Clearance for a RV-xA would probably vary a bit.
But in my case with a 74" dia prop (cut down from 76) I had 4-5" clearance depending on fuel/passenger/baggage load. I did tend to keep the stick back on my belly button whenever taxing especially on non-paved areas. Mostly make full flair landings with nose gear touching after main gear. Made it a point not to keep the nose gear off too long to preclude it from slamming down as airspeed died off.
Ed
------ Original Message ------
Sent: 10/20/2021 3:50:29 PM
Subject: [FlyRotary] Re: Props and gear boxes
Thanks for the real prop knowledge and feedback Charlie and Ed.
Ground clearance is another area I’m left wondering about after hearing what Charlie has played with. I think I’ll have just over 6.7 inches when the 72” MT goes on. I wasn’t game to go for 74”. What clearance does an RV-xA have with a 72” prop?
Steve Izett
When I had Tracy's 2.17 PSRU, I swung a 67 dia x 72 pitch prop. Climb and cruise were fine but takeoff (especially from a short runway on a hot day left something to be desired).
When I got Tracy's 2.85 PSRU, I had a 76 x80 wood prop build by Performance propellers. I flew a few times with it as such and while performance was much better rpm was only around 5800. Plus the prop was only a couple inches from the ground on my RV-6A Nose gear aircraft - was concerned about landing of grass strips with possible mole holes. So I had the diameter reduced to 74 Inches. That was what the doctor ordered.
There was a considerable difference in aircraft performance particularly during take off. With the new combination, I had to tap the left brake to keep runway alignment (if I cobbed full power from a standing start) until airspeed reached approx 40 mph at which time the rudder authority became adequate to hold the nose down the runway. Starting acceleration was considerably better with the new combination, it literally push me back in the seat. Climbout improved up to around 1700 fpm previous had been around 1000 fpm.
I had thought I would probably have to give up a few knots on the top end, but it turned out I actually increased airspeed by approx 4 mph. So it was really a win win for me. I could get up to around 6200-6250 WOT at cruise, so apparently at high airspeed the prop/gearbox unloaded the engine to pick up a few more HP.
It appears that for takeoff there is nothing better than the volume (mass) of air your prop is able to push behind it. So even though the prop turned slower with the 2.85, the much larger prop and the engine increase from a take off rpm of around 56-5700 rpm to 6000 and the increased torque pushed a much larger volume (mass) of air.
I really like the change the 2.85 gear box made possible - oh, yeah, now the prop also turned in the conventional direction.😄
Ed
------ Original Message ------ Sent: 10/20/2021 9:01:32 AM Subject: [FlyRotary] Re: Ag Ops
Hi Steve,
I can't claim any expertise, but I do have some experience with various prop diameters on RVs. Van started out recommending 68" dia props on the 2 seat RVs. My 1st RV4 had a Warnke 72x72. With 160 HP, it had 1200-1500fpm climb and would cruise at 170 kts @ ~9.3 gph rich of peak, ~75% power for a Lyc 160. The cruise matched Van's numbers, and climb was at least as good, if not better than Van's numbers. After my partner in the RV damaged the Warnke, I replaced it with a 68" from another mfgr and the plane never performed as well in either cruise or climb. At one point, I tried a 76" dia IVO. Climb was incredible, but the plane would hit a wall at about 145-150 kts (known problem with IVO's blades; they don't have enough twist for high speed operation). Another data point: Van's current catalog lists a Hartzel 74" dia c/s prop for the 2 seat a/c and the f/p props are in the 72" dia range.
My calculations on optimum dia & ratio for the Renesis are similar to yours. If you start with the Lyc-standard 2700 prop rpm, with the realization that 74" is not too large a diameter (for tip speed issues) at 2700, then the optimum ratio would be determined by your desired max engine rpm. With a controllable prop, you can give up a little bit in diameter & get back low speed thrust by going to flatter pitch & more HP. But if fixed pitch, I try for the largest dia I can safely fit on the plane. With a typical fast 2 seat homebuilt, gear leg length, not tip speed, will almost always be the limiting factor even at 2700+ rpm. The bigger the diameter, the better the low speed mass flow (thrust), and there will be little to no penalty at any cruise speed under the 180-190 kt range (well above 200 mph).
The above assumes a properly designed prop, of course. One reason to desire that 2.4-2.5:1 ratio is that it gets prop rpm where the prop carver is used to working, and we're much more likely to get a usable prop on 1st try if he's in familiar territory. If you tell him, say, 200 HP @ 2700 prop rpm & cruise at 75% will be 170 kts, he'll likely be able to get it right, 1st try. If, on the other hand, you're telling him the same HP & speed, but you want, for instance, 76" dia & 2300 prop rpm, he's going to be guessing on how to carve the prop.
Bottom line: I fit the largest dia I can safely swing, regardless of f/p or c/s (because low speed thrust will always improve with diameter increases), knowing that ground clearance will be a problem long before tip speed becomes an issue. If you haven't already bought the 72", I'd be asking them for a 76" if they'll sell you one and you can safely swing it on your plane. And be sure to tell them the actual rpm range where it will operate. Their 'stock' 72" for a Lyc will be sub-optimal turning in the ~2000 rpm range with a 2.85 drive, unless you intend to really flog the engine hard all the time & pour a lot of fuel through it.
FWIW,
Charlie Hi Andrew
Continuing to fine tune the Renesis cooling system and aircraft. Completing a new exhaust system after a leaking gasket warped a flange. Last flight temps max’d at 91C (196F) on takeoff with 20C (68F)OAT and we got her up to 189knots but only 2600 feet due to cloud. Learning that increasing the IAS by ~10knots in climb has a significant effect on cooling.
Also in the process of changing the current 66.5” Airmasterto for a 72” MT prop. I only read after going with our gearbox/prop combination that Tracy recommended only using the 2.85:1 with props of >=74” For those with expertise in this area, is it true that we should choose: 1. Largest prop dimeter with acceptable clearance 2. Target cruise tip speed of ~0.8-0.85 speed of sound for best efficiency If this is correct then with a 72” prop and a chosen engine redline of 7500 RPM (4 port Renesis) a better gearbox ratio would be ~2.5:1 The 2.85:1 would be better suited to a 74-76” prop at that redline.
Hope you have a great harvest and get back in the air soon Andrew.
Steve
Hi Steve, hows the Glasair going? I been away from flying for awhile, hopefully back into it in new year. Did start mine recently after it been sitting idle since feb 2020, So happy I persevered with the mazda, it started & ran so sweet I contemplated doing a couple of circuits but for the legalities. No MR and I need AFR first.
Neil, not sure I agree with your rotor setup proposal, ( not that mine is better) can get some weird aerodynamics with intermeshing rotors. Whole thing of nurse cart & drone need to be a package, rotors will never be stopped during refill so drone needs to land on roof of truck or trailer for refill from underneath to keep blades clear of dumbasses like me. Truth be told, I dont think we can get anywhere near the efficiency of a ground rig or Ag plane, we easily average 80 ha/hr each machine + nurse cart, going to need a swarm of drones to get near this. But could be a fun project. Andrew Good to hear your voice Andrew. My son-in-law in Kojonup would be very interested in what you guys are talking about.
Cheers
-- Regards Andrew Martin Martin Ag
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