[Rolly9]

I seem to remember that there was a "Rule of Thumb" sort of thing that went something like this: 24" X 2400 rpm (drop the zeros) = 48 or 75%Power.
Anyone remember this. Thx
Rolly Clark
RV-9 Phase I Flying.

 

[n5lp]

Doesn't work

Some people think it is accurate enough. I don't.

Here is one of the threads. http://www.vansairforce.com/community/showthread.php?t=22194&highlight=rule+thumb

 

[Steve Sampson]

Here you go:http://www.vansairforce.com/community/showpost.php?p=199988&postcount=25

I think its a great rule. Presumably you are at the break in stage and want to know how you are doing keeping the power up. While you are managing a new engine and aircraft worrying about airspace and ATC you will probably manage to add two numbers together and ensure they still add to 48 or greater. Getting the graphs out at that stage for a more accurate check is quite unrealistic!

 

[Webb]

3% per 1000

75% of what? Don't forget the 3% per thousand foot rule.

At 10K - your 200hp engine is now a potent 140hp monster.

Hmmm.. at 12K, the MP will be less. Lets give it a generous 20 inches at 2,200. So If I get this right, for the 12K flight, this would be 55% power (42 = 55%). 55% of 128hp is 70.4hp (of course assuming it's a new engine with full set of horses under the cowl).

Vans gives the service ceiling 24,500 for a 7A with a 200hp engine. That means you only have 43hp to work with out of that pristine 200hp motor. MP would be so low I doubt you could get 50% out of that engine but let's say you could. 21.5hp and you're still flying. Does this mean that about a 20 hp engine might keep us afloat?

What's the guestimate on how much power it really takes to keep an RV at a steady altitutde?

 

[pierre smith]

Dunno, but....

What's the guestimate on how much power it really takes to keep an RV at a steady altitutde?

....it's often been said that an RV is one of the very few airplanes that'll do 100 MPH on 3.5 GPH. If 180 HP burns 15 GPH wide open, calculate the HP at 3.5 but as you know, the airplane will fly comfortably at 80.

Regards,

 

[Steve Sampson]

75% of what? Don't forget the 3% per thousand foot rule.

At 10K - your 200hp engine is now a potent 140hp monster.

QUOTE]

Webb, with respect you have rather got hold of the wrong end of the stick there. At 10K you would not have the manifold pressure to get to 75% I expect.

The point about the rule is you are using manifold pressure and the rpm. Now you may have trouble getting the manifold pressure but if you add whatever you have to rpm/100 it gives you a great rough and ready indication of where you are on the power curve. I am on the second hour with a new engine and am finding it a very quick and useful way of checking I still have enough power going in for break in. Obviously I am not trying to do it at 10K because the rule would say i am failing!

 

[hevansrv7a]

Using the CAFE data..

....it's often been said that an RV is one of the very few airplanes that'll do 100 MPH on 3.5 GPH. If 180 HP burns 15 GPH wide open, calculate the HP at 3.5 but as you know, the airplane will fly comfortably at 80.

Regards,

Using CAFE data on the 6A and a little guesstimation..

Assume prop efficiency at 80% (it might be better). Assume LOP BSFC is .40.
The 6A can do 80 mph (CAS=True for this) on 2.8-3.0 gph. That's the speed, within a couple of mph, that takes the least horsepower or fuel flow. The least-sink speed is the speed at which the lowest fuel flow can be obtained; best glide is more efficient, but the gph is higher. And yes, it will do >100 mph on 3.5 gph according to CAFE's data and my two working asssumptions.

Since prop efficiency may be higher, it can maybe do a little better. The BSFC is from the GAMI folks and is, in my experience, pretty realistic.

The -9/9A CAFE data includes fuel flow and it may do even better.

 

[tin man]

Doubt if you can get 80% + prop efficiency. Its all about rpm and manifold pressure. You need both to make rated hp, check the engine manual.
tm

 

[Webb]

75% of what? Don't forget the 3% per thousand foot rule.

At 10K - your 200hp engine is now a potent 140hp monster.

QUOTE]

Webb, with respect you have rather got hold of the wrong end of the stick there. At 10K you would not have the manifold pressure to get to 75% I expect.

I don't know about the wrong end of the stick, but I was speaking in hypothetical terms and thinking outloud. The 3% rule only suggests that your possible max is about 140 ponies at 10,000. Unless you do something to such as turbo your engine, we both know that MP just isn't gonna get there. I never suggested you could get that kind of MP (at least I don't think I did).

