[N941WR]

OK, I'm not an engineer and all the talk about engine size over weight has had me thinking.

There has to be some breakeven point for adding weight when adding HP. In other words, how heavy must an engine be before the power gain it provides is limited by the weight of the installation?

It is a bit like the Vette vs. Lotus argument.

Do any of you know how to calculate this?

 

[Geico266]

The simple ratio calculation is aircraft weight -v- total HP.

 

[N941WR]

The simple ratio calculation is aircraft weight -v- total HP.

Yes, but that doesn't really tell me how the airplane performs.

Say you take one RV-7 with an empty weight of 1050 lbs with a 160 HP engine, that comes out to 6.6 lbs per HP.

Now swap that engine with 180 HP O-360 that weights 20 lbs more. Everything else being equal, that comes out to 5.9 lbs per hp.

That tells me the 180 HP should outperform the 160 HP version but I guess what I'm looking for is how well does one handle over the other.

Wing loading gives you an indication of how many pounds each sq-ft of wing has to lift but somewhere there has to be a combination of the two.

It might be that the difference is so small it is not noticeable. Take for instance a guy who mounts an IO-540 on an RV-7 vs. 160 HP RV-7. I hear the lighter -7 will fly better but how do you quantify that?

 

[Mel]

It also depends on how you define performance.

Some things depend on where the extra weight is. Everything else being equal, an airplane balanced toward the aft end of the CG will fly better than a nose heavy one.

 

[ergie63]

For handling use wing loading. In all your examples, the wings are identical and have the same capacity to generate lift. So a heavier model will require a larger turning radius or a higher angle of attack.

Force = Mass * Acceleration
To get the same acceleration (turn) out of a heavier aircraft requires more force.

The IndyCar guys are all up in arm over Danica Patrick because she has a 40lb weight advantage over them. EDIT: For the 2008 season the weight rule was changed to include the driver. Wikipedia

 

[L.Adamson]

Yes, but that doesn't really tell me how the airplane performs.

Say you take one RV-7 with an empty weight of 1050 lbs with a 160 HP engine, that comes out to 6.6 lbs per HP.

Now swap that engine with 180 HP O-360 that weights 20 lbs more. Everything else being equal, that comes out to 5.9 lbs per hp.

That tells me the 180 HP should outperform the 160 HP version but I guess what I'm looking for is how well does one handle over the other.

Wing loading gives you an indication of how many pounds each sq-ft of wing has to lift but somewhere there has to be a combination of the two.

It might be that the difference is so small it is not noticeable. Take for instance a guy who mounts an IO-540 on an RV-7 vs. 160 HP RV-7. I hear the lighter -7 will fly better but how do you quantify that?

I'll try not to hijack the thread too much....
And this way, I don't need to hijack your "build light" thread at all!

So lets take performance. It appears that my "heavier" 1172 lb. 6A with it's 180 HP C/S prop, is kind of running like a scalded dog, in comparison to a few 9A's with 150/160 C/S prop setups. I'm more maneuverable in comparison to being more sedate. Faster on the level, better climb rate, and less to worry about when it comes to density altitude. I may use a bit more fuel, but I much prefer the performance, as well as a livelier feel..

I won't compare mine to a lightweight "3" that's great for short flights around the countryside with minimal everything. Mine does very well for some long cross-country rides.

Yet, myself and the 9's have been going on quite a few 200 mile brunch runs lately. I get a lot of comparison that way. I've also got about 40 hrs in the two 9's myself.

The point is, mine is heavier, but it appears the horsepower difference, really DOES make a difference. It feels like a sports car, and I didn't have to give up a lot on the comfort end.

L.Adamson ---- RV6A

 

[ergie63]

The point is, mine is heavier, but it appears the horsepower difference, really DOES make a difference. It feels like a sports car, and I didn't have to give up a lot on the comfort end.

L.Adamson ---- RV6A

I disagree with your conclusion. Instead its the wing that is making the difference. The -6 wing is capable generating much more force, evidenced by its greater g load rating, than the -9 wing. This much larger force is apparently sufficient to overcome the additional weight noted.

 

[N941WR]

LA,

You missed the point of this post completely. I wasn't comparing a -9 to a -6 or any other plane.

The point is to find out where the tradeoff of power over weight cross.

I'm sure your -6 handles like a sports car to you, just like the guy who has only driven one type of sports car, say a Vette thinks his car is the best thing in the world. But there are other cars out there that can perform just as well, maybe better on less power, say a Lotus Exige which can stomp the Vette with only 240 HP because it is lighter. It is this light weight that allows the Exige to accelerate faster, stop quicker, and turn in faster than the Vette.

Of course you give up a lot with the Lotus for these performance numbers. The same tradeoffs are there in airplane construction, I just thought it might be nice to know where.

