I hope we get to discuss and learn more about prop wash here.

Cheers.

Mitch.
Oh, I don't think there is any doubt that you will get your wish! No one else is as hard hearted as I am. And since I'm not knowledgeable enough to present gyro performance/design info I'll just continue to occasionally point out what non-PRA members of this forum should be thankful for. In the mean time I'll be trying to learn with the rest of you while I send in my PRA dues!
 
I say . . . sell the magazine apart from membership and watch numbers grow but what do I know?
Sensei is about to loose his title with me.
Heron
Hey, Heron, I just reviewed the PRA chapter list and found Bolivia, Canada (3 chapters), France, Germany and Japan but guess what, I didn't see a chapter in Brazil! Get 5 people together that are PRA members and then ask me to send you a chapter starter packet. Then, when your charter is issued, you will have the right to suggest to the PRA what should be done. Having said that, you still can make the suggestion if you are a PRA member, are you?
 
Doug, I am a little confused on the drawing or perhaps there is another typo. Point 4 as I read it is 2/3 dia. from center but perhaps this should be 2/3 radius. But what is intuitively confusing to me is why would the wind not be fairly consistantly strong at that distance all the way around the center of the prop and why would it be strong in the center of the prop?
 
One Solution: PROMOTION!

One Solution: PROMOTION!

My comments weren’t directed at you as such since you are on the fringe of what I was talking about. But your post did give me an opportunity to point out that PRA members, which are the PRA, are providing a lot of valuable info here that non-PRA Forum members, and lurkers, get without supporting the organization.

Dean ... you are not the only one who feels this way. Cirrus owners have an owners group that charge a yearly membership fee to get on their forum. If you don't pay, your user name and password doesn't work. So you can't take part, use the wikipedia, download support documents or use any other tools.

If that was done here, you'd see just how important this forum is to people (or is not).

I am not a member of the PRA -- mainly because the most valuable part of it (this forum) is provided free.

I wasn't aware of any benefits to joining? For those of us relatively new here, is there a place I can read about the benefits? I don't recall membership being advertised or promoted anywhere. Except for your emotional appeal in this thread.

I was at Bensen Days and did not see even a glimmer of promotion. No place to even sign up! Wouldn't that have been a nice place to have a booth to promote PRA? One central theme I have noticed in the gyro world is that it seriously lacks promotion.

There is not a welcoming spirit. Newbies are practically scared off by overly-critical commentary and red-neck drawling posts about "The ground cummin up to killya".

Product displays are dismal at the air shows and trying to get information about products from so-called sales reps is often met with grunts and groans.

I know from experience that gyro people are really nice ... but I had to almost force myself to stay with it.

You all say you'd like the sport to grow ... but from a newbie's perspective, that is not always the impression we get.

When I signed up for this forum, I should've gotten a welcome email (automatically generated) inviting me to sign up with the PRA. It should've outlined the benefits and due structure.

This forum captures many email addresses, why not market to them? And it has many "lurkers" (eyeballs), why not advertise to them?

Just a thought from someone who makes their living from people who do advertising and promotion.
 
No Mr Dolph, I can not afford it any longer . . .in deep trouble down here.
If I had to choose I would first help Sunstate and then PRA, once I helped 3 entities with fees and work . . .but as you said . . .that is past and dont give you no credit!
Same discussion again, does not matter what is in your heart if you have nothing in your pockets . . .
I am going back to officiating because at my age not too many options here and many times I heard the proverbial "overeducated" line after an interview. Gyros are now fading away . . .
So as you say . . .gotta pay to express your opinion even if this is a free participation Forum and many here try what they can to help.
I wish I could pay!
Heron (just lost another bit of hope)
Sorry y´all this is not the right place for this kind of posting, just realized it
 
In an attempt to terminate the thread hijacking I instigated, I'm in the process of starting a new thread and have extracted Tom's post for that effort. However, don't go looking for that thread just yet! I'm dogsetting for my step daughter while my wife is in China with her and have to take care of that chore at the moment.
 
A meeting place for rotorheads

A meeting place for rotorheads

Doug said:
"I posted the results of the tests here in a summary form, right after we did them. That was in April, 2007. The bottom line is that, in the vicinity of the tail surfaces, the propwash is a cylinder that's 1/2 - 2/3 of the diameter of the prop itself.

At WOT, the speed of the wash in its fastest zones is a little higher than the gyro's own maximum airspeed.

Important conclusion: a HS on the keel provides zero stabilization at low airspeeds. Something like a T-tail or tall tail would appear to be best."