Do I have this wrong??

If you are at 10,000 feet MSL, according to the 3% rule, this means that you engine has 140hp available. Does this mean the max you could run the engine is 70% (140/200) and your engine is producing 140hp OR does it mean that if you ran the engine at 70%, it would be out of the 140hp which means the engine is producing 98hp?

The question I put out is how much HP is needed to keep one of our planes aloft in level flight.

If use the second supposition, it means that at ceiling, our max output (43hp) is 21.5%.

The way gas is going, maybe we need 2 engines like a bass boat. The big one to get us up, and a trolling motor equivalent to keep us moving.

 

[hevansrv7a]

How much power to fly level?

...

The question I put out is how much HP is needed to keep one of our planes aloft in level flight.
...

I thought I answered this. Let me try again..

For the CAFE 6A, 33-37.5 Thrust HP. For my 7A, 37.5 THP. For a -9/9A, less. Now the BHP needed to produce that THP is larger in inverse relation to the prop efficiency. If you use 80%, then 36 becomes 36/0.80= 45 BHP required. The fuel flow required for the 45 BHP will vary according to the BSFC (lower BSFC means lower fuel flow for the same HP).

Sea level is one thing and high altitude is another. The numbers at cieling could be a lot different but I have no idea how to predict them. The power is what it is, but the engine's performance would be wierd because of the high rpm needed to get the thrust and engine HP at very low MP.

 

[elippse]

Doubt if you can get 80% + prop efficiency. Its all about rpm and manifold pressure. You need both to make rated hp, check the engine manual.
tm

All of the testing I've done on my Lancair indicates 82% efficiency on my ELIPPSE 3-blade in a climb at 2410 rpm, 110 mph IAS, 1500-1550 fpm. 1350 lb, and at least 90% efficiency in cruise at 10,000' dalt, 2800 rpm, 201 mph TAS. My engine burns 3 gph at 150 mph TAS at 9000' dalt. Typical cruise is 197 mph TAS at 14,500' dalt, 2760 rpm, 5.6-5.7 gph.

 

[tin man]

All of the testing I've done on my Lancair indicates 82% efficiency on my ELIPPSE 3-blade in a climb at 2410 rpm, 110 mph IAS, 1500-1550 fpm. 1350 lb, and at least 90% efficiency in cruise at 10,000' dalt, 2800 rpm, 201 mph TAS. My engine burns 3 gph at 150 mph TAS at 9000' dalt. Typical cruise is 197 mph TAS at 14,500' dalt, 2760 rpm, 5.6-5.7 gph.

Can't blame a guy for little free marketing. Always wondered why a lancair builder/ prop manufacturer would popup on an RV site. Potential customers? I hope your prop can work at at least 90% efficency, there's always room for a better mousetrap. Any RV people out there with first hand experience to confirm his claims against a Catto or whatever?
tm

 

[hevansrv7a]

Catto Efficiency

Can't blame a guy for little free marketing. Always wondered why a lancair builder/ prop manufacturer would popup on an RV site. Potential customers? I hope your prop can work at at least 90% efficency, there's always room for a better mousetrap. Any RV people out there with first hand experience to confirm his claims against a Catto or whatever?
tm

I have no expertise in this whatsoever. However, I did ask Craig Catto for a number to plug into calculations like these (I was using 85%) and he said..

"The 85% figure seems perhaps realistically a bit high.. probably down around 78 to 82 might be more realistic. This figure is actually buried in my code when it runs through thousands of calculations. The main figure I am looking at in design is the operating angle of attack in determining blade pitch and area."

My Prince P-Tip's efficiency is roughly comparable to the Catto's, plus or minus 1%. I base this on knots versus fuel flow.

I can get 155 kts TAS at low DA's on 7.2 gph (LOP:107 BHP) but my best L/D is around 96 kts CAS and my Carson's speed is therefore 126 kts CAS.

The Lancair numbers are way better than I can do with my -7A. Page 11 of the 2002 CAFE report on the Lancair Legacy factory prototype has cruise data. It got over 20 sm/g at 260+ sm/h but the report is data-rich; that's just a bit of it. Someone with more math skills could convert these numbers to prop efficiency, perhaps.

