Jerry Hey wrote:
Comparing the 2.17 gear box to the 2.85 gear box does not present a
clear cut victory for the 2:85. For example, when cruising with a
prop rpm of 2750, the 2.17 gear box engine is turning 5967 rpm.
With the 2.85 gear box the engine rpm is 7837. I think the rotary
can handle this rpm just fine, but more fuel is being consumed and a
whole bunch more heat is being generated. This latter is an issue.
Enough sustained heat and inconel exhuast components will be the only
option. There is also the problem of heat build up inside tight
cowls.
An exhaust system that can handle the heat easier to imagine than a
cooling system that can handle it. Remember, thus far no one is
claiming to be able to launch at a maximum rate of climb on a 100
degree day when using the 2.17 gear box. Actually, most reports are
of slowing down and lowering the nose even when the outside temps are
only in the 90 degree range. Cranking nearly 2000 more rpm is
probably not the answer. It is pointless to talk of bigger
radiators. There is no room for them.
Next concern is the matter of oil pressure. According to what I
heard at SevenStock, the rotary oil pressure should increase about
10 psi for every thousand rpm up to 6000 rpm. Above that,
lubrication requirements grow more rapidly. Race engines can
require 120 psi. I'm not worried about my Setrabs at 60 or 70 psi
but oil pressure sustained above 100 psi...
Just generating enough oil pressure could be an issue with the stock
oil pump. This is an area I know nothing about, but it should be
investigated before making a gear box choice.
When cruising, one cannot just throttle back the 2:85 engine to 6000
rpm. If you did, the prop would be slowed to a snail like 2105 rpm.
In other words, if you have to back off, the 2.85 gear box is likely
to cruise slower than the 2.17.
It is exactly equivalent to being stuck in second gear.
Recalling the graphs showing bearing loads, it seems that 6000 rpm
is the 13B's sweet spot.
Two advantages of the 2:85 gear box are the POWER it allows the
rotary to develop--so nice for heavy short field take offs over
power lines-- and the slow prop rpm available for touch down and
taxiing.
Jerry (waiting to be shot out of the saddle)
Too simple Jerry.
2750 prop RPM sounds a bit high for cruise.
That sounds like a take off RPM. The heat is a really a function
of HP and not RPM. With a fixed pitch prop a small drop from max
RPM is a major reduction in prop load HP. Prop load is proportional
to at least the square of RPM probably the cube. One reason
is because the tips are in the transonic range. See the graph
below. Assume for the moment it is indeed the square and 2750 RPM
at top speed. Drop the prop RPM to 2500. 6,250,000/7,562,500 83% power.
If you blind fold someone they cannot tell the difference with a rotary
between 6000 and 6500 RPM. The increased load on the engine and the
increase in fuel burn are small. We tend to think piston engine
when it comes to our thinking on RPM. The second gear analogy
is too extreme.
Also when leaning the engine for cruise the EGT actually
comes down as Tracy will confirm if he is not stuck somewhere
between Florida and Arizona :-)
With a 2.85 gear box you can put more pitch in the fixed pitch
prop so you are going faster at a given prop RPM. You have more
HP to pull the higher pitch.
The table below assumes the max HP available at that RPM and NOT the
prop load HP. In other words it is more of a climb chart rather
than a cruise chart. The tip velocity is given in feet per second.
To get MPH multiply by .68. 600 MPH (about Mach one) would be
880 feet per second.
The top row is the diameter of the prop in inches. For the 2.85
perhaps a fixed pitch prop with a smaller diameter and with more
pitch might be optimum. That would tend to sacrifice
a small amount of takeoff performance with the benefit of cruise
or top speed performance.
An even larger pay off for the 2.85 is when we get a variable pitch
prop. Then we will have the best of all worlds. We are working on
it.
Paul Lamar
The AirCraft Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site. http://home.earthlink.net/~rotaryeng/
Copyright 1998-2002 All world wide rights reserved.
RPM HP PROP 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86
4000 124 1843 466 483 499 515 531 547 563 579 595 611 627 643 660 676 692
4100 127 1889 478 495 511 528 544 561 577 594 610 627 643 660 676 693 709
4200 130 1935 490 507 524 540 557 574 591 608 625 642 659 676 693 709 726
4300 133 1982 501 519 536 553 571 588 605 623 640 657 674 692 709 726 744
4400 136 2028 513 531 549 566 584 602 619 637 655 672 690 708 725 743 761
4500 140 2074 525 543 561 579 597 615 633 651 670 688 706 724 742 760 778
4600 143 2120 536 555 573 592 610 629 647 666 684 703 721 740 758 777 795
4700 146 2166 548 567 586 605 624 643 662 680 699 718 737 756 775 794 813
4800 149 2212 560 579 598 618 637 656 676 695 714 734 753 772 791 811 830
4900 152 2258 571 591 611 631 650 670 690 709 729 749 769 788 808 828 847
5000 155 2304 583 603 623 643 664 684 704 724 744 764 784 804 824 845 865
5100 158 2350 595 615 636 656 677 697 718 738 759 779 800 820 841 861 882
5200 161 2396 606 627 648 669 690 711 732 753 774 795 816 836 857 878 899
5300 164 2442 618 639 661 682 703 725 746 767 789 810 831 853 874 895
5400 167 2488 630 651 673 695 717 738 760 782 803 825 847 869 890
5500 171 2535 641 664 686 708 730 752 774 796 818 840 863 885
5600 174 2581 653 676 698 721 743 766 788 811 833 856 878
5700 177 2627 665 688 711 734 756 779 802 825 848 871 894
5800 180 2673 676 700 723 746 770 793 816 840 863 886
5900 183 2719 688 712 736 759 783 807 830 854 878
6000 186 2765 700 724 748 772 796 820 845 869 893
6100 189 2811 711 736 760 785 810 834 859 883
6200 192 2857 723 748 773 798 823 848 873 898
6300 195 2903 735 760 785 811 836 861 887
6400 198 2949 746 772 798 824 849 875
6500 202 2995 758 784 810 836 863 889
6600 205 3041 770 796 823 849 876
6700 208 3088 781 808 835 862 889
6800 211 3134 793 820 848 875
6900 214 3180 805 832 860 888
7000 217 3226 816 845 873
7100 220 3272 828 857 885
7200 223 3318 840 869 898
7300 226 3364 851 881
7400 229 3410 863 893
7500 233 3456 875
7600 236 3502 886
7700 239 3548 898