Thanks Doug, always the gentleman. I'll locate the article and follow the discussions that may evolve. I do recollect the info from last year. A forum search will get that for me. Again thankyou Doug for helping to educate me in my journey with gyros.:)


Dean, you want to give me an english lesson now. Thanks! Abrupt, harsh, blunt or plain old crutchety! :lol:

Yes I am a fully paid member and pilot with the ASRA. Yes I get a 4 magazines a year.
No, people who come to this forum do not have an obligation to the PRA. I do not believe this is a PRA forum.

You say ......."But it is obvious that the ASRA isn't giving you everything you need so it needs to be supplanted by the PRA members who create articles and participate here. The ASRA has some knowledgeable people like Tim McClure and I appreciate his participating here and on the U.S. ASTM gyro standards committee. But you know what, I haven’t had to go to the ASRA site to get my questions answered." Ohhh! All the knowledge is here Eh! :usa2:
A little or largely concieted wouldn't you agree. Of course you wouldn't. I get accussed of having no knowledge or being unqualified to even speak about gyros on this forum yet I am a gyro pilot. I sense a similar attitude from you.

Bit like being told Queensland is like Texas not the other way around eh!

I'm not convinced the woes of the PRA have passed but I'll take your word as the definitive response on that one.

"Mitch, with all due respect I don't care how much respect I get! I'm too old to worry about such trivialities.

ME TOO.:)

And when you say ‘freely’ does that mean non-PRA members get their info free as in freeload?!!

IF THAT'S HOW YOU WANT TO LOOK AT IT. ANYONE WHO GOES TO ANY FORUM AND GET INFO FOR FREE, YOU WANNA GO AHEAD AND CALL THEN FREELOADERS?:drama:

I'm getting into a confrontation with you that I didn't intend when I responded to your initial post. This is what has become increasingly common to call unintended consequences. What that means is, whoops, I didn’t see that coming!

LIKE COLLATERAL DAMAGE EH! :twitch:

My comments weren’t directed at you as such since you are on the fringe of what I was talking about.

THATS ME A NON DESCRIPT FRINGE DWELLER.::painkiller:

But your post did give me an opportunity to point out that PRA members, which are the PRA, are providing a lot of valuable info here that non-PRA Forum members, and lurkers, get without supporting the organization."


HAPPENS WITH MOST FORUMS AROUND THE W.W.W. DEAN.

"Dean ... you are not the only one who feels this way. Cirrus owners have an owners group that charge a yearly membership fee to get on their forum. If you don't pay, your user name and password doesn't work. So you can't take part, use the wikipedia, download support documents or use any other tools.

If that was done here, you'd see just how important this forum is to people (or is not).

I am not a member of the PRA -- mainly because the most valuable part of it (this forum) is provided free."


If you blokes wanna charge for your forum go ahead lets see how many use it then. I would suggest Cirrus owners can afford such extravagancies.
I like you Dean dont mind paying for what I need but if I need to, I'll ask Tim Mc or Birdy or Rob P (who manufactures props and blades and has this topical knowledge, just looking to compare notes and add something positive if possible.) or a host of others in OZ with the knowledge who unlike yourself dont have a 'Join up, pay now, get your info in the magazine maybe, if youre lucky' attitude.

"I wasn't aware of any benefits to joining? For those of us relatively new here, is there a place I can read about the benefits? I don't recall membership being advertised or promoted anywhere. Except for your emotional appeal in this thread.

I was at Bensen Days and did not see even a glimmer of promotion. No place to even sign up! Wouldn't that have been a nice place to have a booth to promote PRA? One central theme I have noticed in the gyro world is that it seriously lacks promotion.

There is not a welcoming spirit. Newbies are practically scared off by overly-critical commentary and red-neck drawling posts about "The ground cummin up to killya".

Product displays are dismal at the air shows and trying to get information about products from so-called sales reps is often met with grunts and groans.

I know from experience that gyro people are really nice ... but I had to almost force myself to stay with it.

You all say you'd like the sport to grow ... but from a newbie's perspective, that is not always the impression we get.

When I signed up for this forum, I should've gotten a welcome email (automatically generated) inviting me to sign up with the PRA. It should've outlined the benefits and due structure.

This forum captures many email addresses, why not market to them? And it has many "lurkers" (eyeballs), why not advertise to them?

Just a thought from someone who makes their living from people who do advertising and promotion. "


I'd agree with everything you said Tom. This is not an unfamiliar entry into the world of gyros. It happens in Oz to.

Suffice to say I visit more than three rotorcraft forums and all are free, with experts commenting freely without any other agendas. I've been to Mentone and paid my dues. I actively seek out info. Bought Paul Bergen Abbotts books, read them 10 times then donated them to ASRA library.............

