The term MAGIC was used in reference to prop science and I think the explanations were excellence.Tommy W.
On Sat, May 10, 2014 at 12:10 PM, Jeff Scott via KRnet <krnet at list.krnet.org > wrote: > Jeff L. > > Yes, I think we are dancing around the same elephant. When I say I > respectfuly disagree, I mean exactly that as I know you are really into > aircraft performance and measurement with a lot of first hand knowledge. I > have a lot of respect for your work. > > My only point was that as a general statement, a longer prop will usually > contribute significantly to both take off and initial climb. As you > stated, aircraft drag becomes the more predominant factor with speed as > more drag is generated with more speed. Prop length becomes much less of a > factor in a small slick aircraft, especially at speed. In fact, for high > cruise performance, too long of a prop becomes a real detriment. But, if > you calculate your prop tip speed and find that it is running down in the > .6 - .7 mach area, you can usually improve your take off, climb and cruise > performance with a bit longer of a prop. If your prop tips are making > close to .7 mach static, a longer prop is likely to help with take off and > initial climb, but may hinder cruise. If you're making .8 mach or more > with your prop tips at cruise, then chances are you'll probably increase > your speed by going to a shorter prop. When calculating the mach number of > the prop tip, you have to factor the aircraft speed into the calculation. > .75 mach static may be over .8 mach at high cruise. For prop tuning, a > faster plane may require a slightly shorter prop. > > Wish I could tell you the measurements from the prop I had on the VW > powered Avid Flyer, but that's been 15 years ago and all that information > was long ago lost. Additionally, since this was all done at over 7000' > altitude, the numbers probably would not be useful for near to sea level > operations. > > -Jeff Scott > Los Alamos, NM > > > ----- Original Message ----- > > From: JL > > Sent: 05/09/14 11:12 PM > > To: Jeff Scott, KRnet > > Subject: Re: KR> Fw: Re: Propellers > > > > Jeff, > > > > Since I don't disagree with almost everything you posted, I suspect we > are commenting about the same elephant, but from different ends ; ) > > > > Sounds like you found the minimum diameter prop for your airplane. May I > ask what that prop was and at what rpm you saw no further gains? > > > > The only thing I would not agree with is the lumping of static thrust, > takeoff roll and rate of climb together. I tend to view them as three > separate phases of flight. I think that there are certainly relationships > there, but just because a plane has a short takeoff distance does not make > it a good climbing airplane. Climb is a function of excess thrust and lift, > where takeoff distance is less influenced by airframe drag and more > influenced by raw mass flow past the cowl and anything else directly behind > it. > > > > That being said, your comment about finding the right compromise is spot > on. Same elephant ; ) > > > > Jeff > > > > Sent from my iPhone > > > > > On May 9, 2014, at 9:53 PM, Jeff Scott via KRnet <krnet at list.krnet.org> > wrote: > > > > > > After a fair amount of experimenting with props over the years, I will > respectfully disagree with you, Jeff, and agree with Tommy. But I will also > state that the statement about longer props equals better climb is only > true within limits until you reach a point where the engine HP gets used up > by the tip drag as I'll discuss further down in this post. > > > > > > One specific instance that comes to mind was a propeller I had on my > KR for a number of years from Performance Propellers. It performed > reasonably well, but after checking and finding that my Tach was reading > 200 rpm high, I knew that with a bit less pitch, I should be able to turn > the engine up another 200 rpm improving both climb and cruise performance. > I pulled the prop off and sent it back to the manufacturer to be repitched > to a lower pitch to allow it to turn another 200 rpm. When I got the prop > back and flew it, I knew without even measuring it that he had cut the tips > off the prop rather than repitching it. It took a lot more RPM to do > anything, but the takeoff and climb performance was seriously off. When I > called the manufacturer, he confirmed that he decided to be lazy and cut 2" > of diameter off the prop rather than repitching it as I had requested (and > paid) him to do. For use on my plane, the prop was essentially ruined. > > > > > > On an Avid Flyer that I built using a VW engine, I simply could not > get it to perform at our 7000' elevation here in Los Alamos. I returned the > prop to Ed Sterba for some adjustment to the pitch. When I got it back, it > really didn't perform any differently, nor did the engine turn up any > better. I called Ed and discussed the issue with him and we formulated a > plan. The issue was that the prop was