 

[elippse]

Can't blame a guy for little free marketing. Always wondered why a lancair builder/ prop manufacturer would popup on an RV site. Potential customers? I hope your prop can work at at least 90% efficency, there's always room for a better mousetrap. Any RV people out there with first hand experience to confirm his claims against a Catto or whatever?
tm

I like the RVs, and lots of my friends have them, plus this is an interesting site! I only go to this and the Lancair site. I don't manufacture props, I just design them. I wasn't trying to plug my design, just giving the actual flight test data. Craig made the three-blade and four-blade racing props that I designed for the biplanes, and he told me that a little of my design characteristics were now in his props. If you know how to determine the prop efficiency from actual performance and aircraft data, I'd be happy to supply that to you. When I gave the data to Peter Garrison that gave the 82% number I got for climb, he calculated more like 84%. I'll stay with my estimate. I have only one design flying on an RV, and that's Jim Smith's three-blade that gave him an average of 191.5 mph TAS from 3 or 4 tests at 7000' dalt, 2740 rpm with his 150 HP O-320. Keep in mind that the three-blade prop I designed for Tom Aberle's Phantom biplane boosted his speed from 220 mph with a two-blade at Reno 2003 to 240 mph in 2004 at 250 rpm less. At Reno 2006 with a four-blade I designed he did 251 mph on the same rpm that gave him the 220 mph with the two-blade. I should think that these numbers obtained at the Reno air races offer some measure of the efficiency of this design. Jack Norris in his new book "Propellers, The First and Final Explanation" describes the increased efficiency obtained with my design.

 

[elippse]

I thought I answered this. Let me try again..

For the CAFE 6A, 33-37.5 Thrust HP. For my 7A, 37.5 THP. For a -9/9A, less. Now the BHP needed to produce that THP is larger in inverse relation to the prop efficiency. If you use 80%, then 36 becomes 36/0.80= 45 BHP required. The fuel flow required for the 45 BHP will vary according to the BSFC (lower BSFC means lower fuel flow for the same HP).

Sea level is one thing and high altitude is another. The numbers at cieling could be a lot different but I have no idea how to predict them. The power is what it is, but the engine's performance would be wierd because of the high rpm needed to get the thrust and engine HP at very low MP.

These numbers seem a little bit on the high side. There's an excellent article in the latest Sport Aviation that goes into these numbers and I think they are somewhat lower. I estimate that my plane requires 24HP at the best endurance speed of 86 mph. I have an AR of 8.05 which reduces my induced loss vs the 4.8 AR of a -6. I wonder how many of you notice how much higher your nose angle is at 10k-14k density altitudes due to the low AR. When I mentioned this to Jim Smith about his -6, he confirmed it! Jim is making some new tips I designed that should help this. We'll see! The -9 has quite an advantage with its higher AR. Something for you to try at the higher altitudes is put in about 2 to 4 deg of flaps and see if that drops your nose down somewhat and gives a little more speed. When I decrease the reflex in my flaps at high weight-high density altitude, the nose goes down and the speed increases about 3 mph!

 

[hevansrv7a]

Justifying 33 hp minimum

These numbers seem a little bit on the high side. There's an excellent article in the latest Sport Aviation that goes into these numbers and I think they are somewhat lower. I estimate that my plane requires 24HP at the best endurance speed of 86 mph. ...

CAFE's 6A test says 134.3 pounds of drag at 106 mph best glide. The HP for best endurance is necessarily 71.6% of the HP at best glide. THP is drag times TAS. 134.3 x 106 x [etc] = 37.961 HP and so for minimum sink (80.5 mph) that would be 33.307 THP. If you use their glide ratio instead you get slightly higher numbers (their numbers don't all agree). If you convert THP to BHP the numbers get bigger depending on prop efficiency losses. The minimum sink for the 1650 pound -6A would be 666 feet per minute if the best L/D drag were 134 pounds.

For a given airplane to have a best endurance speed of 86 mph and need only 24 THP it would have to be much lighter and/or cleaner. Since 86 is 6.8% faster and 24 is 38.75% less THP, the pounds of drag would have to be about 90.4 or 67.6% of that for a -6A. Maybe so - I don't know anything about your airplane. I think that my numbers for the -6 are based on accepted theory and widely respected test data.

What sink rate and weight would you need to have to come out with 24 THP? If the weight were the same the sink rate would have to be about 444 fpm, rougly estimated. Can yours do that?

 

[elippse]

For a given airplane to have a best endurance speed of 86 mph and need only 24 THP it would have to be much lighter and/or cleaner. Since 86 is 6.8% faster and 24 is 38.75% less THP, the pounds of drag would have to be about 90.4 or 67.6% of that for a -6A. Maybe so - I don't know anything about your airplane. I think that my numbers for the -6 are based on accepted theory and widely respected test data.

