Engine choices

Dear folks, this forum is of most outstanding source of information for everything about RVs. Flying , building, so on...

I am faced with a difficult choice, I plan to operate a RV7A on a very high altitude field (4500ft having common days of @6500ft ) and with a grass runway 1650ft long (510m) .

I am buying an assembled RV7A that until now has by contract a Lycoming YIO-360-M1B 180hp c/s prop Hartzell C2YR-1BFP/F7497? 72

But I can still change that. My main goal is:

A) to maintain SAME fuel consumption because in Brazil some regions the facilities that carry fuel are very distante from one another

B) I don't want to increase much weight to the plane nose, so I have to consider that on planing a new engine

C) I want the most horse power with the lowest consumption and weight possible.

That will made my take off in such a high field more safe on a hot day. I did some experience with 180hp from a friend and the landings are dificult in such high place, but the take-off are leaving to little margin for safety. Taking about 75% of the field on our trials.

So I was considering to put instead the Lycoming either:

- Superior 360 with high compression pistons and cold air induction sump (185hp)
or
-AeroSport Power IO-375 (high compression with 205 HP)

I HEARD that due to balanced crankshafts (with less friction) and other improvements both this engines yields more power and less fuel consumption than the stock IO360 Lycoming, I will like to read your opinion in comments does figures are real?

I will have more power with less fuel consumption and thus be able to maintain my range and take off run more safe ?

Many thanks in advance for all the opinions that are based on your Experience in flying one of those.

Engine power is important, but don't forget that light weight is even more important. If you increase the engine power by 10%, the take-off distance should decrease by 10%. But if you decrease the weight by 10%, the take-off distance should decrease by 20%.

Once you have your new aircraft, study every possible way to safely reduce its weight. Also consider a diet for the pilot, unless he is already a super lightweight guy.

Fuel consumption: fuel injection is a big advantage if you cruise at low power, as it makes it practical to run lean of peak EGT. If you are prepared to fly 20 kt slower than maximum cruise speed, the distance traveled per litre of fuel will increase very significantly.

Kevin Horton said:

Engine power is important, but don't forget that light weight is even more important. If you increase the engine power by 10%, the take-off distance should decrease by 10%. But if you decrease the weight by 10%, the take-off distance should decrease by 20%.
y.

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I'd be interested in seeing a derivation (or reference) for those numbers.

It's classical physics:

V^2 = 2 a S, or

S = V^2/(2 a)

S = the distance required
a = the average acceleration
V = the ground speed at lift off

I.e. The distance required to accelerate to a given speed is proportional to the square of the speed divided by the average acceleration.

Acceleration, a = F / m

a = acceleration
F = force (i.e. the thrust)
m = mass

i.e., the acceleration is proportional to the thrust divided by the mass (I'll use the term weight from here on instead of the more technically correct mass). The thrust comes from the propeller, and the thrust should be roughly proportional to the horsepower, if we keep the propeller unchanged. Thus if we keep the weight the same, the acceleration is proportional to the power.

Combining the two above equations, we get:

S = m V^2 / (2 F)

If we keep the weight the same, the take-off speed should be the same, so V^2 does not change. Looking at last equation, we see that the distance S is inversely proportional to the thrust (or power). So, a 10% increase in power should yield approximately a 10% decrease in take-off distance, assuming everything else is the same.

==============
Now let's look at the effect of weight.

It is reasonable to assume that the angle of attack at lift-off will be approximately the same for different weights, thus the lift coefficient should be approximately the same.

The classical equation for lift is:

Lift = 1/2 rho V^2 A Cl

rho is the air density
A is the wing area
Cl is the lift coefficient.

At lift-off, the lift produced equals the weight. Thus V^2 at lift-off is proportional to the weight.

From above we saw that:

S = m V^2 / (2 F)

If we decrease the weight 10%, the mass is 10% lower and V^2 is 10% lower. The thrust, F, is unchanged. There are two terms that reduce by 10%, so S will reduce by approximately 20%.