BTW, this thread was not aimed at you and your well documented 180HP CS sports car of an RV.

 

[Kyle Boatright]

LA,

You missed the point of this post completely. I wasn't comparing a -9 to a -6 or any other plane.

The point is to find out where the tradeoff of power over weight cross.

<snip>

BTW, this thread was not aimed at you and your well documented 180HP CS sports car of an RV.

One hp does 33,000 lb*ft/minute of work. That means it can either lift 1 lb 33,000 feet in a minute or lift 33,000 lbs one foot in a minute.

In our situation, we factor this by prop efficiency. Say 80%, so an engine HP will lift 1 lb 80% x 33,000 feet in a minute = 26,400 ft.

If you have a 160 hp engine and a 1600 lb plane, theory is that it should climb at 160 x 80% x 33,000/1600 = 2640 fpm. But that's wrong since we didn't consider the power it takes to simply hold the aircraft in the air. Let's call that 50 hp off the top.

So for climb performance, you have a 110 surplus hp x 80% efficiency x 33,000 ft*lb/min = 1815 ft/min.

Modify the equations with the new aircraft weight and the new hp and see what happens.

Boil it all down and if you increase the ratio of surplus hp vs aircraft weight and you'll improve climb rate. In the example above, the ratio was 110/1600.

Hope this helps..

 

[L.Adamson]

BTW, this thread was not aimed at you and your well documented 180HP CS sports car of an RV.

I know that. Yet your other "build light" thread, naturally caught my eye. The problem is, we can get into this notion, that building light is going to be the best of all things. When actually, it may not be.

I have a slider, I have a C/S prop. I have the 180HP engine, the two axis auto-pilot, and the leather seats. I've got the electric elevator & aileron trim. As it is, I didn't need to give these things up; and would not give them up a second time around. Besides, my RV probably penetrates wind better than the light weights.

L.Adamson

P.S. ---- If I had the time and money, I'd like to build a Cub Crafters Cub with a 180 HP engine and tundra tires. A different mission, but fun. The guy in the hangar across from me is building one, to go with his Cessna 180 & Pitts. It's a nice plane!

 

[rvbuilder2002]

I have a slider, I have a C/S prop. I have the 180HP engine, the two axis auto-pilot, and the leather seats. I've got the electric elevator & aileron trim. As it is, I didn't need to give these things up; and would not give them up a second time around. Besides, my RV probably penetrates wind better than the light weights.

L.Adamson

I think within the context of Bobs original question, the reality is that if your airplane was closer to a typical weight for an RV-6A equipped like yours ( I am familiar with quite a few 6A's with everything you listed except for leather interior, with empty weights between 1075-1085 lbs) it would have even better performance. That doesn't mean there is anything wrong with your airplane weighing 1172 lbs. The important thing is that you are happy with it.
The flip side of that though, is that it doesn't mean everyone else should be building a highly appointed airplane also.
Case in point...
I have an RV-6A (currently with no exterior paint) also with an O-360 and its empty weight is 1042 lbs. It has a fixed pitch prop. Even so, it also runs like a scalded dog speed wise.
I don't have an auto pilot, or a fancy leather interior, but I would bet that if we did a performance comparison with equal fuel loads and equal weight pilots, my 6A would be about as fast, would climb nearly as fast (130 lbs lighter) and is even more maneuverable than your sports car (once again lighter weight, and probably a little more mid range C.G. because of the lighter prop).

Main point, everyone needs to build for what they want (and can afford) depending on the choices they make they may even be able to get nearly equal performance without some of the extras.

 

[the_other_dougreeves]

OK, I'm not an engineer and all the talk about engine size over weight has had me thinking.

There has to be some breakeven point for adding weight when adding HP. In other words, how heavy must an engine be before the power gain it provides is limited by the weight of the installation?

It is a bit like the Vette vs. Lotus argument.

Do any of you know how to calculate this?

Hmmm. You could probably develop some kind of formula(e) to predict engine output vs weight, but that would probably involve compression ratio and RPM. Money can do a lot to reduce weight, though (think Formula1).

Before we start throwing in all the variables, though, what are your constraints? Pistons only, 'cuz turbines offer a lot of power for very little weight - just a lot of cost? Mogas compatible? Frontal area small enough to fit in a RV? Reliability? Rotaries Ok?

I might suggest looking at it another way - maybe define what you need in terms of power first, weight and other limits, and that's going to narrow down things quickly. With a car, you also have a lot more leeway to add weight - your Lotus vs Vette anology.

