PM 38 Why the difference in prop length

[Randy Haskin]

Re: PM 38 Why the difference in prop length

Thom, thanks so much for posting here and for your openness on the subject. You are a great addition to the sport, and I know a lot of folks are impressed, supportive, and hoping for you and your team's continued safety and success.

[Randy Haskin]

Re: PM 38 Why the difference in prop length

I could be that certain airplanes do better, at a certain speed range and G than others. We know the Bear for example does better on the straightaways etc, etc.

FWIW, this concept is codified in the military fighter world as "Energy Maneuverability Theory".

In layman terms, it plots out comparisons of energy produced vs energy demanded by various combinations of airspeed and G. These results are called "specific excess energy" (annotated as P-sub-s), and it simply shows where energy available-vs-demanded is a positive number, a neutral number, or a negative number. A positive number means the airplane can have that particular combination of airspeed and G and still accelerate due to "excess power". The neutral number means that performance can be sustained, and the negative P-sub-s means that combination of airspeed and G will cause the airplane to slow down (or have to descend -- or use potential energy - to maintain speed).

Different aircraft have different P-sub-S curves, and thus different capabilities under different combinations of airspeed and G.

In designs that have had this information tested and plotted, you can locate precise combinations of airspeed and G which will, for example, maximize turn rate in degrees-per-second or minimize the turn radius, and what those bits of performance will cost in terms of energy.

These analyses allow fighters to figure out how to fly to best optomize that performance in, say, a dogfight, as those "friendly" numbers can be plotted against similar analyses of threat aircraft.

Although it would take gobs of cash to test a particular racer thoroughly enough to plot an accurate E-M diagram, it would provide highly useful data with which to form strategy about how fast you want to fly and how much G to pull around the corners. These analyses would also show the differences that Thom pointed out amongst the different racers -- why some gain ground in the corners, and why some do it in the straightaways.

Unfortunately, contrary to how it appears, with respect to understanding aircraft turn performance, faster is not always better, nor is higher G.

[Thom Richard]

Re: PM 38 Why the difference in prop length

Thanks Randy,

That's exactly right, a perfect example would be the L-39 vs. the Iskra. I had a couple of duels in 2010 and the L-39's took me on every straightaway, but I beat them in the turns every time. And luckily, since there are more turns, the "little-jet-that-could" ended up victorious... A big engine on the Iskra would make in very hard to beat.

But back on subject, there are people that argue that different props are better in the turns as well. Anyone want to elaborate?

[Leo]

Re: PM 38 Why the difference in prop length

Thom,
Thank you sir!
It is so nice to hear it all from the source than from the WAG's and theories of the "experts" that speak from the heart and not the brain.

[spacegrrrl]

Re: PM 38 Why the difference in prop length

Thom,

I want to say how impressed I am with your approach to developing your incredible racer! You clearly understand the need for rigor and process as you refine your aircraft.

Many racers have taken the "shotgun changes all at once" approach using what a friend called "demon tweaks" not every really knowing the affect of changes they made at random. Your approach will definitely yield great results over time as you learn more and more about your unique one of a kind racer.

So congratulations on a great showing this year! I cannot wait to see your racer improve as you clearly have a plan and I believe there is an amazing future for Precious Metal now that she is in the right hands!

Spacegrrrl

[440_Magnum]

Re: PM 38 Why the difference in prop length

. We wanted to stay in front of Dread and only used enough power to do so.

Isn't that kinda like passing your college entrance exams? You're not in the big leagues until you pass the Buick. Congratulations.

[Lockheed Bob]

Re: PM 38 Why the difference in prop length

Thom thanks for the insight. I've been watching air racing since 1946 & you & the crew are taking the right course in your progress. I hope I get to see you win the Gold as you do deserve it.

[wingman]

Re: PM 38 Why the difference in prop length

Hello Thom -- good to see you over here. Some of us older folks don't like or use Facebook, so miss some of the stuff posted over there.

It's been a real pleasure to watch the evolution of your Racing career! First you turned up at Reno with a well proven Silver class IF1. I remember asking you if you'd changed anything on the airplane. You said the only change was a new larger canopy because "My head's too big to fit in the old one". I remember thinking then that this guy sounds like he belongs in an Unlimited, LOL...

Then you brought another IF1 -- one which had always been in the low to middle Gold. You proceeded to blow the field away and solidly take the IF1 Gold!

Then came a move to the jet class, in an Iskra of all things! You fit right in.

And now you've rescued one of my all time favourite Unlimiteds, and are actually RACING the airplane.

Enough praise -- I have a couple of questions I hope you will address...

