Cary, That was a fantastic post.  Lots of hard work and great info.  Yea, I knew those were pipe dream speeds.  But here is my latest trial...  207 KTAS at 17500 (238 mph with at ground speed of 257kts, 295mph)  now I just need to wait for one of those really really high tail wind days.  Interesting, that was still only 6500 RPM and 28" MAP, though it was almost full throttle.

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David Leonard

Turbo Rotary RV-6 N4VY
http://N4VY.RotaryRoster.net
http://RotaryRoster.net

On Sun, Nov 7, 2010 at 12:35 AM, cbeazley <cbeazley@innovista.net> wrote:

Hey David;

I'm not an AE either.
@201knots TAS + 860fps prop tips (3060rpm x 2.17 x 64") your tips are already near 920fps or ~Mach0.83
The 900fps prop tips recommendation is an airfoil efficiency thing due to (mach) compressibility effects. Other things like maximum lift coefficient change as well.
Your prop tips are becoming less efficient and the prop (Betz?) lift distribution is also changing (similar to the ideal elliptical lift distribution of a wing) as the prop section inboard of the tips are loading up. This will be somewhat offset by the increased HP output of the engine at higher rpm.
A balanced and nick free prop is best - but who knows how much that effects things?


Here is a quick example of the transonic drag rise due to compressibility - Mach effects. Thinner 6% airfoil tips do better than thicker.
From theory of Wing Sections, a 6% and 9% thick airfoils - sorry, I don't have Clark Y info handy.
Mach .6 .7 .8 .9 .94
Cd 2306 0deg .01 .011 .014 .031 .049
Cd 2309 0deg .011 .013 .018 .048 .071

Here is a link for Phil Johnson's homebuilt prop. I think I read that Al Wick built his own prop as well. Phil's site seems to be offline but some of his presentation slides here have some structural/load info:
http://eaa245.org/Presentations/CozyProp/index.html
If Craig Catto is not willing to help, we can only speculate.

Do you know the pitch of the prop? That will affect top speed as well.
Is the Catto prop a flat bottomed airfoil - RAF/clarkY/USA-35?
Some of the racers chop the tips to get full rpm.
You might consider trying another higher pitched speed prop.
A 2.85 redrive ratio might help as well.

My SWAG is that you will be hard pressed to reach 240knots, particularly with that prop/redrive.
From CAFE report - Sport Avaiation Apr,1997. RV-6 Wetted area = 427.2 ft^2, Cd = 0.0054
427.2 x .0054 =~ 2.32 ft^2 equivalent flat plate drag area

An old speed estimation formula (in mph) from Bruce Carmichael:
201Knots * 1.1515 = 231mph
231mph=52.73mph*cubed root[(HP * PropEfficiency)/flat plate drag area]
4.39=cubed root[(HP * PropEfficiency)/flat plate drag area]
So, [(HP * PropEfficiency)/flat plate drag area] = 4.39^3 =~ 84.6

Assuming a straight line HP increase from 6700rpm to 8000rpm (8000/6700) -> HP will increase 20% (without boost increase)
84.6 * 1.2 = 101.5
cubed root of 101.5 = 4.66
4.66 * 52.73 =246mph /1.1515 = 213knots
And that is assuming the prop efficiency remains the same.


Factory tested Vne aside. I haven't seen a V-n diagram for an RV yet, so I have no idea what the true structural limits are.
Control slaps at Vne is asking for it. These should be limited to maneuver speed whatever that is.
Here is an example of glider testing, they may have been doing aerilon doublets.
http://www.youtube.com/watch?v=kQI3AWpTWhM

As a heads up, a buddy of mine was killed this year when the rudder and part of the vertical stab departed his RV-7A. The final report is not out, but high speed flutter is suspected.

Another buddy lost a prop on a lancair years ago - he got the power off quickly but had to land in a field with the plane pitching violently up and down due to the imbalance. You might consider safety strapping the engine in case a blade failure rips the engine mount off the plane. This is probably more of a concern with heavier metal props.

Drag cleanup is your friend for speed.
Borrow a chute and test at altitude.

