Ken Powell wrote:
Paul,
I have been following the discussion of prop tip speeds but none of
the charts presented an answer that I could see to the following
question? (I am a programmer and I am sometimes mathamatically
challenged!)
-- On a 200 MPH airplane (RVx), at what propeller RPM do the tips
enter the high drag transonic speed that we need to avoid?
1. for a 68" prop
2. for a 70" prop
3. for a 72" prop
I also noticed last week that one of the charts seemed to present
evidence that prop diameter changes of this small magnitude were not
significant, but the results of real world tests seem to indicate
that they do. What is the 'real' story here?
Please give me some guidance here; there doesn't seem to be an easy
answer. I know that 'thinning' the tips will allow higher RPM, but
I would like the issue addressed for standard wood props like we
would buy from most of the prop makers.
Thanks,
Ken Powell
Here are the numbers Ken.
According to the data Vance gave me there will be a slight advantage
in static thrust for the 72 verses the 68 inch prop but little or no
advantage at top speed.
Just to cloud the issue further.. for most aircraft engines, for the same tip mach
number, you will get slightly more HP with the smaller prop because the engine
is running faster.
BTW a Cessna 150 takes off pretty good with a fixed pitch prop as the ratio
of top speed to takeoff speed is not much so a fine pitch prop can be
chosen. Ultra lights are in the same category as the takeoff, cruise and stall
speeds are all the same :-)
It is when you are confronted with a wide speed range airplane such as
a Lancair 360 or an RV that can cruise over 200 MPH that constant speed
props show a huge advantage in much faster takeoffs.
Vance neglected to mention this and I was too stupid to pick up on it. :-)
As I recall Vance used a fixed pitch prop optimized for only 180 MPH in his chart.
If you optimize a fixed pitch prop for 220 MPH the takeoff thrust will
suffer greatly. I still believe the Air Master chart for the Lancair with
the constant speed prop.
I spoke to Tracy Saylor, owner of the fastest RV6 in the world at 228 MPH, about this
and he said he reduced the fine pitch stop on his constant speed prop so the engine
could reach 2800 RPM for max power and it does so within a second of starting to
taking off.
It takes off in less than three seconds. It all happens so fast it is difficult
to know exactly when the engine reaches 2800 RPM or to get accurate data with a hand
held stop watch. I intend to spend the money and put a constant speed prop
on my turbo 13B powered KIS cruiser. As far as I am concerned it is the best
investment in overall aircraft performance and takeoff safety.
Paul Lamar
The Aircraft Rotary Engine Newsletter. Powered by Linux.
http://home.earthlink.net/~rotaryeng/ http://www.linux.org
dia = 68
RPM Tip FPS Vel FPS Vel MPH Mach
2200 653 716 488 0.70
2250 668 729 497 0.71
2300 682 743 506 0.72
2350 697 756 516 0.74
2400 712 770 525 0.75
2450 727 784 534 0.76
2500 742 798 544 0.78
2550 757 811 553 0.79
2600 771 825 563 0.80
2650 786 839 572 0.82
2700 801 853 582 0.83
2750 816 867 591 0.84
2800 831 881 601 0.86
dia = 69
RPM Tip FPS Vel FPS Vel MPH Mach
2200 662 724 494 0.71
2250 677 738 503 0.72
2300 692 752 513 0.73
2350 708 766 522 0.75
2400 723 780 532 0.76
2450 738 794 541 0.77
2500 753 808 551 0.79
2550 768 822 560 0.80
2600 783 836 570 0.81
2650 798 850 580 0.83
2700 813 864 589 0.84
2750 828 878 599 0.86
2800 843 893 609 0.87
dia = 70
RPM Tip FPS Vel FPS Vel MPH Mach
2200 672 733 500 0.71
2250 687 747 509 0.73
2300 702 761 519 0.74
2350 718 775 529 0.76
2400 733 790 538 0.77
2450 748 804 548 0.78
2500 764 818 558 0.80
2550 779 832 567 0.81
2600 794 847 577 0.82
2650 809 861 587 0.84
2700 825 875 597 0.85
2750 840 890 607 0.87
2800 855 904 616 0.88
dia = 71
RPM Tip FPS Vel FPS Vel MPH Mach
2200 682 742 506 0.72
2250 697 756 516 0.74
2300 713 771 525 0.75
2350 728 785 535 0.76
2400 744 799 545 0.78
2450 759 814 555 0.79
2500 774 828 565 0.81
2550 790 843 575 0.82
2600 805 857 584 0.83
2650 821 872 594 0.85
2700 836 886 604 0.86
2750 852 901 614 0.88
2800 867 916 624 0.89
dia = 72
RPM Tip FPS Vel FPS Vel MPH Mach
2200 691 751 512 0.73
2250 707 765 522 0.75
2300 723 780 532 0.76
2350 738 794 542 0.77
2400 754 809 552 0.79
2450 770 824 562 0.80
2500 785 838 572 0.82
2550 801 853 582 0.83
2600 817 868 592 0.85
2650 833 883 602 0.86
2700 848 898 612 0.87
2750 864 912 622 0.89
2800 880 927 632 0.90