How much of the huge speed increase this year do you feel is due to the changes in the propellers?

What research or data did you use in deciding where and how much to cut? The changes certainly look pretty radical, and noone has ever tried such a thing before on a Griffon Racer. You certainly must have had a plan for the changes and they seem to have worked remarkably well!

Was there any "retwisting" or changes to the airfoil? (You obviously don't have to answer any of these questions if there are secrets involved).




One thing that struck me is the apparent similarity between your prop profile and the prop run these days on Strega.



Obviously they are not the same, but they look similar to me, with a relatively straight leading edge, relatively narrow tip, and much more curved trailing edge -- all quite unlike any stock propellers I've seen. Rumour has it there there has also been retwisting and maybe reprofiling on Strega's. Tiger said at the time that the new prop was just cut down after he modified it with a runway but I've always wondered, given just how fast the airplane has been running with the new prop.

Thanks again for being willing to post here, and you've got a lot of people rooting for you and Precious Metal next year!

Neal Nurmi

-- Wingman Photo --

[xmh53wrench]

Re: PM 38 Why the difference in prop length

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
I am soooooo building a 1/24 model of this

Neal your photos always deliver, and a big thanks to the PM guys for their candidness

[C_roundy]

Re: PM 38 Why the difference in prop length

Obviously they are not the same, but they look similar to me, with a relatively straight leading edge, relatively narrow tip, and much more curved trailing edge -- all quite unlike any stock propellers I've seen. Rumour has it there there has also been retwisting and maybe reprofiling on Strega's. Tiger said at the time that the new prop was just cut down after he modified it with a runway but I've always wondered, given just how fast the airplane has been running with the new prop.


Once upon a time, somewhere out on the left coast, I had a heart to heart talk with an outstanding woman at a prop shop. I admitted to her that all I think about is propellers.

Long story short, 2 hours later, she had admitted to me, just which kind of racers ask for what twist, where....

Bottom line, they ALL think they can get a free ride with some certain twist somewhere.

What Thom is doing, is WAAY beyond that regular garbage.

C.R.

[Thom Richard]

Re: PM 38 Why the difference in prop length

Gents,

I appreciate the praise, not really sure how to handle it, but thank you. I'm just a flight instructor/mechanic trying to do my little part in aviation.

The design of the prop came about by default. I always felt the old design was the biggest boat anchor ever put on a prop shaft. Without a doubt the number one thing we could do to make this airplane faster. You could even hear it when it flew by. Being under-propped and the horrendous vortices made a real racket. Noise is wasted energy. The idea behind the old, smaller diameter was much higher RPM than I am willing to turn. And as history has showed us, the engine wasn't willing to do it either...

So how do you go about designing a prop? Well, I certainly had no experience so I read about it. And read about it. Spoke to lot's of people "in the know". Read about it. Aerodynamicts, prop designers, racers etc. Read about it. Looked at many fast designs. Did I mention I read about it?

It turns out modern propeller thinking for high speed applications is basically the Scimitar. It slows down the progression of shock waves at high mach numbers, which is all drag. (And noise...)

I could not afford building a prop from scratch and the only two props that fit this installation is the Dowty-DeHavilland and the Dowty-Rotol. The Rotol is the higher speed of the two, but meant for the Spits and Seafires. Even if I could find one, I would never cut one up for racing. So the only option was the Shack prop. I acquired a new one this summer from my great friends in England. Kennet Aviation.

The diameter is based on something called "Helical tip speed". I found the formula and plugged in to a spreadsheet where I could vary all the parameters. I played with it a lot. The optimum tip speed is mach 0.88-0.92. That can be tough to stay within for our purposes. Based on all the numbers I decided (or defaulted) to a 132" diameter on the front. The difference in length between the front and the back was already determined by Dowty. I used the same number. About 4" shorter per blade. If they figured out what distance was needed to avoid the vortices from the front blade, good enough for me.

There is nothing new under the sun. The leading edge curvature was basically stolen from the C-130J prop. Those guys know what they are doing and I don't have a wind tunnel. They operate in relatively high mach numbers as well. I can't make a true scimitar out of a straight blade, but I can try to get as far in that direction as possible. I'm sure the design is not optimum, but it's as good as I know how to make it.

The trailing edge radius is simply for structure. I was not comfortable with the propeller coming to a sharp point. It seemed a bit thin to me. Probably wasn't, but erred on the side of caution. (Though it would have looked really cool!)