Kevin Horton said:

and the thrust should be roughly proportional to the horsepower, if we keep the propeller unchanged. Thus if we keep the weight the same, the acceleration is proportional to the power
.

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All of your equations depend on the Force (thrust) being constant. Unfortunately, the above quote is a very poor approximation. A better (although still not great for cruise props) approximation is to assume that the prop efficiency is constant, in which case F=P/v. You can then solve Newton's law to get that the takeoff distance S is proportional to mass, velocity cubed, and inverely proportional to power.

So you got the right functional dependencies for the weight and power, but for the wrong reasons!

BobTurner said:

All of your equations depend on the Force (thrust) being constant. Unfortunately, the above quote is a very poor approximation. A better (although still not great for cruise props) approximation is to assume that the prop efficiency is constant, in which case F=P/v. You can then solve Newton's law to get that the takeoff distance S is proportional to mass, velocity cubed, and inverely proportional to power.

So you got the right functional dependencies for the weight and power, but for the wrong reasons!

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Prop efficiency is not even constant - it'll be fairly low at the start of the take-off run, and increase as the air speed increases (the prop efficiency data I have from Hartzell and MT show this clearly). A proper assessment of take-off distance vs weight and power would likely require a numerical solution, using data from prop efficiency maps. The only way to describe it in simple terms is to make gross assumptions.

But, I'm glad we agree that weight is a much more important than horsepower if you want to decrease take-off distance.

It was interesting to look at the STOL take-off and landing competitors from Valdez at Oshkosh. The extremely lightweight Cub-like one with an O-200 had a shorter take-off distance than the O-360 powered ones.

Simple answer, go to Barret Precission Engines, ask for a parallel valve high compression engine (most power and lower fuel burn) and fit it with the Hartzel BA prop.

You will get light weight (of the stock engine) and better HP than a stock 180, and have better fuel burn, but it will need Avgas not mogas.

Keep your engine but change your wing.

If you are going to operate out of a high density airport, go with the -9 rather than the -7.

The long wing and unique airfoil on the -9 take the worry out of high and hot operations.

As I understand it, Aerosport Power simply assembles kit engines or rebuilds cores. Thus you could end up with a Superior, ECI, Lycoming, or a mix of all three.

Prop

I see you have listed the B/A prop good thing, but in a 72 inch. Go with 74 inch. Much more bite in thin air.

I have an Aerosport Power engine in my purchased 7. I Have about 50 hours on it in the past 6 weeks. I love the power of the engine but haven't owned it long enough to tell you how it will perform over a longer time frame. But the extra power is very nice.

I cruise at about 23 squared at 175mph TAS or better and burn about 8.6 GPH for what it's worth.

Andy

EDIT: I also have a Hartzel CSP.

I would say if you want to understand the difference between weight/ horsepower/ takeoff/ and landing , go to the mothership. They have performance numbers and variations for each and also for different model numbers. I presume that these numbers are for sea level performance, but you can see the differences.
( http://www.vansaircraft.com/public/rv7perf.htm and http://www.vansaircraft.com/public/rv9perf.htm )
I have not had a chance to play with the physics from Kevin and Bob but it looks like a good exercise for tonight. Also remember to consider landing distance.

On fuel economy, hp is directly comparable (proportional) to fuel, by definition. I have noticed however that engine efficiency is better when not running at peak HP and/or peak RPM. Hence 180hp from a 180hp power plant is less efficient than 180 hp from a 200 hp power plant. I can think of some reasons why that is true, but I do not have any specific data to point you to.

RV10inOz said:

Simple answer, go to Barret Precission Engines, ask for a parallel valve high compression engine (most power and lower fuel burn) and fit it with the Hartzel BA prop.

You will get light weight (of the stock engine) and better HP than a stock 180, and have better fuel burn, but it will need Avgas not mogas.

Click to expand...

TH

Thanks RV10inOz do you operate one of that? What are your impressions on fuel consumption, have trustfull numbers to tell?

AndyRV7 said:

I have an Aerosport Power engine in my purchased 7. I Have about 50 hours on it in the past 6 weeks. I love the power of the engine but haven't owned it long enough to tell you how it will perform over a longer time frame. But the extra power is very nice.