TODR

 

[tommylewis]

weight vs power

everyone needs to build for what they want (and can afford) depending on the choices they make they may even be able to get nearly equal performance

I strongly agree with Scott's comment. If keeping it light is what you want, then do it. When I built my RV7a, I wanted a traveling machine that my wife and I and 100 lbs of stuff could travel cross country. I wanted IFR capability and comfort. Therefore, my machine is on the heavy side, BUT I can easily carry my intended load and be within the weight and balance guidelines. It meets the published Van's speed numbers, and I am happy. We have put 468 hours on our RV7a in the last 18 months and have seen alot of the US. I can camp out of it because I can carry a load in the baggage compartment.

 

[airguy]

I strongly agree with Scott's comment. If keeping it light is what you want, then do it. When I built my RV7a, I wanted a traveling machine that my wife and I and 100 lbs of stuff could travel cross country. I wanted IFR capability and comfort. Therefore, my machine is on the heavy side, BUT I can easily carry my intended load and be within the weight and balance guidelines. It meets the published Van's speed numbers, and I am happy. We have put 468 hours on our RV7a in the last 18 months and have seen alot of the US. I can camp out of it because I can carry a load in the baggage compartment.

That's it exactly - build your aircraft to fit your mission. In my case, building a 9A, I know it's going to come in a bit heavier than most. I'll have a IO360 and C/S prop, plus extended range tanks and a full glass IFR panel, slider canopy, onboard oxygen and 2-axis AP. BUT my mission is not aerobatic, or local sightseeing, or just generally goofing off - it's primarily long XC business travel. The high aspect ratio wing of the 9 extracts a very low induced drag penalty for added weight, so the extra weight will only shave 1 mph (according to Vans) off the max speed, and lets be realistic - none of us make long cruise legs at max speed anyway due to the high fuel burn (racers excepted of course). I will have all the items I want or need onboard at the expense of that 1 mph top end speed. The large majority of my flight time will include cruise in the mid-teens, and the aircraft will be built for that.

 

[Bill Wightman]

Simple steady-state climb equation

Hey Bill,

Like Kyle pointed out, an airplane climbs on excess power. We define "excess power" as the power available in excess of that required to maintain steady-state flight (level, 1g, unaccelerated condition).

Keep in mind that we're talking about power here, not force. Power is equivalent to moving a force through a distance in a certain amount of time. For an airplane climbing, this is equivalent to:

Excess thrust (TE): thrust above that req'd for steady state (pounds)
Force moved (W): The airplane's weight (pounds)
Distance: Altitude gained (feet)
Time: Time during the climb (seconds)

The hardest part of this problem to solve is knowing how much excess thrust you have on hand. Best way I know to get this is by test flying your airplane and taking down some data.

Fly a data point at a steady state that's just below your cruise speed. Record the speed (TAS), your GW (lbs) and RPM and MAP since we need to know how much crank HP you're making at that point.

Now go to your climb power, and hold the airspeed steady. Record the climb rate (fpm), the MAP and RPM.

We need to back out your excess thrust from this data. Here's how to do that. The basic equation for steady state climb looks like this (for small climb angles where the tangent of the angle is small, less than 3 degrees or so):

Climb rate (feet/second) = [excess thrust (lb) x true airspeed (ft/sec)] / weight (lb)

Solving for TE:

TE = Rate x W / TAS

We need to keep consistent units, so remember to convert your TAS to feet per second and climb rate to feet per second also.

You now have TE (excess thrust in pounds) for that data point. If you want to factor in propeller efficiency, for a good prop it will be close to .85. For a not so good prop, maybe .80. The prop efficiency is basically the difference between "crankshaft thrust" and "propeller thrust".

If you want to know actual excess power, in HP, then you need to convert units again. Since one HP is defined as 550 ft-lb/sec, take your excess thrust, divide by 550 and then multiply by your climb rate in fps. The result will be excess horsepower (propeller horsepower, not crank HP).

Take several data points like the one above. Develop a database of your airplane's performance. Now you can goof around with answering your original question: "If I install more HP, but weigh more, how will it affect performance?"

Keeping all other factors equal, and just changing TE and weight you can solve for the new climb rate, using the basic climb equation.

And, if you like, just go fly and take the data like I said. Send it to me and I'll be happy to work up a spread sheet for you. I used to do performance analysis for General Dynamics. But this will cost you a ride in the plane some day

 

[merookie]

And, if you like, just go fly and take the data like I said. Send it to me and I'll be happy to work up a spread sheet for you. I used to do performance analysis for General Dynamics. But this will cost you a ride in the plane some day

I was thinking along these lines. Take some data in your plane and post that, and someone should be able to graph it out for you pretty easily. If you have the time and the desire, do it with both engines (but not at the same time). If math isn't your thing, this is definitely the way to go.

If I might impose my own opinion, fuel economy seems to be the thing these days. If you can measure speed and fuel consumption at three different power settings (again, for both engines), it should be pretty simple to decide which motor is going to give you the best of both speed and fuel consumption.