The last obstacle in the design was the ever twisting helicoidal shape of the cut. We tried simple shapes on the computer and even test cut a few blades, but no luck. The only way to do it was to 3-D scan the existing blade and then model the existing airfoil in to ever smaller chords towards the tip, following the existing twist of the blade and using the existing leading and trailing edge radii. You can't just shrink an airfoil. It gets too thin on the leading and trailing edges, but since the scimitar is essentially an ever increasing sweep, it makes up for it in the presentation to the relative wind. Aren't we lucky?

The computer modeling and machining services was provided by one of our sponsors, Track-Air here in Kissimmee, FL.

Once the model was made in Solidworks, we made another few test cuts on a 4-axis CNC machine. Eventually we were comfortable enough with the design and attached the actual blades to the fixture. This is a very risky step. The blades are a matched set. If you screw one up, the entire prop is junk... Let's just say "Measuring twice and cutting once" was not enough for our operation... Once the RH blades were done, we did the whole thing over for the LH blades. It's not a mirror image since they are not cut at the same station.

I was asked what NACA airfoil I used in the design. I said, it's a TR-1...

Once the blades were cut, I brought out my 50 grit beltsander and started smoothing out the CNC cut. It took a lot of shaping to make that happen. At the same time, the prop had to be statically balanced. Mounted in the hub (which is an undertaking) with the correct preload and blade twist drag etc. Let's just say we learnad a lot about these props... I stepped down to finer grits, got the props balanced, painted and polished. Each took about a week to do. It was hot this summer so we assembled them in our tool room. We had a four inch clearance in the door diagonally when we hand carried them out to the hangar. The whole thing weighs in around 650 lbs. It's a monster to manhandle.

The installation took about a day and when we eventually got to test run it, a huge sigh of relief. It didn't vibrate! We must have gotten very lucky, both in airfoil design and mass distribution. Just with our static balance, we managed to get it to 0.30IPS. Much smoother than the old boat anchor. After a dynamic balance on the way home from Reno we ended up at 0.08IPS. Anything below 0.20 the dynamic balance is satisfactory. Very pleased with the results.

The twist is stock. Here's my understanding and perhaps someone could enlighten me, but if you have a constant speed prop, the twist is ever changing by means of feathering. On a fixed pitch prop I understand the need for re-twisting based on different installations, but not on a variable pitch. The twist is set at optimum already. Why change it? The twist is based on the optimum AOA along the entire blade and since the entire blade turns as one piece, that is a set rate of twist. If you change it, you will have some parts of the blade be more efficient than others. At least that's my thinking. Input? Maybe I could learn something really cool here...

Did I miss anything?

[Thom Richard]

Re: PM 38 Why the difference in prop length

Oh, the similarity to Strega's prop is incidental. Or perhaps default. I'm guessing similar thinking was used in its design. But I did not use it as a model for what I did. I actually remembered the tips being more round than they are which is why I didn't use it. I don't believe in rounded tips. Had I looked closer and remembered correctly, I would have probably stolen from that design as well...

[wingman]

Re: PM 38 Why the difference in prop length

Now that is an impressive answer! I've read it a couple of times now and I'm not at all sure I understand it all, but I do understand just how much thought and work went into this prop project. I don't know how you find time to run your business while doing this.

There is much talk among us who don't know about such things about retwisting and reprofiling propellers. Your comments on twist make sense, and it'll be interesting to see if anyone else weighs in here on the subject.

I'm very impressed with the fact that you not only took this on, but that you seem to have gotten it so right. Propeller design seems to be such a "black art"! Both Destefani and Pardue threw money at new designs from reputable shops without much success.


Another question: You and others have commented that the Whittington cockpit design and layout was an ergonomic nightmare, with guages and controls in almost unreachable places. Are you fairly pleased with the cockpit changes you've made so far?

Neal

[Thom Richard]

Re: PM 38 Why the difference in prop length

Well, we don't know if we've gotten it "right" yet. Wait till next year for judgment... We still have 40mph to go. The hardest ones from what I understand...

The cockpit is marginally better. I've done some changes but the basic layout is the same. I've learned to live with it but it's far from desirable. If time and budget allows, I'd like to make some changes, but that's way down on the priority list.

[John]

Re: PM 38 Why the difference in prop length

Thank you for this thread. You all have done something that many out here keeping track of the Unlimited world have been hoping for for many many years on the Griff Mustang -a good treatment of the prop blade tips. Less slapping the air into submission and more efficiency.

The new prop looks just great! So much better than the older design. My hats off to you for the knowledge, persistence and effort.

And to think that PM goes great without blade cuffs as on the Superstangs and the old Rare Bear 3 blade.

What is the next step for speed improvement on PM that will not break the bank?