But you are right of course Dean, this discussion gets us no where and so now seeing as you dont care or need respect, I feel you can stick you elitest attitude where the sun dont shine.:rapture: Or perhaps it does.........:tape:

Have a good day.:yo:
 
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Hmmm, well that got elevated into a meaningful discussion.

Guess a few other than Calvin like the Sugar Frosted Chocolate Bombs that get one a sugar high and totally wired.

But about that propwash. Am I understanding correctly that the vortex ring coming off the prop is most effective with a T tail with a T 2/3rds of the prop diameter centered with the prop hub, and that the low horizontal stabs are not as effective?
 
Attached is the corrected diagram for Mike Gaspard's machine.

Hi Doug,

the results you got from your measurements don't quite add up in my mind. Maybe you can help me understand them a bit better. Here is the data from your graph in a table:

Nr........r (in).......v (mph)
================
1...........0..........80
2..........16...........0
4..........35..........95
5..........36..........90
6..........38...........0
3..........52...........0
7..........52...........0
8..........52...........0
9..........54...........0
10........74...........0

"Nr" is the point number in your graph, and I calculated the radial distance for each measurement point, also from the information provided in your graph. This should be a meaningful exercise assuming rotationally symmetric flow velocity. In other words, the flow velocity at the same radial distance from the hub doesn't depend on the circumferential position you measure it. In yet other words, the flow at 3 o'clock has the same speed as the one at 9 o'clock or anywhere else at the same distance from the hub.

(Aside: I know the flow has some cork screw component to it, but if you take only the speed of the flow it should only depend on the radial distance. And I assume you were only measuring the flow component directed perpendicularly away from the prop disc.)

What I can't come to grips with is points #1 and #2. There is substantial flow velocity at the center (#1) out to at least 36 inces (#5). Only point #2 is the oddball out with no measureable flow. Or -- the second possibility -- there is no flow in the center and it only picks up between 16-35 inces, in which case the data for #1 would appear to be wrong. The latter seems to make more sense to me and also mesh with your verbal description of a "cylinder" shaped velocity pattern. Either way, it seems to me that one of the two measurements ought to be wrong.

Here is a diagram I made from the data table above to illustrate my point:

Diagram.jpg

Another question I had: at what distance away from the prop hub did you measure the flow. And is my assumption correct, that all points were measured at that same distance?

-- Chris.
 
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In with John R. Doug, i cant see why theres so much of a windspeed difference at the same distance from the hub in different areas of the disc??????????
I can see how anythn upwind of the prop can make a difference, but 0 to 90mph AS?????
Wot am i missn?
 
Yes, the term should be "2/3 prop radius," not "2/3 prop diameter." The boys who took the notes got it right, and yours truly messed it up in transcribing the notes to the diagram. I re-posted the chart with this correction.

Guys, you cannot take two data points out of around 30 and expect to draw any conclusions. That picture was one of three in the article (one for each test gyro), and we recorded 9-12 readings for each of the three. Our tools were primitive -- a pitot tube on a hand-held stick and tape measures for distance. We merely eyeballed the centerline of the prop hub. Needless to say, we didn't employ the tape measure while the engine was running; we measured, say, 16" with the power off, picked a "landmark" (e.g., to the top of the N-number) and then fired up and measured the flow.

Holding a stick out at arm's length in a 100 mph wind while squatting a foot away from a lethal buzz-saw introduces its own set of errors. You don't linger.

Two-thirds of a 52" prop's radius is 17.4." Given our measurement methods, there's no significant difference between 16" and 17.4" The best you can say is that outer edge of the fast air was somewhere between 16" and 18" out on Mike's gyro.

Propwashes are readily deflected by ambient breezes. They are not quite symmetrical side-to-side. There are any number of things that could cause a 1.4" anomaly.

The point of the project was to determine how small the wash is, relative to the prop's diameter, in the area of the tail surfaces. We found 2/3 to be a good rule of thumb, based on three different gyros with quite different power loadings and performance envelopes.

Those who want more precision are most welcome to try their own experiments.
 
Doug and his elves have verified that Bernoulli was correct when he stated something like; “the energy in a fluid stream remains constant if friction if ignored,” which is a variation of conservation energy law.

The pressure on the outlet side of the propeller disc is higher than ambient and in the process of equalization, the slipstream accelerates. As the slipstream accelerates, it must contract if energy is to remain constant. If it did not contract as it accelerated, we would have invented a perpetual motion machine.
 
Doug and Chuck,
These findings were similar to mine when I checked my RAF. I couldn't bring myself to do the full power thing after being once smitten. Thanks for the reprint of Chucks very detailed article on the physics of gyro flight. I think this should be reprinted every year for the new members and made required reading.
Walt
 
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