Fly Safe
Cheers
Cary

From:   David Leonard <wdleonard@gmail.com>
Subject:        Re: [FlyRotary] Re: Vne is the question was Re: How fast is it safe to turn a Prop. Opinion Poll
Date:   Mon, 1 Nov 2010 15:14:30 -0700
To:     Rotary motors in aircraft <flyrotary@lancaironline.net>

       Message Header <http://mail.lancaironline.net:81/lists/flyrotary/Message/52752-H.txt>

Undecoded Message <http://mail.lancaironline.net:81/lists/flyrotary/Message/52752-P.txt>

Hi Ed,
Now that you bring it up, I have always taken exception to that article. At the start it basicly makes 2 assumptions without giving any sutable support:
1) that Vans published Vne speed is due to flutter.
2) Flutter is based on TAS not IAS.
The rest of the article basicly explains the difference between TAS and IAS and gives examples of aircraft that might be exceeding Vne unknowingly.
Where is the reference to the data that shows flutter is purely a function of TAS? Where is Vans Flutter analysis data? At one point it says that flutter is not resonance, but a function of "energy input to the system" - but then wouldn't dynamic pressures (read IAS) be primarily responsible for adding energy to the system. He then goes on to use the Tacoma narrows bridge as an example of flutter, but that is the pent-ultimate example of resonance.
It also just makes sense to me that dynamic pressures must play some role in flutter. Some examples: suppose you had an aircraft that was susceptible to flutter, you could make it less-so by beefing up critical areas (supposedly resisting the forces of flutter). Suppose you look at the extreme case, and aircraft going very fast in a very very thin atmosphere. If you make the atmosphere thin enough, you will not be able to generate enough force to destroy a plane (from flutter) no matter how fast the molecules are traveling.
Now I am definitely no AE, and I am not saying that TAS does not play a role in flutter, if not possibly the predominate role in normal atmospheric conditions. I am just saying that Ken's article is far from the final word on the subject and has never really convinced me of anything.
It does however have one excellent piece of advise: that if you want to go raging around at those higher airspeeds you are better off buying an aircraft built for the purpose. For me, it is not about raging around at those airspeeds on every trip, but about positing some big numbers to fulfill some underlying need to legitimatize my choice of a rotary engine. Like Tracy did in the SUN 100. And to do that, I have to outrun, in a public forum, the guys who are using lycs and paid 3x the price. And to do that, I have to fly nearly 250mph. (and then fly home at 150kts).
But I don't want to wreck my engine, plane, prop, or myself in the process.
--
David Leonard

Turbo Rotary RV-6 N4VY
http://N4VY.RotaryRoster.net
http://RotaryRoster.net

On Mon, Nov 1, 2010 at 2:15 PM, Ed Anderson <eanderson@carolina.rr.com <mailto:eanderson@carolina.rr.com>> wrote:
Hi Dave,
From what you said, I'm not certain whether we should take issue with the article or Van's limit for Vne. We all know that Van is pretty damn conservative on his limits. So I for one am not surprised that his Rv's can push past his published Vne without ill effect. But, I'm just not certain that invalidates the substance of the article - but, then I'm not a aerodynamic engineer either.
Just wanted to bring it to the list's attention, because I was surprised at what I read and really had not the knowledge to accurately assess it - it appeared to make sense.
Ed

*From:* David Leonard <mailto:wdleonard@gmail.com>
*Sent:* Monday, November 01, 2010 4:51 PM
*To:* Rotary motors in aircraft <mailto:flyrotary@lancaironline.net>
*Subject:* [FlyRotary] Re: Vne is the question was Re: How fast is it safe to turn a Prop. Opinion Poll

Well yes, that is the the jist of the article, but one article does not a 100% fact make. Here is a link for those interested:
http://www.vansaircraft.com/pdf/hp_limts.pdf
The limiting factor for Vne can be any of number of factors which may or may not be fluttler, dynamic pressures, tran-mach waves etc. Vans Vne number represents the upper limit of factory testing and imply an untested zone above that number. There is little reason to suspect that some flutter boundry lies 'not too far' beyond Vne. True, there are a couple of isolated reports of something that may have been flutter (but likely not true destructive flutter). OTOH, there are a lot more reports of RV's being tested well beyond Vne without incident. This includes Dave Anders RV-6 and many of the other cross country race planes, a number of super-6 and super-8 RVs, and my own RV-6 that I tested to 205 KIAS (about 235KTAS - but I did not do elevator and aileron slaps at that speed). (for those of you without RV's, Vans suggests 210mph (187kts) as Vne. In light civil aviation, Vne is usually expressed in terms of indicated air speed)
Not that I want to encourage anyone to exceed Van's Vne limit, go there at your own extreem peril.
So, Vne issues asside, what say you about the prop speed issue?
--
David Leonard

Turbo Rotary RV-6 N4VY

http://N4VY.RotaryRoster.net <http://n4vy.rotaryroster.net/>
http://RotaryRoster.net <http://rotaryroster.net/>

On Mon, Nov 1, 2010 at 12:24 PM, Bryan Winberry <bryanwinberry@bellsouth.net <mailto:bryanwinberry@bellsouth.net>> wrote:

Ed,

That’s the way I recall the article.