What sink rate and weight would you need to have to come out with 24 THP? If the weight were the same the sink rate would have to be about 444 fpm, rougly estimated. Can yours do that?

Here's some formula and the results;These do not account for any residual propeller thrust or drag:
Q=rhoxV^2/2, CL= W/QxAw, CDI=CL^2/pixARxO, HP=Qx(CDIxAw+Afp)/550, ROD=60xHPx550/W
At best L/D, CDIxAw=Afp
On my plane Aw=77, W=1350, AR=8.05, O=0.82=Oswald eff.factor, Afp=1.45, but C.A.F.E. said, I think, 1.65. Solving for Q, V, HP, and ROD for each Afp gives:
1.45: 28.1 lb, 104.8 mph, 22.7 HP, 556 fpm.
1.65: 26.3 lb, 101.4 mph, 23.5 HP, 574 fpm.
Since best endurance is at 76% best L/D, then that would give 79.6 mph and 77.1 mph. Here the total drag area will be four times as much, so the HP for each will be 20.0 and 20.6, and ROD will be 488 fpm and 504 fpm.

 

[hevansrv7a]

We almost agree

Here's some formula and the results;These do not account for any residual propeller thrust or drag:
Q=rhoxV^2/2, CL= W/QxAw, CDI=CL^2/pixARxO, HP=Qx(CDIxAw+Afp)/550, ROD=60xHPx550/W
At best L/D, CDIxAw=Afp
On my plane Aw=77, W=1350, AR=8.05, O=0.82=Oswald eff.factor, Afp=1.45, but C.A.F.E. said, I think, 1.65. Solving for Q, V, HP, and ROD for each Afp gives:
1.45: 28.1 lb, 104.8 mph, 22.7 HP, 556 fpm.
1.65: 26.3 lb, 101.4 mph, 23.5 HP, 574 fpm.
Since best endurance is at 76% best L/D, then that would give 79.6 mph and 77.1 mph. Here the total drag area will be four times as much, so the HP for each will be 20.0 and 20.6, and ROD will be 488 fpm and 504 fpm.

I don't have your engineering or math skills, but I think this is also correct and easy to do:

1350 pounds times 556 feet per minute gets 750,600 foot-pounds per minute which, divided by 33,000 foot pounds per minute per HP is 22.75 horsepower. for the higher sink rate it's 23.5 horsepower.

I don't know about drag AREA but the difference in drag FORCE between best endurance and best L/D is 0.866 or 1.1547.

The thrust horsepower needed is directly proportional to the weight and the rate of sink. The RV-6A test weight at 1650 was 22.2% higher than yours so for weight alone, your 23.5 THP becomes 28.7 THP for the higher weight. Using 33.307 THP for the RV, your airplane is 28.7/33.307 = 86.2% as draggy as the RV. That's really not a surprise. The differences between my numbers and yours are more due to the lighter weight of your aircraft than the slickness of the airframe, but from these numbers, the Lance is 16% cleaner.

The numbers do seem to be reasonable, then, for both aircraft. At least they do to me. I think, then, that the numbers I am alleging for the 1650 pound RV-6A minimum THP for level flight are still supportable and explainable.

Obviously, if the RV-6A were flying at minimum weight the THP required would be lower but not in direct proportion. Best endurance speed would change. Drag at best endurance speed is 75% induced and that varies with the square of the weight. That would change the sink rate. In other words, fly it light and it needs less power to stay up there.

CAFE used a zero thrust method to remove the prop from the measurements. Your ROD appears to be based purely on theory. Am I reading it right? If I am, can you provide some data points to verify the assumed values? For example, fuel flow running LOP at three different CAS's (or TAS's with density altitude)? Have you verified your best L/D speed? I'm not being insulting (not on purpose) but I'm always thirsty for more data! Thanks.

 

[elippse]

From my testing I know that my best L/D is about 105 mph IAS. I calculated the numbers for the -6A based on: 2.32 Afp(C.A.F.E.), 1650 lb., 110 sq. ft., 0.81 Oswald and get:
107.5 mph, 137 lb drag, 39.3 HP which verifies the numbers you gave. I guess my comments about the numbers being somewhat high were based on the number I came up with for Jim Smith's -6 using 1440 lb and 2.2 Afp which gave 101.8 mph for best L/D, which is what I had him do his climb tests at.