simply too long for the engine as the > tip drag was using up all the horsepower rather than generating thrust. I > took a ruler and marked the prop tips in 1/8" increments. I would take the > plane out for a test ride, then come back and saw 1/8" off both prop tips, > reducing the diameter of the prop by 1/4" increments. With each pass, the > engine turned up more, with some gain in performace through about 3 or 4 > iterations. The next iteration I saw more rpm, but no change in > performance. Then the next iteration the engine again gained rpm, but the > take off and climb performace was showing a definite decrease. I would have > loved to have continued cutting on the prop to see how much more it would > drop off, but since I couldn't add back onto the prop, decided to stop > there. > > > > > > I also built and fly a SuperCub. It's pretty common knowledge and has > been demonstrated over and over that if you want a 160 HP SuperCub to get > off the ground short, you take off the stock 74 x 58 McCauley prop and > install an 84 x 43 McCauley prop. The rpms are about the same, but the > plane will get off the ground much quicker (roughly half the distance!) and > climb significantly better. It's pretty obvious that the longer prop pulls > better. The thrust difference is quantifyable by pulling static against a > scale. A number of the SuperCub guys have done just that to prove it out. > Craig Catto is in the process of develping an STC to use his long 82 - 84" > props on certificated SuperCubs. However, everything with a prop is a > compromise. To get that super take off and climb performance out of a > SuperCub, the tip drag is high enough at cruise speeds that the SuperCubs > lose roughly 10 mph off the top end of their cruise. Why not go to a 93" > prop? The tip speed is high enough that the tips are creating so much drag > that all of the HP gets used up just driving the tips around in a circle, > so the thrust drops off. > > > > > > From Valley Engineering (Culver Props): The efficiency of a propeller > is reduced as the tip speed approaches the speed of sound. Beyond 80% of > the speed of sound, further increases in RPM has little effect on thrust. > Thus, it is important to keep tip speeds below 75%-80% of Mach. > > > > > > Bottom line, you tune your prop for the performace you want. > Everything on a prop is a compromise. Within reasonable limits a lower > pitch, longer prop will provide more thrust for initial take off and climb. > > > > > > -Jeff Scott > > > Los Alamos, NM > > > <http://jeffsplanes.com> > > > > > >> ----- Original Message ----- > > >> From: schmleff . via KRnet > > >> Sent: 05/09/14 11:39 AM > > >> To: tommy waymack, KRnet > > >> Subject: Re: KR> Fw: Re: Propellers > > >> > > >> Not to start an argument, but I have to disagree ; ) > > >> > > >> How a prop works on a particular airplane is based on the amount of > > >> air moved (past the cowl, not smashing into it) and the velocity of > > >> that air. I have never been able to find a real reference to the > > >> ?large diameter prop=better climb? theory nor have I seen it work in > > >> real life. > > >> > > >> On Pete?s KR, the Cloudcars 52x54 out climbs the 56x52 Sterba. Lots of > > >> factors there, but to sum up, the smaller diameter allows the engine > > >> to spin up more and make more power. The increased pitch makes up for > > >> the lesser disk area. > > >> > > >> On my SI, I have tried about 2 dozen prop variations from a 54x42 down > > >> to a 47.75x50. Climb performance between the two are about the same, > > >> but the top speed is radically more with the later. I?m not saying > > >> that a prop that small would work on a KR since the frontal area is > > >> greater. I would be curious to hear from someone that has experimented > > >> to find just how small of a prop is still effective on a KR. > > >> > > >> I recently put up some prop, climb and speed data on my blog here: > > >> http://schmleff.blogspot.com/2014/04/skye-racer-history-part-1.html > > >> > > >> Jeff Lange > > >> Race 64 - Skye Racer > > >> Blog: http://schmleff.blogspot.com > > >> Youtube Channel: https://www.youtube.com/schmleff > > >> > > >> On May 9, 2014, at 10:47 AM, tommy waymack via KRnet > > >> <krnet at list.krnet.org> wrote: > > > > > > _______________________________________________ > > > Search the KRnet Archives at http://tugantek.com/archmailv2-kr/search. > > > To UNsubscribe from KRnet, send a message to > KRnet-leave at list.krnet.org > > > please see other KRnet info at http://www.krnet.org/info.html > > > see http://list.krnet.org/mailman/listinfo/krnet_list.krnet.org to > change options > > > _______________________________________________ > Search the KRnet Archives at http://tugantek.com/archmailv2-kr/search. > To UNsubscribe from KRnet, send a message to KRnet-leave at list.krnet.org > please see other KRnet info at http://www.krnet.org/info.html > see http://list.krnet.org/mailman/listinfo/krnet_list.krnet.org to change > options >