Bryan

------------------------------------------------------------------------

*From:* Rotary motors in aircraft [mailto:flyrotary@lancaironline.net <mailto:flyrotary@lancaironline.net>] *On Behalf Of *Ed Anderson

*Sent:* Monday, November 01, 2010 3:19 PM

*To:* Rotary motors in aircraft
*Subject:* [FlyRotary] Vne is the question was Re: How fast is it safe to turn a Prop. Opinion Poll

Interesting article back a couple years ago in Van's RVator about Vne. If I understood it properly, unlike every other airspeed of interest which is based on Indicated air speed or Q factor or dynamic pressure, Vne is not based on IAS but TAS.

According to the article (as best I recall) when flying high (where IAS is low) and fast - it is True Air Speed that is the limit factor on flutter and other dastardly happenings. Apparently the excitation of the airframe components is due to the true air speed at which the molecules of air are moving across said component and not the dynamic pressure which is a combination of air density and true airspeed.

The message was you could find yourself in trouble particularly at high altitude with low IAS (and thinking you are safe because its below stated Vne) and high TAS.

So unlike just about all other airspeed limitations which are based on IAS (dynamic pressure, Q factor, etc) , apparently the Vne is True air speed based - always assuming I understood the article.

Ed

*From:* Mark Steitle <mailto:msteitle@gmail.com>

*Sent:* Monday, November 01, 2010 3:07 PM

*To:* Rotary motors in aircraft <mailto:flyrotary@lancaironline.net>

*Subject:* [FlyRotary] Re: How fast is it safe to turn a Prop. Opinion Poll

Dave,

What is Vne for the 6A? Van's site only shows "Top Speed" as 208 mph with the 180hp engine. Is this considered Vne?

Mark

On Mon, Nov 1, 2010 at 1:49 PM, David Leonard <wdleonard@gmail.com <mailto:wdleonard@gmail.com>> wrote:

So I am getting a little more bold with my new turbo. On the way home today I decided to open it up a little bit. At 8500' and MAP around 34", I was at 6700 RPM and 201 KTAS (about 3060 RPM at the prop with the tips doing 860fps). Temps still less than 160, plenty of throttle left. I just couldn't accept turning the prop any faster, not knowing what the limit really is.

My prop is a 64" diameter (pretty short) Catto (wood/composite) with a number of nicks in the leading edge and has not been dynamical balanced to date.

I talked to Craig Catto about the issue and what he told me completely makes sense, but is utterly unhelpful in setting my racing redline. He said that I could turn it as fast as I want, but the risk of separating the prop. increases as I go faster. I just depends on too many factors.

I realize the the final number is going to depend on my risk tolerance and how much I really want to post some big numbers in a race. But it is hard for me to balance that without better understand or the factors on the prop, hence, the opinion poll, on various factors..

I have heard that it is important to keep the prop tips under 900 fps, while others say it is not a factor. Is there some important number for the max tip speed? Is it a safety issue or an efficiency issue?

Most of my nicks in the paint and composite layer are in the outer 1/3 of the prop. Are these a big deal for safety? Efficiency?

If I were to run the engine at 7000 RPM that would give me 3225 at the prop, tips doing 900 FPS an an aircraft speed of around 209 kts (240mph). This seems pretty reasonable to me as an upper limit, but it is scary a little.

But what about 8000RPM at the engine (if I have the power to get there)? With a cleaned up and balanced prop, that would be almost 3700 RPM at the prop, tips doing 1020fps, aircraft going something less than 240kts (275 mph). Stupid though? Or give it a try and see how close to those numbers I can get?

Any issues with the gear-box going that fast, other factors I am not considering?

Anyway, it feels really cool to have broken 200kts in level flight.

--
David Leonard

Turbo Rotary RV-6 N4VY

http://N4VY.RotaryRoster.net <http://n4vy.rotaryroster.net/>
http://RotaryRoster.net <http://rotaryroster.net/>

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