I cruise at about 23 squared at 175mph TAS or better and burn about 8.6 GPH for what it's worth.

Andy

EDIT: I also have a Hartzel CSP.

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Thanks AndyRV7!!

Did you have to make any modifcations to VANS standard mounts , fairings our cowls or its is all straightforward?

Another question if you will... What exhaust and exactly outfit engine and acessories you bought.

Thanks again in advance.

N941WR said:

If you are going to operate out of a high density airport, go with the -9 rather than the -7.

The long wing and unique airfoil on the -9 take the worry out of high and hot operations.

As I understand it, Aerosport Power simply assembles kit engines or rebuilds cores. Thus you could end up with a Superior, ECI, Lycoming, or a mix of all three.

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Thanks N941WR but I was operating a RV9A in this runway easily (read my profile after my msgs) now I sold it because I need aerobatics.

So I have to manage safe operation for RV7 on this already critical runway. Landings are OK I am trying to manage my take offs by adding some more power!!!

ALMARTON said:

Thanks AndyRV7!!

Did you have to make any modifcations to VANS standard mounts , fairings our cowls or its is all straightforward?

Another question if you will... What exhaust and exactly outfit engine and acessories you bought.

Thanks again in advance.

Click to expand...

I bought the plane already built but I believe it is as stock as you can get. I am no expert. I only know what I have learned from the builder and in speaking with the manufacturers of the equipment that was in my plane. The O-375 is a stroked O-360, so the mount and size of the engine should be the same as the 360. It is also carburated so no fuel injection or horizontal induction to deal with. I believe the exhaust is Vetterman.

I am a new owner, and not a builder so I wish I could offer more input on the engine itself in terms of cost or quality. I have not heard anyone say a bad thing about Aerosport Power though, so I feel good about owning it. My "standard" takeoff is about 400' at sea level. I'm sure with a few degrees of flaps and the stick back, I could shorten that a fair amount. But that is off of pavement. I did some training in a fixed pitch RV6 with an O-360, and some test flights of similar planes I was interested in. I have to say, this definitely feels a lot stronger on the runway than any of those, but I'm sure only some of that is the extra power. The constant speed prop is night and day different from the fixed pitch prop I flew when it comes to takeoff roll.

One other thing I think I benefit from a little more now is the ability to go around. I've gotten a little out of shape on some cross-wind landings and full throttle really gets you back to flying speed in a hurry.

Good luck with your choice.

Andy

Engine

In your case I would be inclined to go with a factory 0 360, preferably injected, because of your location. Parts and service would appear to be best with a stock engine.
The engines with higher than stock compression tend to use extra fuel for the takeoff and climb. This will likely be counter to your desire for fuel economy.

jrs14855 said:

In your case I would be inclined to go with a factory 0 360, preferably injected, because of your location. Parts and service would appear to be best with a stock engine.
The engines with higher than stock compression tend to use extra fuel for the takeoff and climb. This will likely be counter to your desire for fuel economy.

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Thanks jrs14855 , I appreciate your input but I'm not sure I do agree with you, I may have indeed problems with maintenance but I think it will be minimum compared to the gains... And I hope the many improvements (eg: counter
balanced crankshaft and roller tappets.) that those engine have would improve my fuel consumption in cruise.

So I may attain the objective a more safe take off without sacrificing range.

But I respect your input , do you actually flown any of those engines (Aero Sport Power and Superior)?

I have experienced Superior in another friends plane, comparing with stock IO-360 was a noticeable gain (fuel and power) but I have not data or experience to compare Superior with Aerosport.

My MAIN drawback in Brazil if choosing for one of those I think would be resale. Because not all buyer here knows yet the benefits and reputation of those custom engines. They will have to rely in what I tell...

Thanks you all , I look forward for more experience and opinons all are being of great help and appreciated indeed.

Can any of you guys that posted here vote and possibly add info to this related thread in the link bellow?

http://www.vansairforce.com/community/showthread.php?t=118919